ML604: Biomedical Instrumentation Manual. Supriya Babu . ... An analog-to-digital converter (ADC) is...

Prepared by: Instructor: Ms. Supriya Babu, Assistant Professor,

Dept. of Medical Electronics, M S Ramaiah Institute of Technology Email: [email protected]

Verified by: Co-Instructor: Mrs. Purnima B. R., Assistant Professor, Dept. of Medical Electronics, M S Ramaiah Institute of Technology Email: [email protected]

Authorized by:

2012-13

M S Ramaiah Institute of Technology (Autonomous Institute, Affiliated to VTU)

Department of Medical Electronics

ML604: Biomedical Instrumentation Lab Manual

Supriya Babu


Supriya Babu Department of Medical Electronics 2012-2013 1

Week No.

Experiment Name Page Nos.

1. Introduction to BMI Lab and Design of experiments 2-4

2. Output Characteristics of recorder for Bio Signal using: X-Y Recorder/ X-t Recorder, Operations of DSO

5-7

3. Determination of Pulse Rate using Photo- and Mechanical Transducer and Measurement of ECG and derivation of heart rate, pulse transit time

8-11

4. Determination of Respiratory Rate using Temperature Transducer 12-15

5. Demonstration of interfacing the following equipment: Spirometer and Non –Invasive Ventilator

16-19

6. Demonstration of interfacing the following equipment: Oximeter and Haemodialysis Machine

20-26

7. Determination of Hearing Loss by Air conduction & bone Conduction using Audiometer

27-31

8. Design of basic Hearing Aid 32-34

9. Determination of percentage Transmittance, Absorbance & Concentration of given solutions using Spectrophotometer

35-38

10. Demonstration of Cardiac Defibrillator 39-42

11. Basic Design of a Cardiac Pacemaker 43-48

12. Demonstration of Physiotherapy Equipment and Electrotherapy Equipment

49-54

13. Principle & Working of Ultrasound Scanner 55-60

14. Repetition of any experiment + final Viva -



1. Introduction to BMI Lab and Design of experiments

Introduction

To acquire the analog data and send it to the PC for further processing one needs to have a signal acquisition system as shown in figure 1.1.

This system in general consists of a Sensor/ Transducer which senses the physiological variable and converts it to electrical signal which can be further processed to obtain information. Biological signals are in general low in amplitude and hence the output of the sensor needs to be amplified. In addition the signal of interest should be filtered before being converted to a digital signal. An analog-to-digital converter (ADC) is a circuit that produces a digital signal that corresponds to an analog voltage. The idea is shown graphically in Figure 1.2.

Figure 1.2: ANALOG TO DIGITAL CONVERSION WITH DIFFERENT SAMPLING RATES

Figure 1.1: BASIC BLOCK DIAGRAM OF A SIGNAL ACQUISITION SYSTEM



An analog voltage (like most “real-world” signals) can have any value (within operating ranges) and is defined at every time. In contrast, a digital signal can only have certain values and is only defined at specific times. An ADC samples the incoming analog voltage at the sampling rate, fs. Now, obviously, if the sampling rate is too low some events could be missed. Thankfully the Shannon-Nyquist sampling theorem gives us a way to know what sampling rate we must use such that the sampled signal accurately represents the original voltage. If the maximum frequency of the incoming voltage is fmax, the sampling rate must be fs=2*fmax. If the incoming voltage is not band-limited (i.e., it has no maximum frequency), then the maximum frequency that can be represented by fs/2, and aliasing will occur. In Figure 2 the sampling frequency is 0.2 Hz, so the maximum frequency that the sampled signal can reproduce is 0.1 Hz.

After sampling, each voltage sample must be converted to a digital representation for storage. In a digital system, voltage values are encoded as binary numbers. In the binary, or base-2, system, the only numerals available are 0 and 1. In contrast, the decimal system that we are used to is base-10 and ten numerals (0 through 9) are used. When counting up from zero, we only need one digit until we have a number larger than 9. Then we reset the first digit to zero and put a 1 as the second digit (the tens place). In a base-2 system, we count from 0 to 1 and then need another digit. Thus 2 in a decimal system is 10 in binary and 25 in decimal is 11001 in binary. The binary number system is easily implemented in electronic circuitry.

When digitizing a sampled voltage value, the number of digits in the binary representation, or bits, is very important. Consider the following example to understand why. First assume a digitizer with a minimum input voltage of -10 V and a maximum of +10 V. If we have only one bit, we might say that any input less than 0 V is 0 and any input greater than 0 V is 1. We would have very poor resolution because each digital value could represent an input with a range of 10 V. If we can use two bits to represent the voltage, we have four possible values (00, 01, 10, 11) and each value could represent an input range of 5 V. Our resolution has improved. If our digitizer has m bits, the total number of possible values is 2m and the resolution is:



In Figure 1.2, a 2-bit ADC is compared to a 14-bit ADC. First note that the ADC voltage only changes when a new sample is acquired. In the 2-bit plot, the ADC voltage is close to the input voltage for many samples, but is further away for others. Note that the four possible values of the digitized voltage are -10 V, -5 V, 0 V, and 5 V. In the 14-bit plot, at the sampled points the ADC voltage and input voltage are indistinguishable. An ADC with more bits has better resolution. Sometimes this extra resolution is unnecessary, like when the input voltage is noisy.

An analog voltage that has been digitized now is stored on the computer as a series of binary numbers. Many of the linear analysis techniques that are used for continuous time systems can be adapted for analysis of the digitized signal. For example the digitized signal can be filtered and we can take a Fourier transform (more precisely a Fast Fourier Transform, or FFT) to see its frequency content.

CLEAN-UP

Please turn off all instruments and put cables in their place. Don't forget to record all the observations in your record book. Be sure to log-off of the lab computer.

POST-LAB ASSIGNMENT

Design an experiment to a hypothesis that you have come up in this lab. It should include the aim, apparatus required, methods, expected results and conclusions.



2. Output Characteristics of recorder for Bio Signal using X-t Recorder

Introduction

Study of X-t recorders provide an opportunity to study the way biomedical signal can be recorded on a paper for filing and analysis. X-t recorders are also called strip chart recorders.

The one you will study in laboratory is a SYSTRONICS made. It is a Potentiometric null balance servo recorder which provides an accurate and permanent graphic record of an input signal. Outputs of analytical or other precision instruments can be graphically recorded by this unit. A null balance Potentiometric servo system and a digitally controlled stepper motor chart drive ensure high precision and accuracy. Reliability is obtained by using solid state devices.

Figure 2.1 shows the simplified block Diagram of the Strip Chart Recorder.

The input signal to the recorder is first amplified to a level which is less susceptible to noise and interference. This signal is later applied to a differential amplifier which continuously compares it to the feedback signal developed by rebalance potentiometer. The difference between these two signals, called an error signal. This error signal is amplified by FET chopper, amplifying chopping at 50 Hz, and is used to drive a servomotor. This servomotor drives the rebalance potentiometer in a direction such as to reduce the error signal to zero.

A recorder pen and the rebalance potentiometer are mechanically coupled to the servomotor, so that the pen position on the chart represents an accurate and continuous graphic record of the input signal. The chart speed is controlled by digitally controlling the frequency of the signal fed to the stepper motor.



Figure 2.1 Simplified Block-Diagram of X-t Recorder

Figure 2.2 Top-View of X- T recorder



PROCEDURE

Refer the Top View of the instrument as given before. 1. Install the chart paper and pen in their respective holders. 2. Set the SENSITIVITY switch at 10 mV. 3. Set ATTENUATOR control to CAL. 4. Toggle PEN LOCK switch upward. 5. Set the EXT/INT switch of the CHART SPEED in INT. 6. Set the CHART SPEED at 16 x 2. 7. Set the cm/min, cm/hour switch at cm/min. 8. Lift up the ‘Pen Lift Lever’. 9. Switch the POWER ON. The POWER indicator should glow. So does the PEN

LOCK indicator. The chart moves at 32 cm/min. 10. Set the EXT/INT switch in EXT to temporarily stop the chart movement. 11. Lower the ‘Pen Lift Lever’. 12. Toggle the PEN LOCK switch downward. Its LED indicator switched off. 13. Press PUSH button and adjust ZERO control to set the position as baseline of

chart. 14. Connect signal to be recorded to the HI and LO terminals. The GND terminal

can be returned to the earthing of signal source. 15. Now select the SENSITIVITY and the chart speed according to the required

resolutions on the Y and t-axis. 16. Start recording the signal by returning the EXT/INT switch of the CHART

SPEED to INT position.

CLEAN-UP

Please turn off all instruments and put cables in their place. Don't forget to record all the observations in your record book. Be sure to switch-off instrument after use. Thanks!



3. Determination of Pulse Rate using Photo- and Mechanical Transducer and

Measurement of ECG and derivation of heart rate, pulse transit time

Introduction: Photoplethysmography Plethysmographs measure changes in volume. The only method to measure absolute changes in blood volume accurately in the extremities is by using chamber-plethysmography. The volume change can be converted into blood flow by using F = dV/dt. However, in some cases, we are only interested in the relative volume, for example to see the pulsation rate of the heart. In that case, information is in the timing and not in the amplitude or shape of the signal and we can use photo-plethysmography (PPG) for its measurement. A PPG sensor can be used in reflection mode (for example on the finger) or in transmission mode (for example on the ear) as shown in Figure 3.1. Normally, a wavelength in the near-infrared is used because there we have the strongest modulation of the signal due to light absorption in the haemoglobin in the blood. If the infrared light is transmitted through the skin of the earlobe or fingertip it will encounter capillaries that perfuse the tissue with blood. The amount of light that is reflected depends on the amount of blood in the capillaries, which increases and decreases with each heartbeat. The photodiode converts the signal from the reflected light into an electrical signal that can be used to monitor a patient's heart rate without the use of electrodes.

Figure 3.1: (a) LIGHT TRANSMITTED INTO THE FINGER PAD IS REFLECTED OFF

BONE AND DETECTED BY A PHOTO SENSOR. (b) LIGHT TRANSMITTED THROUGH THE AURAL PINNA IS DETECTED BY A PHOTO SENSOR [Medical Instrumentation by

Webster]



Diodes are non-linear devices that are vital components of most modern electronics. In this lab, students will assemble a biomedical device (a photoplethysmograph) that uses an infrared LED and diode to measure changes in peripheral blood volume and pulse rate. Like all diodes, LEDs have non-linear I-V characteristics (Figure 3.1). When the voltage across the two terminals of a diode is less than the turn-on voltage (Von), no (or very little) current flows through the diode and the diode has very high resistance. When the voltage is above Von, the resistance of the diode drops dramatically. When the diode is ``on'', the current through the diode is determined by the rest of the circuit.

Figure 3.2-DIODE CURRENT-VOLTAGE CURVE

Diodes can also be used to detect light. In this mode, the diode has no voltage bias, or it is reverse biased (i.e., the voltage is negative). The diode housing is clear which allows light to hit the semiconductor material. If a photon has a short enough wavelength, an electron in the diode will be energized creating a current. The current-light intensity relationship is linear over a wide range as shown in Figure 3.3.

Figure 3.3-PHOTODIODE CURRENT-VOLTAGE CURVES FOR DIFFERENT LIGHT

INTENSITIES



Pulse Transit Time (PTT) is the time it takes the pulse pressure waveform to propagate through a length of the arterial tree. The pulse pressure waveform results from the ejection of blood from the left ventricle and moves with a velocity much greater than the forward movement of the blood itself. To measure pulse transit time, record the time difference between peak of the QRS-wave and the peak of pulse waveform at the fingertip.

Figure 3.4: THE MEASUREMENT OF PULSE TRANSIT TIME (PTT)

PRE-LAB QUESTIONS

Read the lab procedure. 1) Describe the function of the photodiode circuit shown in Figure given below. What is

the output voltage (Vpd) as a function of photodiode current?

2) Find the specification sheets of the infrared LED and photodiode on the Google.

What are the peak wavelengths of the infrared LED and a red LED? Which do you think the photodiode should be more sensitive to?

LAB PROCEDURE



1. Have one volunteer attach disposable pre-gelled ECG electrodes to his/her upper arms favourably on a vein (where you feel the pulse) and left leg just above the bone. Connect the left arm, right arm and left leg electrodes to the ECG amplifier.

2. Connect the Photo-Pulse Transducer to the finger tip, such that the photodiode and detector are on the dorsal side.

3. The student volunteer should remain still and not touch any of the measurement equipment or circuit while the batteries are plugged into the circuit.

4. Connect the batteries and wait a few seconds for the ECG signal to go to baseline. 5. Connect the block for measuring ECG and PPG. 6. Note the different amplitude and time periods of both ECG and PPG. Do this for

about 9 – 10 complete waves and calculate the average values. 7. Calculate the Pulse Transit Time for about 9 to 10 ECG and PPG cycles and

average them.

CLEAN UP

Please put parts back into their places and return all the components that you have taken. Thanks! POST-LAB QUESTIONS

Please read clinical applications of Photoplethysmography. a. The photoplethysmograph has both AC and DC components. What is the

physiological basis for each? b. What are three factors that determine what wavelength of light to use? Briefly

describe each. c. What factors can affect reproducibility of the photoplethysmograph? Did you

experience any of these in lab? d.What is the clinical application of photoplethysmography? How is

photoplethysmography used in this experiment? What part of the waveform is used (e.g., amplitude, shape, pulse time)? What information do clinicians hope to gain?



4. Determination of Respiratory Rate using Temperature Transducer

Introduction

Human respiration rate is measured when a person is at rest and involves counting the number of breaths for one minute by counting how many times the chest rises. Respiration rates may increase with fever, illness, or other medical conditions. When checking respiration, it is important to also note whether a person has any difficulty breathing.

From physiological monitoring point of view, monitoring respiration rate is involved to:

1. Check the level of saturated oxygen in blood. As sPO2 decreases, respiration rate increases

2. Detect respiratory arrest or apnea.

3. Prolonged, minimally intrusive measurement of the breathing pattern is particularly important for polysomnography, sleeping disorders, stress monitoring, biofeedback techniques, and circadian rhythm analysis.

Respiration rate can be monitored in different ways.

• Impedance Pneumograph, which measures ac impedance-changes across chest that occur due to movement of chest can be used to measure respiration rate (RR).

• Pneumograph that uses piezoresistive strain gauges can be used in a Wheatstone bridge configuration with temperature compensation and offset control.

• Temperature transducers like thermistors and some fast response thermocouples can be used to monitor the nasal airflow. Since the expired air is warmer than inhaled, RR can be monitored.

• Gas flow rate can be determined by rotating-vane, ultrasonic, and thermal-convection flow meters and this can be in turn used to monitor RR.

In this lab you will use nasal temperature sensor to measure RR.



Temperature sensors:

A thermistor is an electronic component similar to a resistor. Unlike a resistor, a thermistor is built using a semiconductor process that makes it extremely sensitive to changes in temperature. Under constant current, a thermistor will change its resistance as a function of the temperature.

Normally, manufacturers of resistors attempt to make their components less sensitive to variations in temperature, since a circuit that is designed with a resistor should have the same resistance regardless of the surrounding temperature. On the other hand, thermistor manufacturers need to make their devices extremely sensitive to temperature, so that small changes in temperature can be transduced into voltages. The relationship of the resistance of a thermistor to temperature is given by the following equation:

R = K e / T

K represents the intrinsic resistance of the device since R=K when T=0, is a sensitivity

constant that characterizes each different type of thermistor, and T is the temperature in Kelvin.

As you notice from the equation above, the relationship between the temperature and resistance is a non-linear equation. In fact, it is exponential. As a result, if measuring a larger temperature range is desired, some linearization will need to be performed. By linearizing the relationship between temperature and resistance, the thermistor is made useful over a broader range of temperatures. A first order linearization is accomplished by connecting a series resistance with the thermistor, creating a current limiting resistance. The equation for this is given by:

RL is the value of the resistance desired for linearization, Rm is the resistance of the thermistor at the midpoint of the temperature range desired, and b is a sensitivity constant that characterizes each different type of thermistor.



Since, Thermistors act like resistors, they can be used in many different circuits. One of the simplest is a voltage divider with one of the resistors replaced by a thermistor. This circuit is not very linear (which is made worse by the non-linearity of the thermistor itself). Sometimes the thermistor is placed in a bridge circuit, but again the output voltage is not linear with change in resistance (particularly because resistance might be expected to vary over a wide range).

One simple circuit is shown below in Figure 4.1. The op amp is in a basic inverting amplifier configuration. The output voltage will change as the resistance changes, but the change is linear. Consider: the current through the input resistor will not change. Because no current (or very little anyway) enters the op amp inputs, that constant current must pass through the thermistor. If voltage across the thermistor is measured and current is constant, Ohm’s Law can be used to find the unknown thermistor resistance.

Figure 4.1: Thermistor Circuit

PRE-LAB QUESTIONS

1) What are the respiratory control centres in human body? 2) Which is the dominant factor that determines the respiratory rate? 3) Define the following:

Apnea, Dyspnea, Hyperpnea, Tachypnea, Hypopnea, Bradypnea, Orthopnea and Platypnea



LAB PROCEDURE

1. In this laboratory, you will monitor the breath rate of the subject at rest and after some mild exercise. First we will measure the breath rate at rest.

2. Collect the respiratory transducer from the instructor and connect it to the measurement device.

3. Connect the pulse output and ground to the positive and negative leads of DSO.

4. Ask the subject to sit in a relaxed position and ask him/her to hold the transducer in front of their nose/ mouth.

5. Set the display settings on DSO, so that you get stable reading.

6. Once you get a stable reading, simultaneously record the RR, by counting the number of pulses and noting the time period from the DSO, manually counting the number of breaths and the RR displayed on the device.

7. Ask the subject to take long deep breaths and monitor RR.

8. Ask the subject to talk and monitor the changes in RR.

9. Always remember to take at least three reading and then note the average.

10. Sketch the circuitry used for measuring RR.

CLEAN UP

Return back the respiratory transducer and put the chairs back in their positions.

POST-LAB QUESTIONS

What are the different types of temperature transducers? Compare the types of temperature sensors based on the following criteria: linearity, sensitivity, speed, ease of use, cost and temperature range.



5. Demonstration of interfacing the following equipment: Spirometer and Non –

Invasive Ventilator

Introduction: Spirometry

The assessment of pulmonary function is important in the diagnosis and evaluation of obstructive and restrictive pulmonary diseases.

The flow-volume loop (also called a spirogram) is a plot of inspiratory and expiratory flow (on the Y-axis) against volume (on the X-axis) during the performance of maximally forced inspiratory and expiratory maneuvers.

The normal expiratory portion of the flow-volume curve is characterized by a rapid rise to the peak flow rate, followed by a nearly linear fall in flow as the patient exhales toward residual volume (figure 5.1). The inspiratory curve, in contrast, is a relatively symmetrical, saddle-shaped curve. The flow rate at the midpoint of exhalation (between total lung capacity and residual volume) is normally approximately equivalent to the flow rate at the midpoint of inspiration.

Obstructive lung disease is clinically identified by a decrease in expiratory flow rates, the anatomical basis of which is airway narrowing, whereas, restrictive lung disease is clinically identified by decreased lung volumes. Pulmonary functions utilized in the diagnosis and prognosis of these diseases includes:

• Forced Vital Capacity (VC) • Flow Rate (FEV1) • Flow-Volume Loops • Maximal Ventilatory Volume (MVV) • Maximum Flow Rates

Thus, in restrictive diseases, the maximum flow rate is reduced, as is the total volume expired. The flow is abnormally high in the latter part of expiration because of increased recoil and in obstructive diseases, the flow rate is very low in relation to lung volume, and a scooped out appearance is often seen following the point of maximal flow.



Figure 5.1: Flow-Volume Loops in Normal and various Diseased Lung states (Image Source: http://www.frca.co.uk/article.aspx?articleid=100023)

PRE-LAB QUESTIONS

1. Learn about various parameters measured for assessing lung function s.

PROCEDURE

You will be taken to M S Ramaiah Memorial Hospital and the Lab Technician will demonstrate the equipment and the procedure used to connect the device and make the measurements. Make a note of all the instruction given by the technician in your record. Paste representative recordings from that demonstration and write your discussions. POST-LAB QUESTIONS

1. How do FEV1 and FVC change in Obstructive and Restrictive Lung diseases?

2. Can you differentiate Obstructive and Restrictive Lung diseases just by looking at flow volume loops?



Introduction: Non-Invasive Ventilators

Mechanical ventilation can be noninvasive, involving various types of face masks, or invasive, involving endotracheal intubation. Mechanical ventilation is generally considered when there are clinical or laboratory signs that the patient cannot maintain adequate oxygenation or ventilation. Concerning findings include respiratory rate > 30/min, inability to maintain arterial O2 saturation > 90% with fractional inspired O2

(Fio2) > 0.60, and PaCO2 > 50 mm Hg with pH < 7.25. The decision to initiate mechanical ventilation is based on clinical judgment that considers the entire clinical situation and should not be delayed until the patient is in extremis.

Selection and use of appropriate techniques require an understanding of respiratory mechanics. Normal inspiration generates negative intrapleural pressure, which creates a pressure gradient between the atmosphere and the alveoli, resulting in air inflow. In mechanical ventilation, the pressure gradient results from increased (positive) pressure of the air source.

Peak airway pressure is measured at the airway opening (Pao) and is routinely displayed by mechanical ventilators. It represents the total pressure needed to push a volume of gas into the lung and is composed of pressures resulting from inspiratory flow resistance (resistive pressure), the elastic recoil of the lung and chest wall (elastic pressure), and the alveolar pressure present at the beginning of the breath (positive end-expiratory pressure [PEEP].

Figure 5.2: Components of airway pressure during mechanical ventilation, illustrated by an inspiratory-hold maneuver. (Image Source: http://www.merckmanuals.com/)



Noninvasive positive pressure ventilation (NIPPV): NIPPV is the delivery of positive pressure ventilation via a tight-fitting mask that covers the nose or both the nose and mouth. Because of its use in spontaneously breathing patients, it is primarily applied as a form of PSV, although volume control can be used.

NIPPV can be given as continuous positive airway pressure (CPAP) or bi-level positive airway pressure (BiPAP). In CPAP, constant pressure is maintained throughout the respiratory cycle with no additional inspiratory support. With BiPAP, the physician sets both the expiratory positive airway pressure (EPAP) and the inspiratory positive airway pressure (IPAP), with respirations triggered by the patient. Because the airway is unprotected, aspiration is possible, so patients must have adequate mentation and airway protective reflexes and no imminent indication for surgery or transport off the floor for prolonged procedures. NIPPV should be avoided in patients who are hemodynamically unstable and in those with evidence of impaired gastric emptying, as occurs with ileus, bowel obstruction, or pregnancy. In such circumstances, swallowing large quantities of air may result in vomiting and life-threatening aspiration. Indications for conversion to endotracheal intubation and conventional mechanical ventilation include the development of shock or frequent arrhythmias, myocardial ischemia, and transport to a cardiac catheterization laboratory or surgical suite where control of the airway and full ventilatory support are desired. Obtunded patients and patients with copious secretions are not good candidates. Also, IPAP must be set below esophageal opening pressure (20 cm H2O) to avoid gastric insufflation.

PRE-LAB QUESTIONS 1. Learn about different types of ventilators and specifically non-invasive ventilators. PROCEDURE Technicians will bring the ventilators to the lab and demonstrate at least one type of the non-invasive ventilators. Make a note of all the parameter settings in your record. POST-LAB QUESTIONS 1. Under what circumstances are non-invasive ventilators used? 2. When is a CPAP device used? And when is a BiPAP device used? 3. What are the different parameters set in CPAP and BiPAP devices?



6. & 9. Demonstration of interfacing the following equipment: Oximeter and

Haemodialysis Machine

Introduction: Oximeter

Measuring the arterial oxygen saturation (SpO2) levels of patients is very beneficial to healthcare providers and has over past few decades become one of the most vital and easiest parameters to obtain and analyze. SpO2 levels can provide valuable information including the efficiency of pulmonary gas exchange and the adequacy of alveolar ventilation, blood-gas transport, and tissue oxygenation. In the past, SpO2 levels were measured by taking periodic blood samples and analyzing them several times a day using large, expensive laboratory equipment. This method was not only inconvenient for doctors, nurses, and patients, but it failed to provide accurate real-time measurements of SpO2. During the 1970’s much attention was paid to developing a cheaper, more efficient way of measuring SpO2. This led to the introduction of non-invasive pulse oximetry in the early 1980’s. Pulse oximetry is a non-invasive method to determine SpO2 by utilizing light emitting diode (LED) technology to measure the differences in red and infrared light absorption or reflection of oxygenated and deoxygenated hemoglobin (Photoplethysmography).

Photoplethysmography is an optical method for measuring volume changes in tissue by analyzing changing volumes of an optically absorbent/transmitting substance such as blood. As arterial pulsations fill a capillary bed, the changes in blood volume modify the absorbance/transmittance of light passing through. These changing light characteristics can be measured and used to determine the heart rate of a patient. Furthermore, understanding the characteristics of oxygenated and deoxygenated blood, this technology can be extended for use as a non-invasive method to determine SpO2.

Most of the oxygen carried by the blood (~98%) is reversibly bound to a protein molecule called hemoglobin that is found in erythrocytes (red blood cells). Hemoglobin has the ability to bind one oxygen molecule to each of its four subunits that are composed of a single heme group containing a central iron atom where oxygen binding takes place. At any time, depending the on the SpO2, there are varying amounts of



completely oxygenated (4 molecules of oxygen bound), partially oxygenated (1-3 molecules of oxygen bound), and deoxygenated (0 molecules of oxygen bound) hemoglobin circulating in the blood stream. Oxygenated and deoxygenated hemoglobin possess unique optical characteristics when exposed to red and infrared wavelengths of light. More specifically, oxygenated hemoglobin absorbs infrared light and transmits red light, while deoxygenated hemoglobin absorbs red light and transmits infrared light. This phenomenon can be utilized to obtain an accurate, non-invasive measurement of SpO2.

Oxygen saturation is the percentage of hemoglobin molecules in the arterial blood that are bound to oxygen. There are at least four forms of hemoglobin present in the blood: oxyhemoglobin (bound to oxygen, HbO2), reduced hemoglobin (unbound, rHb), carboxyhemoglobin (bound to carbon monoxide, COHb), and methemoglobin (a form of hemoglobin that cannot release bound oxygen, metHB). Oxygen saturation can be calculated from:

Pulse oximetry can be performed in two ways, absorbance pulse oximetry and reflection pulse oximetry (Figure 6.1). In absorption pulse oximetry, two LEDs at two separate wavelengths (red: 660 nm, infrared: 940 nm) are cycled on and off at up to 480 times per second. After the transmitted red and infrared signals pass through the test site (usually a reasonably translucent area with good blood flow, such as a finger or earlobe) they are received at the photo diode. The ratios of transmitted red and infrared light are calculated and compared to tables provided by the manufacturer that are based on a calibration curve derived from various known SpO2 levels. The numeric SpO2 value returned by the pulse oximeter is the hemoglobin saturation percentage, or the percent of hemoglobin passing through the test area that is completely saturated with oxygen. Normal arterial hemoglobin saturation percentages are anywhere from 95 to 100%. In reflection pulse oximetry, the only difference lies in the fact that the photo diode is on the same side as the LEDs. The reflection of the light from test area is measured instead of the transmitted light through the test area as in absorbance



measuring. Similarly, the ratios of the lights detected are calculated and compared to a calibration curve made from normal healthy subjects.

Figure 6.1: Absorbance and reflection pulse oximetry. (Source: www.pulmolink.co.uk)

There are many different types of sensor leads for pulse oximetry. In adults it is most common to obtain the measurement through the tip of the finger using a finger clip pulse oximeter. However in some cases, such as with infants who would constantly pull the lead off, this is not reasonable. In these cases, pulse oximetry measurements can be taken from the earlobe or toe.

Figure 6.2: Both a finger clip sensor (right) and a finger wrap sensor (left) may be used for pulse oximetry readings. The difference is that the finger clip has all components embedded inside while the disposable finger wrap design requires the user to place the photosensor inside the wrap prior to use. (Source: pettibonsystem.com and www.boston.com)

Regardless of the test area, there are always constant light absorbers such as tissue and venous blood present that add a constant DC component to the detected signal. After each heart beat, there is a surge of blood into the area that momentarily increases the



blood volume across the test site. The constant DC signal is subtracted from the time-varying ac component caused by the momentary increase in arterial blood volume. This ensures that only arterial blood is used when calculating SpO2 levels, eliminating the error that would be caused by the presence of tissue and venous blood.

In early pulse oximetry systems, artifact could be a major problem in obtaining an accurate reading. A common source of artifact was motion. If a subject wearing a finger clip pulse oximeter waved their finger around in space, it would have a turbulent effect on the blood flow in the finger. This in turn would have an impact on the measurement. However, in recent years, improved algorithms from manufacturers have limited the amount that artifact can corrupt the pulse oximetry measurement

Since its inception, pulse oximetry has become a standard in the clinical environment because of its ease of use and the importance of the information it provides. Pulse oximetry is especially useful in surgical procedures and emergency situations, where the SpO2 levels can be monitored to ensure that the brain is receiving adequate amounts of oxygen.

In the this laboratory demonstration, you will observe heart rate, SpO2, respiration effort, and one channel of electrocardiography (ECG) data in order to gain a better understanding of the basics of pulse oximetry.

Introduction: Haemodialysis Machine

Kidney is one of the important organs to remove waste from the body such as potassium and urea, as well as free water from the blood. Under the healthy conditions, a kidney removes waste products from the blood and also removes excess fluid in the form of urine. Dysfunctioning kidney could loss these functions which is a dangerous situation because this may also prove fatal for a human being. Dialysis is a renal replacement for kidney dysfunction.

Dialysis is the diffusion of dissolved solutes across a selectively permeable membrane against a concentration gradient in an effort to achieve equilibrium. While small solutes pass through the membrane larger solutes are trapped on one side. By exchanging the



dialysate buffer on the outside side of the membrane, you can continually pull away the smaller solutes to purify the trapped larger molecules.

Haemo is the Greek word for blood. Dialysis means a filtering process. Haemodialysis (HD) means the process of filtering blood. In haemodialysis, the filtering process takes place in a machine outside the body. Haemodialysis is a method for removing waste products such as creatinine and urea, as well as free water from the blood when the kidneys are in renal failure. When the kidneys do not work well, dialysis is needed to remove extra fluid and waste products from the body. Hemodialysis is a type of dialysis that uses a machine with an artificial filter to remove wastes and extra fluids from the blood. This treatment also helps control the chemical balance in the body and blood pressure. Each treatment takes about 4 hours and is done three times each week.

In general, dialysis will be most effective when the buffer is replaced a few times over the course of a day and then left overnight at room temperature on a stir plate. A standard protocol for dialysis is 16 to 24 hours. Many factors affect the rate dialysis, including: diffusion coefficients, pH, temperature, time, concentration of species, sample volume, dialysate (buffer) volume, number of dialysate changes, membrane surface area, membrane thickness, molecular charges and dialysate agitation (stirring).

A general Block diagram of the process that you are going see during the demonstration:



Figure 6.3 (Source: http://coep.vlab.co.in/?sub=25&brch=78&sim=1124&cnt=1)

PRE-LAB QUESTIONS

1. Describe the physiological effects of carbon monoxide poisoning. 2. Which ion (hydrogen from the acid or hydroxide from the base) will migrate

through the dialysis tubing bag (membrane) the fastest? 3. How could the migration rate of the ions be increased? 4. How could you use a series of ions to determine the size of the pores in the

membrane?



POST-LAB QUESTIONS

1. Normal resting SpO2 values for adults are between 95 and 100%. What was the resting SpO2 of the test subject? What changes (if any) did you observe in SpO2 when instructing the subject to hold his/her breath? Explain any changes that you observed.

2. What changes did you observe in heart rate when instructing the test subject to hold his/her breath?

3. Why is it important in a clinical setting to measure both ventilation and arterial oxygen saturation?

4. How does dialysis work and how long does it take to complete? 5. Which membranes minimize protein binding the best? 6. How good is the mass transfer across the membrane if the osmolarity is equal on

both sides but concentration gradients still exist?



7. Determination of Hearing Loss by Air conduction & bone Conduction using

Audiometer

Introduction:

An audiometer is a machine used for evaluating hearing loss. Audiometers are standard equipment at ENT (ear, nose and throat) clinics and in audiology centers. Audiometry is the technique to identify and quantitatively determine the degree of hearing loss of a person by measuring his hearing sensitivity, so that suitable medical treatment or one of the appropriate hearing aids and assistive devices can be prescribed. In audiological investigations, the hearing sensitivity is tested for pure tones, speech or other sound stimuli. The result, when plotted graphically, is called an audiogram. The electronic instrument used for measuring the hearing threshold level is called an audiometer. Using it, the test tones of different frequencies and levels are generated and presented to the patient and hearing thresholds are determined on the basis of patient’s response. The auditory system and its disorders are described. Different audiometric tests, techniques and various audiometers are discussed.

You have already learnt about the hearing mechanisms and about basic audiometers in your previous semester. Here is a summary again for your reference.

The sounds reaching the ear are characterized by loudness (intensity) which depends on the amplitude, pitch which depends on frequency and in quality which depends on combination and interaction of the waves. An octave is the interval between one pitch and another with half or double its frequency. It may be derived from the harmonic series as the interval between the first and second harmonics. Human ear responds to frequencies from 20 to 20 kHz. The ear is not equally sensitive to all frequencies, particularly in the low and high Frequency ranges. The frequency response over the entire audio range is shown as as a set of curves showing the sound pressure levels of pure tones that are perceived as being equally loud. The curves are plotted for each 10 dB rise in level with the reference tone being at 1 kHz, also called loudness level contours and the Fletcher-Munson curves, as shown in figure 7.1. The lowest curve represents the threshold of hearing, the highest the threshold of pain. The curves are



lowest in the range from 1 to 5 kHz, with a dip at 4 kHz, indicating that the ear is most sensitive to frequencies in this range.

Figure 7.1: The average dynamic range of the human hearing

Figure 7.2: Range of Audibility of human ear



Audiometry tests identify the nature of hearing loss and to determine the threshold of hearing by recording responses of the patient after presenting him with auditory stimuli with varying intensity levels. There are different audiometric techniques and procedures used for achieving this. For air conduction testing, stimuli are presented to each ear independently with specialized earphones. For bone conduction testing, a bone vibrator is placed onto the mastoid process of either right or left temporal bone.

In audiometry, both ears are tested separately. During the air conduction test, the stimulus while passing from test ear to cochlea of the non-test ear gets attenuated. This loss of sound energy is called interaural attenuation and varies between 45 to 80 dB. However, during bone conduction test, the cochleae of both sides are equally stimulated i.e. the inter-aural attenuation is of 0 dB. Hence, cross hearing is a serious concern in case of bone conduction test than it is for air conduction. A bone vibrator is placed over the mastoid process of the appropriate ear and pure tones are transmitted. Factors such as vibrator placement and pressure may influence results. Fewer frequencies are tested: 250, 500, 1000, 2000, 3000, and 4000 Hz. In addition, audiometer output is limited to approximately 80 dB due to distortion and other technical factors. Interrupted signals in an ascending series are again preferred. Whenever cross hearing is suspected, it is necessary to remove the non-test ear from procedure.

A simple procedure by which this can be done is to deliver a noise to the non-test ear in order to remove it from the test procedure by masking. Here masking noise which is loud enough to prevent the tone reaching and stimulating the non-test ear, but at the same time it should not mask the sensitivity of the test ear overmasking. Wide-band noise has uniform power density spectrum over all the audible frequency range i.e. from 250 Hz to 8 kHz. However the masking effect is actually contributed by frequency components centered on the test tone frequency, over a bandwidth of about 1/3 to 1/2 octave, known as critical band.

PRE-LAB QUESTIONS

1. What are the thresholds of hearing and pain, both in dB and W/m2? 2. What is frequency range of speech? 3. What are the different types of audiometers? How are they classified?



4. What are the normal levels of hearing at different frequencies?

PROCEDURE

Puretone Audiometry: The audiometer in lab (PME portable audiometer) is very simplest model. This model facilitates for AIR and BONE conduction with WHITE NOISE masking and TONE decay test.

1. There are nine Puretone signals available to cover full spectrum of test frequencies. Select test frequency through electronic interrupter, with two computer soft-touch key tops, marked ‘AUTO/SPCH’. The first manual/reset gives a continuous tone output on the headphone as long as the key is pressed. In ‘Auto’, pulsing (1 sec ON, 1 sec OFF) appears if key is pressed once.

Puretone Audiometry for Air Conduction

2. Plug the earphones into the jack socket on side panel. 3. Switch ON the Audiometer 4. Turn the mode switch either to ‘Right’ or ‘Left’ depending on the ear to be tested. 5. Set the test frequency, turn the FREQUENCY dial ‘o’ to match the frequency

reading against shiny surface of panel. It is advised to start test with 1000 Hz. Next go to higher frequencies up to 8000Hz and come back down to 250 Hz.

6. Provide a masking noise on the non-test ear. The device is provided with white masking level of up to 100 dB SPL arranged in 10 dB steps.

7. The intensity of the test tone can be controlled by rotating the hearing loss attenuator (intensity dial)

8. While testing each frequency, the tone should be given at higher intensity level to make subject aware of tone he/she is going to hear, and then gradually the intensity reduced to his/her threshold. Threshold is the level where subject gives 50% response through switch.

9. Repeat the procedure to test the other ear.

Puretone Bone Conduction Audiometry:

10. The jack of bone conduction vibrator is plugged into the socket on the side panel of the audiometer.

11. The mode switch is positioned to BC (Bone Conduction) upto 4000 Hz can only be tested.

12. Plot the Audiogram of frequency vs. threshold (in dB)



Block Diagram of AUDIOMETER

CLEAN UP

Please put parts back into their places and return all the components that you have taken. Switch the power off of all the instruments used. Thanks!

POST-LAB QUESTIONS

1. How does excessive noise cause deafness? 2. What is the intensity of loudness for different frequencies of speech?



8. Design of basic Hearing Aid

Introduction

Speech is the most important form of communication. Some people are unlucky to lose their sense of hearing, greatly impairing their ability to communicate. The loss of hearing may be due to Conductive losses due to problems in middle ear pathology or due to Sensorineural damage at the inner ear (cochlea) or Mixed where both cochlear damage & outer/middle ear pathology problems.

According to FDA the hearing loss can be quantified based on the following criteria:

0 - 20 dB HL: Within normal limits (WNL)

20-40 dB HL: Mild

40-70 dB HL: Moderate

70-90 dB HL: Severe

> 90 dB HL: Profound

There is a lot of research being done to invent devices that improve the quality of life of hearing-impaired patients. Hearing aids are sound-amplifying medical devices to aid individuals with hearing loss. Hearing aids may be useful for treating loss that may or may not be medically treatable.

Selection of hearing aids should be based on the type and severity of hearing loss, listen-ing needs, and lifestyle.

Hearing Aids consist of following components as shown in figure 8.1:

• Microphone • Amplifier circuitry • Miniature loudspeaker/receiver • Battery



Figure 8.1: Basic Components of hearing aid

The goal is to build a simple device that will amplify sound to compensate for hearing loss, followed by a set of filters that retain the optimal frequency range. The next stage is to make the device capable of stimulating the cochlear nerve if the inner hair cells (they are responsible for conversion of mechanical energy in the sound wave into electrical input to the brain via the cochlear nerve) are destroyed.

Procedure:

1. Instead of an input sound signal, use a sine wave as the input. You will vary the input amplitude as well as the frequency to characterize the device that you have constructed.

2. Construct an amplifier with a gain of 20. You may select an inverting or a noninverting amplifier in your design. Ensure that you are obtaining the desired amplification before moving on to the next step.

3. Construct an active filter with a pass band between 200 Hz and 4 KHz. This can be done by cascading separate low pass and high pass filters. Use op amps in filter construction.

a. Build the high pass filter first with the cutoff at 200 Hz and a gain of 1 in the pass band. The output from the amplifier will serve as input for this stage.



b. Build the low pass filter with the cutoff at 4 kHz and a gain of 1 in the pass band.The output from the high pass filter will serve as input for this stage.

c. Compare the output waveforms at each stage of the circuit (amplifier, high-pass filter and low-pass filter) as you vary the input frequency of the sine wave.

d. Draw a Bode plot of your circuit performance. This is done by plotting the ratio of the output voltage and input voltage as a function of frequency. Plot your resultson a dB vs. log10 (freq). Now you should have a functional simple hearing aid that amplifies and filters the signal.

4. Half-wave rectify your signal by including a diode at the output. 5. Make a comparator with a reference voltage of 0 V. Check the input and output of

the comparator. There should be a variable-width pulse for each half cycle of sound. Bonus: Set the reference voltage so that your circuit does not produce pulses for low amplitude noise.

6. The comparator will give a ±12V output (rail voltage). Make a voltage divider to reduce this to ±6V. Then use a voltage follower to buffer the output.

Pre-Lab Questions

1. Telephones use a filter with a pass-band approximately in the range of 200 Hz to 4 kHz. Why is this frequency range chosen?

2. Draw the circuits for each stage of the lab. Use the resistor and capacitor values that are available in the lab in planning your circuit. Show your calculation of theoretical gain and cutoff frequencies of the pass-band.

3. Sketch a theoretical frequency response for your circuit.

Post-Lab Questions

1. What is meant by cut-off frequency of a circuit? Apart from the cut-off frequency, What additional parameters characterize the filter response?

2. What are the advantages of an active filter over a simple RC filter? 3. Why is a logarithmic amplifier better than a linear amplifier for this device?



10. Determination of percentage Transmittance, Absorbance & Concentration of

given solutions using Spectrophotometer

INTRODUCTION

Spectrophotometry is a technique used for measuring the quantity of light that is absorbed or that passes through (is transmitted) by a sample solution or mixture. While spectrophotometry techniques are available for all portions of the electromagnetic spectrum, in this lab, our spectrophotometers will detect electromagnetic waves that are between 400nm and 700nm in wavelength – the spectrum of visible light.

The data provided by the spectrophotometer takes on two general forms: percent transmission and absorbance. Transmittance (T) is defined as the fraction of incident light which is transmitted (i.e. passes through) a sample. Thus, T = I/Io, where Io equals the intensity of light which strikes the sample and I is the intensity of light after passing through the sample. Transmittance is usually expressed as a percentage: %T = (I/Io) x 100 Absorbance (A), or optical density is a logarithmic function of T and is expressed as: A = log10 (1/T) = log10 (Io/I)

Because absorbance is plotted on a logarithmic scale, there is a direct linear relationship between the concentration of the absorbing substance and absorbance. Each substance within a given sample will absorb light at characteristic wavelengths. The color displayed by a substance represents the wavelengths reflected. An absorption spectrum can be generated by measuring the absorbance (after zeroing on a blank) over a range of wavelengths. This is best depicted by a line graph such as the one shown in figure 10.1. Figure 10.1: Absorption spectra for three different pigments commonly found in plants.



By measuring the absorption spectrum of a substance (i.e. all the wavelengths at which it absorbs) it is possible to identify it or at least place it in a particular class of compounds. The wavelength at which peak absorption occurs, the absorption maximum (λ max), is very useful when trying to identify an unknown. By creating and measuring a series of standards (e.g. serial dilutions), it is possible to quantify the amount or concentration of a substance in a sample. For example, in their pure form, the nucleic acids can be quantified by absorption measurements in the UV range.

The absorbance and transmittance of each dilution at the λ max is measured. The graph of absorbance or transmittance vs. concentration for the serial dilutions will describe a direct relationship, known as Beer’s Law.

The relationship between absorbance and transmittance is illustrated in the following diagram:

So, if all the light passes through a solution without any absorption, then absorbance is zero, and percent transmittance is 100%. If all the light is absorbed, then percent transmittance is zero, and absorption is infinite. PROCEDURE

1. The wavelength on a Spec 20 is selected by adjusting a prism within the instrument such that only a narrow range of light wavelengths are directed through the sample.

2. In order to measure the absorbance of a particular substance in a reaction mixture, it is necessary to first "zero out" the spectrophotometer such that only the absorbance of the substance of interest is measured. This is done with a blank - a cuvette which contains the carrier solvent without the substance of interest.



3. Zeroing the Spectrophotometer

a. Turn on the Spectrophotometer and allow it to warm up for 5 - 10 minutes (left front knob). Set wavelength using the dial on top of the Spectrophotometer.

b. Prepare a blank cuvette by adding the solvent (DI water) without the absorbing substance.

c. Raise the sample holder trapdoor and insert the cuvette in the sample holder. Close the lid.

d. Using the right front knob, adjust the meter needle to read absorbance = 0.0 (= 100 % transmittance). This step is called setting the "full scale".

The spectrophotometer is now calibrated to this blank at this wavelength. 4. Measuring Absorbance on the Spectrophotometer

a. Remove the blank and insert the cuvette containing the test solution. Close the lid.

b. Read the absorbance (lower scale) or transmittance (upper scale) as appropriate for your sample. Since you are creating an absorption spectrum, you should note the absorbance at each wavelength.

c. Create an absorption spectrum for the test solution. Measure the absorbance from 400 to 700 nm at 10nm intervals. Remember that you must re-zero the Spectrophotometer with the blank every time you change the wavelength.

d. Plot the absorbance values on a graph with the absorbance on the y axis against wavelength on the x axis to create an absorption spectrum for the test solution.

5. Make 10 dilutions of test solutions using the formula CfVf = CiVi Cf: is the final concentration Vf: is the final volume Ci: is the initial concentration Vi: is the initial volume Calculate the test substance concentration of the serial dilutions and enter those values in the results table. Each dilution is a 1:1 dilution, so the test substance concentration is halved from each dilution to the next.

6. Record all of the absorbance values for each serial dilution of the test solution. 7. Plot a graph of absorbance against concentration. Apply Beer’s Law to estimate

the concentrations of the unknown samples. Enter those values in the table.



PRE-LAB QUESTIONS

1) What is the relationship between percent transmission and absorbance? 2) Why is absorbance data most frequently used to determine the concentration of a

substance in a solution? 3) If an unknown substance has a green color, what wavelengths would you expect

that it does not absorb? POST LAB QUESTIONS

How did you use the standard curve that you generated for the serial dilutions to determine the quantity of test substance in concentrations lesser than the least concentrated solution you used for obtaining calibration curve?



10. Demonstration of a Cardiac Defibrillator

Defibrillation is a common treatment for life-threatening cardiac dysrhythmias, ventricular fibrillation, and pulseless ventricular tachycardia. Defibrillation consists of delivering a therapeutic dose of electrical energy to the affected heart with a device called a defibrillator. This depolarizes a critical mass of the heart muscle, terminates the dysrhythmia, and allows normal sinus rhythm to be reestablished by the body's natural pacemaker, in the sinoatrial node of the heart. Defibrillators can be external, transvenous, or implanted, depending on the type of device used or needed. Some external units, known as automated external defibrillators (AEDs), automate the diagnosis of treatable rhythms, meaning that lay responders or bystanders are able to use them successfully with little, or in some cases no training at all.

An automated external defibrillator (AED) is a lightweight, portable device that delivers an electric shock through the chest to the heart. The shock can stop an irregular rhythm and allow a normal rhythm to resume in a heart in sudden cardiac arrest. Sudden cardiac arrest is an abrupt loss of heart function. If it’s not treated within minutes, it quickly leads to death. Most sudden cardiac arrests result from ventricular fibrillation. This is a rapid and unsynchronized heart rhythm starting in the heart’s lower pumping chambers (the ventricles). The heart must be “defibrillated” quickly, because a victim’s chance of surviving drops by 7 to 10 percent for every minute a normal heartbeat isn’t restored.



Figure 10.1: a) Defibrillation Electrode Position b) Automated External Defibrillation (AED)

Mechanism of operation of AED

When turned on or opened, the AED will instruct the user to connect the electrodes (pads) to the patient. Once the pads are attached, everyone should avoid touching the patient so as to avoid false readings by the unit. The pads allow the AED to examine the electrical output from the heart and determine if the patient is in a shockable rhythm (either ventricular fibrillation or ventricular tachycardia). If the device determines that a shock is warranted, it will use the battery to charge its internal capacitor in preparation to deliver the shock. This system is not only safer (charging only when required), but also allows for a faster delivery of the electrical current.

When charged, the device instructs the user to ensure no one is touching the patient and then to press a button to deliver the shock; human intervention is usually required to deliver the shock to the patient in order to avoid the possibility of accidental injury to another person (which can result from a responder or bystander touching the patient at the time of the shock). Depending on the manufacturer and particular model, after the shock is delivered most devices will analyze the patient and either instruct CPR to be given, or administer another shock.



Many AED units have an 'event memory' which store the ECG of the patient along with details of the time the unit was activated and the number and strength of any shocks delivered. Some units also have voice recording abilities to monitor the actions taken by the personnel in order to ascertain if these had any impact on the survival outcome. All this recorded data can be either downloaded to a computer or printed out so that the providing organization or responsible body is able to see the effectiveness of both CPR and defibrillation. Some AED units even provide feedback on the quality of the compressions provided by the rescuer.

Latest advancements in external Defibrillators is the wearable cardioverter defibrillator (WCD) is worn by patients at risk for sudden cardiac arrest (SCA), allowing their physician time to assess their long-term arrhythmic risk and make appropriate plans.

The WCD is lightweight and easy to wear, allowing patients to return to their activities of daily living, while having the peace of mind that they are protected from SCA. The WCD is non-invasive and consists of two main components – a garment and a monitor. The garment, worn under the clothing, detects arrhythmias and delivers treatment shocks. The monitor is worn around the waist or from a shoulder strap and records the arrhythmias. The WCD continuously monitors the patient’s heart and, if a life-threatening heart rhythm is detected, the device delivers a treatment shock to restore normal heart rhythm. The entire event, from detecting a life-threatening arrhythmia to automatically delivering a treatment shock, usually occurs in less than a minute. Timely defibrillation is the single most important factor in saving an SCA victim’s life. A treatment shock must be delivered within a few minutes after an event to be effective; with each passing minute, a patient’s chances of survival drops 10 percent. The WCD does not require bystander intervention and has a 98 percent first treatment shock success rate for resuscitating patients from SCA.The WCD may be prescribed for patients at high risk of SCA following a heart attack, before or after bypass surgery or stent placement, as well as for those with cardiomyopathy or congestive heart failure that places them at particular risk.

The WCD is a therapeutic option, listed along with implantable cardioverter defibrillators (ICDs), in the ACC/AHA/ESC Guidelines for the Management of Patients with Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death. It is also a



Class I recommendation in the International Society for Heart and Lung Transplantation Guidelines for the Care of Cardiac Transplant Candidates.

Figure 10.2: Wearable Cardioverter Defibrillator (Image Source: http://www.fda.gov/)



11. Basic Design of a Cardiac Pacemaker

INTRODUCTION

A Cardiac Pacemaker consists of a battery, a pulse generating circuit and leads to deliver these pulses to the heart. This lab focuses on design of a pulse generating circuit and the pulses are delivered through a single strand wire. The function of the output circuit is to deliver periodic voltage pulses to the heart muscle. High standards of quality and durability are required for the design and implementation of the circuitry. A patient’s quality of life, and sometimes his/her life itself, depend upon the proper operation of the pacemaker. Malfunction of the unit generally results in an unhealthy and unhappy customer. Design of the output circuit has evolved since the first pacemaker was implanted. Many early units had RC oscillators and transformer-coupled outputs. Quite a few of these devices were also constant current output sources. As time has progressed, these units have fallen out of favor and been replaced by devices with constant voltage capacitor discharge outputs and crystal oscillators. Other advances in pacemaker design, such as triggered and inhibited sensing, have created new problems which require solving. CONSIDERATIONS FOR OUTPUT DESIGN

Cardiac pacemakers are required to deliver a stimulus pulse of sufficient magnitude and duration to cause heart muscle contraction. A typical stimulus pulse has a charge in the range of 0.1 to 50 µC and duration of 0.1 to 2 ms. To achieve a sufficient stimulation value and maximize the battery longevity, a pacemaker requires output pulse programmability of about ten amplitude and ten duration values. All new implantable pacemakers are constant voltage sources, designed for typical load impedances between 400 and 1000. When designing an output circuit, it is important to understand the basics of heart stimulation in order to more effectively address the problem of maximal stimulation with minimal use of pacemaker energy. You have studied this in theory. Here is a brief explanation of some of these basic concepts concerning pacemakers.



Strength–duration curve Varying the pulse duration is a common and effective method of controlling the pacemaker output. Understanding the relation between the pacemaker pulse voltage amplitude and duration required for effective heart stimulation can help increase the pacemaker lifespan. In order to gain an understanding of what occurs during the output pulse, it is necessary to understand the strength–duration curve, shown in Figure 11.1(b). Figure 11.1(a) shows a stimulation device (the pacemaker) in series with a cell membrane. Depolarizing (i.e. achieving a voltage drop of 20 mV across Rm) the membrane with different current values results in the strength–duration curve of Figure 11.1(b). In reality, while the load seen by a pacemaker is more complex than a simple membrane, the resulting strength–duration curve has the same characteristics as a cell membrane, with different current and voltage values. Thus, for the sake of simplicity, the membrane model will be used to explain the response of the heart to electrical stimulation.

Log Pulse Duration

L o g C u r r e n t

chronaxie time

rheobase

(b)

(a)

Rm Cm

Cell Membrane

S t i m u l a t o r

Figure 11.1 (a) A simplified version of a pacemaker stimulating a load. In this case, the load is simply a cell membrane. (b) The strength–duration curve associated with the circuit in part (a). While this specific curve relates to the depolarization of a single cell



membrane, the strength–duration curve for stimulation of the heart has the same shape and characteristics. The rheobase is the minimal current that will cause the membrane to depolarize. The chronaxie time is the pulse duration required to cause depolarization when the stimulating current magnitude is twice the rheobase. The stimulus threshold curve results from the interaction of the pacemaker patient’s heart and the electrode and is influenced by many factors. These include the size and nature of the stimulating electrode, the nature of the heart, the placement of the lead, epinephrine levels in the body, etc. Since numerous factors affect the threshold for cardiac pacing, a 100% safety margin is generally set. This means that the energy delivered while pacing should be 100% higher than the minimal amount of energy necessary to excite the heart. This can be easily determined using the strength–duration curve and energy equations previously mentioned. The threshold for excitation increases by a factor of two to three after implantation due to inflammation and the development of scar tissue but then stabilizes after a month or so. Output waveform characteristics Pacemakers should ideally deliver a square wave pulse from the cathode with characteristics similar to Figure 11.2(a). The duration and amplitude of the pulse should be adjustable. All modern pacemakers are of the constant voltage output type. Thus, the load impedance has a large effect on the stimulus current. A load with high impedance will have an excessively high threshold of stimulation, while low impedance will result in large currents and premature battery drain. There are four different pacemaker output designs that are available.

0 V

Vdd

(a) Ideal (b) ActualT T

Figure 11.2 (a) A representation of an ideal stimulation pulse from a constant voltage stimulator. The voltage is measured from the stimulating tip to the reference (either the ring or the pacemaker can). Vdd can either be a fixed value, as in single voltage devices, or a programmable value, with variability dependent upon the pacemaker model. The period of stimulation T is variable in all devices. (b) A realistic depiction of a waveform appearing across the heart emitted from a capacitor discharge output circuit. Note that the drop in pulse voltage magnitude is dependent upon the size of the output capacitor.



A larger capacitor will have a waveform which more closely resembles an ideal constant voltage. The small rise and exponential decay after the stimulus pulse is after-potential.

Basic circuit

Figure 11.3 shows an example of a simple capacitor discharge system. The capacitor is periodically discharged across the load resistance (the pacemaker lead and heart) to stimulate the heart muscle. During operation, if the voltage at the base of the transistor is low, then the transistor will be in its nonconductive (off) state. In this situation, the capacitor charges to Vdd through the pull-up resistor. Upon reception of a signal at its base, the transistor goes into its conductive state, and ground voltage appears at the collector. Since the collector is now at ground, the voltage drop across the capacitor causes a voltage of –Vdd to appear at the load resistance. This causes stimulation and subsequent contraction of the heart muscle. After the pulse signal is removed from the base of the transistor, the transistor turns off, and the output capacitor recharges to Vdd.

Vdd

Pulse signal

Load resistance

(a) Circuit (b) Output

Time

Voltage 0

Figure 11.3: A simple version of the capacitor discharge output circuit. The capacitor initially charges to the voltage Vdd. Upon reception of a signal from either the microprocessor or a separate oscillator, the transistor is switched into its conductive state. This causes ground voltage to appear at the collector. Since the output capacitor is already charged with voltage Vdd, and the load resistance is connected to ground, a negative voltage –Vdd appears across the output resistance. This voltage induces current flow which stimulates the heart muscle to contract. (b) The output which appears across the heart.



Unipolar stimulating circuit

A slightly more sophisticated circuit shown in Figure 11.4. To understand the function of the circuit, assume that it is initially at steady state: all three capacitors have a charge of Vdd across them (noting that the 3.3-µF electrolytic capacitor has a polarity opposite that of the + sign), and both transistors are not in the conducting state.

Vdd

33 kž33 µF

15 kž

33 µF

33 kž

3.3 µF

33 kž Zener 8.2 VOutput to heart

Q1

Q2

Micro- processor

(A)

(B)

(C)

Figure 11.4: A basic unipolar output circuit. A command from the microprocessor causes an output of -2Vdd to appear at the output. Adapted from Blaser (1980).

If a pulse is applied at the base of Q1, with sufficient magnitude to place the transistor in the conductive state, then the voltage at the collector of Q1 will effectively go to ground. Since capacitor A cannot discharge immediately, this forces the emitter voltage of Q2 to be shifted below the ground potential of the circuit. The base of Q2 is connected through the 15-kΩ resistance to ground and is switched into the conductive state when the emitter voltage drops. This changes the voltage at the collector of Q2 toward –Vdd. Just prior to Q2 being switched, the 3.3-µF capacitor has a charge of positive Vdd appearing at the negative side of the capacitor. When the collector voltage of Q2 is abruptly changed toward –Vdd, the 3.3-µF capacitor already has a voltage drop of Vdd across it. This causes a voltage close to –2Vdd to appear at the output electrode. The circuit acts as a voltage doubler. The zener diode is in place to limit the output



voltage fluctuations, while the 33-F decoupling capacitor B connected from the supply rail to ground is in place to stabilize Vdd when a pulse is being delivered.

The duration of the output pulse can be controlled by adjusting the length of time that Q1 is in the conductive state. A controller attached to the base of the transistor could supply pulses with the duration adjusted internally by some logic implementation, or externally through telemetry. As an even more basic design, attachment of a fixed rate oscillator to Q1 and the battery directly to the supply line Vdd, would provide an output with a fixed voltage and fixed duration.

PROCEDURE:

Get the components for the basic and unipolar stimulating circuit. Connect the circuits and verify the results. Observe the waveforms generated by the circuits and record.

PRE-LAB Questions

1. What parameters decide the design of a Cardiac Pacemaker? 2. What are the different types of Artificial Pacemakers? 3. What parameters have you considered in designing the circuit? 4. How is the Electromagnetic interference overcome?

POST-LAB Questions

1. How can you improve the design further to make it programmable?



12. Demonstration of Physiotherapy Equipment and Electrotherapy Equipment

Different Treatments Used by Physiotherapists

The types of physiotherapy treatments vary and depend on a particular muscular or skeletal condition. Often, physiotherapy treatments are specifically made for one person and may not be the same for another with the same condition.

Cold Therapy or Cryotherapy

Cold therapy is effective in minimizing pain and swelling, especially in acute (immediate) injuries. It may also be used just after deep kneading massage (DKM). Examples of cold therapy are ice pack application and ice massage. Heat Therapy

Applying heat can help improve your blood flow especially at the site of injury, thus, speeding up healing. In addition, it can help soften tight tissues and relieves pain. There are several heating modalities and devices available including

• Hot packs • Paraffin wax bath • Infrared heat • Ultrasound (US) • Diathermy

Electrical Stimulation

Electrical stimulation is used to prevent muscle atrophy (wasting) in people with paralysis and assist in gaining some degree of muscle strength, such as after a knee surgery. In electrical stimulation, electrodes are placed on the surface of the skin, which can cause muscles to shorten.

TENS

TENS, or transcutaneous electrical nerve stimulation, is a type of electrical stimulation. It is a small battery-powered device that sends low current through the electrodes place on the skin surface. A TENS device does not cause the muscles to contract, but rather helps temporarily relieve pain.



Range of Motion (ROM) Exercises

Range of motion exercises are often prescribed to increase or maintain flexibility of your joints and to reduce stiffness. There several types of range of motion exercises often prescribed including the following

Passive Range of Motion (PROM), Active Assistive Range of Motion(AAROM), and Active Range of Motion (AROM) Exercices

Strengthening Exercises

Certain conditions can make your muscles to become weak. Strengthening exercises is an important part of physiotherapy rehabilitation to prepare you for your return to your original performance level or highest possible function.

Soft Tissue Mobilization

Soft tissue mobilization or therapeutic massage may be a part of your physiotherapy treatment to relax your tight muscles, relieve pain and reduce swelling.

The following are other physiotherapy treatments.

Joint Mobilization, Gait (Walking) Training / Assistive Device Training, Postural Training, Ergonomic training, Balance Exercises, Traction, Taping, Bandaging and Fitting of Orthosis. You will be observing two Physiotherapy treatments:

1. Shortwave Diathermy 2. Transcutaneous Electrical Nerve Stimulation (TENS, electrotherapy)

Short wave diathermy Short wave diathermy has been a viable modality in physical therapy for many years. Originally, it was utilized for musculoskeletal conditions which require increased circulation and other benefits of thermal increase. It has proven an effective modality in the management of non-orthopedic conditions such as pulmonary, urologic and gynecological conditions. Heat generation through short wave diathermy depends on the molecular activity within the tissue exposed. Differential heating is noted within bone, muscle and fat



because of the different densities of these tissues. More the dense tissue more it becomes warmer than the less-dense one. Also, shorter the wavelength or higher the frequency is, greater is the penetration. The frequencies assigned for short wave diathermy operations are 13.66, 27.33, and 40.98 megahertz. Most commercial machines operate at a frequency of 27.33 megahertz, a wavelength of approximately 11 meters. Physiological Effects: The principal effect of shortwave diathermy on the body is the production of heat in the tissues due to the rise of temperature. 1. Increased metabolism: Heating tissues accelerates chemical changes such as metabolism, so oxygen and food staffs are used up. As there is increased metabolism, there will be increased output of waste products. 2. Increased blood supply: As a result of increased output of waste products such as metabolites, which act on the walls of the capillaries and arterioles, these tissues dilate. In addition, heat has direct effect on blood vessels, causing vasodilatation. Heat also produces stimulation of superficial nerve endings, causing reflex dilatation of the arterioles. Vasodilatation increases blood flow through the area, so that the necessary oxygen and nutritive materials are supplied and waste products are removed. 3. General rise of temperature: As blood passes through the heated tissues, it also becomes heated and carries the heat to other body parts. As a result, heat reduces the excitability of nerves, provided that heat is not excessive. Furthermore, rise of temperature induces muscle relaxation and increases the efficiency of muscle action. 4. Fall in blood pressure: The generalized vasodilatation reduces the peripheral resistance to blood flow. Heat also reduces blood viscosity, which helps in the reduction of blood pressure. 5. Increased activity of sweet glands: If a general rise of temperature occurs, there will be increased activation of the sweet glands. SWD is used to treat: - Myositis and arthritis, Pelvic inflammation, Otitis media, Bronchitis, Sinusitis etc.



Transcutaneous Electrical Nerve Stimulation (TENS, electrotherapy)

Transcutaneous Electrical Nerve Stimulation (TENS) is the most commonly used electrical stimulation device to apply electrotherapy to the body for the treatment of back pain. A TENS unit has electrodes that can be placed over the painful area, over the nerve supplying the painful area or on the opposite side of the body. The user can adjust the intensity of electrical stimulation and chose either high or low frequency. The exact mechanism of electrical stimulation’s beneficial effect is not known, although it is thought that TENS may effectively treat pain by directly blocking transmission of pain signals along nerves. In addition, electrical stimulation has been shown to promote the release of endorphins, which are natural painkillers produced by the body.

TENS is used for relaxation of muscle spasms, prevention and retardation of disuse atrophy, increase of local blood circulation, muscle rehabilitation and re-education, maintaining and increasing range of motion, and management of chronic and intractable pain.

The main treatment variables which are available on modern machines are:

The current intensity (A) (strength) will typically be in the range of 0 - 80 mA, though some machines may provide outputs up to 100mA. Although this is a small current, it is sufficient because the primary target for the therapy is the sensory nerves, and so long as sufficient current is passed through the tissues to depolarise these nerves, the modality can be effective. The machine will deliver discrete ‘pulses’ of electrical energy, and the rate of delivery of these pulses (the pulse rate or frequency (B) will normally be variable from about 1 or 2 pulses per second (pps) up to 200 or 250 pps (sometimes the term Hertz or Hz is used here). To be clinically effective, it is suggested that the TENS machine should cover a range from about 2 – 150 pps (or Hz). In addition to the stimulation rate, the duration (or width) of each pulse (C) may be varied from about 40 to 250 micro seconds (ms). Recent evidence would suggest that this is possibly a less important control that the intensity or the frequency and the most effective setting in the clinical environment is probably around 200 - 250 micro sec.

Mechanism of Action:

The type of stimulation delivered by the TENS unit aims to excite (stimulate) the sensory nerves, and by so doing, activate specific natural pain relief mechanisms. For convenience, if one considers that there are two primary pain relief mechanisms which



can be activated: the Pain Gate Mechanism and the Endogenous Opioid System, the variation in stimulation parameters used to activate these two systems will be briefly considered.

Pain relief by means of the pain gate mechanism involves activation (excitation) of the A beta (Aβ) sensory fibres, and by doing so, reduces the transmission of the noxious stimulus from the ‘c’ fibres, through the spinal cord and hence on to the higher centres. The Aβ fibres appear to appreciate being stimulated at a relatively high rate (in the order of 90 - 130 Hz or pps). It is difficult to find support for the concept that there is a single frequency that works best for every patient, but this range appears to cover the majority of individuals. Clinically it is important to enable the patient to find their optimal treatment frequency – which will almost certainly vary between individuals. Setting the machine and telling the patient that this is the ‘right’ setting is almost certainly not going to be the maximally effective treatment, though of course, some pain relief may well be achieved.

An alternative approach is to stimulate the A delta (Aδ) fibres which respond preferentially to a much lower rate of stimulation (in the order of 2 - 5 Hz), which will activate the opioid mechanisms, and provide pain relief by causing the release of an endogenous opiate (encephalin) in the spinal cord which will reduce the activation of the noxious sensory pathways. In a similar way to the pain gate physiology, it is unlikely that there is a single (magic) frequency in this range that works best for everybody – patients should be encouraged to explore the options where possible.

TENS Modes:

Traditional TENS (Hi TENS, Normal TENS) Acupuncture TENS (Lo TENS, AcuTENS)



Modulation Mode TENS:



13. Principle & Working of Ultrasound Scanner

Introduction

Ultrasound is a sound wave that has a frequency greater than 20 KHz. It is generated by applying an alternate current to a piezoelectric crystal (found in the transducer in the sound head). This crystal contracts and expands at the same frequency at which current changes polarity. The sound field generated by this crystal in turn makes the molecules in the sound field vibrate and oscillate. The crystal commonly used in US units is synthetic plumbium zirconium titanate (PZT). Crystal quality to be used depends on the following:

• Beam Nonuniformity ratio: ranges from 2 to 6 – the smaller the better. • Effective Radiating Area: as close to sound head area as possible • Therapeutic ultrasound has a frequency range of 0.7 and 5.0 MHz. • Most clinics will have 1 MHz and 3 MHz sound head.

Sound Wave Physics Solids and liquids consist of molecules held together by elastic forces that behave like rubber bands connecting each molecule to each of its nearest neighbors. If one molecule is set in vibration, then it will cause its immediate neighbors to vibrate, and in turn their neighbors, and so on until the vibration has propagated throughout the entire material. This is a wave. A sound wave is sound energy that is transmitted from one molecule to the next. It cannot travel by itself. It needs a medium for transmission (solid, liquid, gas). Energy contained within a sound beam is decreased as it travels through tissue. Energy is lost to: Reflection or scattering of the sound beam when it strikes a reflecting surface Absorption – energy lost by the sound wave as it overcomes internal friction that exists in tissue while traveling through it. Higher the frequency, the more rapidly the molecules are forced to move against this friction. The more they move, the more energy is consumed (absorbed); the sound beam will therefore have less sound energy



available to propagate further through the tissue. The velocity of the wave travel depends on the closeness of the molecules of the medium. The closer the molecules, the quicker they collide with each other and sooner they respond to disturbance, the faster they loose energy in a short distance. Energy final = Energy initial – (E reflected + E absorbed) So a 3MHz sound head will affect more superficial tissues while a 1MHz sound head will affect deeper tissues. Frequency dependence The attenuation coefficient is a function of ultrasound frequency. As shown in figure 13.1, the coefficient is roughly proportional to frequency for a variety of tissues. In fact, a good rule-of-thumb is that the attenuation coefficient for soft tissue is 1 dB cm-1 MHz-1. Note the implications: at 1 MHz, transmission through 10 cm of tissue reduces intensity by a factor of 10, but at 10 MHz, transmission through 10 cm of tissue reduces intensity by a factor of 10,000,000,000! This frequency-dependent attenuation imposes limits on the performance of imaging systems. Most ultrasound imaging is based on the localization of “echoes” or scattered waves from structures within the tissue. Intuitively, we would expect that it is easier to figure out where a short pulse came from because longer pulses would lead to temporal overlap in the echoes and greater ambiguity about their source. Hence it is generally desirable for the source to emit a short pulse, but this is not what we get from a resonating transducer:

The emitted pulse lasts a long time and looks like a pure “tone”. This corresponds to a narrow frequency spectrum. If the transducer is “damped” so that it does not resonate, the pulse is much shorter, and the spectrum is correspondingly broad.



Acoustic fields of ultrasonic transducers A very small transducer (compared to the ultrasound wavelength) would act like a point source (and detector). This would be useless for imaging because we would not be able to direct the ultrasound pulse to a specific region, nor would we be able to tell which direction echoes were coming from. A-Scan and B-Scan Images: A- Scan Image is simply the plot of ultrasound echoes. To obtain a complete B-Scan image it is necessary to get sufficient A scans. In the early days of ultrasound imaging this was accomplished by moving a single element transducer over the surface of the patient. Before considering modern methods, it is instructive to examine the resolution attainable with the older method.



Resolution in Ultrasound Images Consider the case of a rectilinear scan where parallel A scans are obtained by translation of the transducer.



The axial resolution is determined by the pulse length. This is why it is important to have well-designed wide-band transducers that emit a short pulse. Typical values of axial resolution are 1 mm or less.

The lateral resolution depends on the focusing properties of the transducer. It can be improved by increasing the frequency or by decreasing the f#. The first option is limited by the clinical requirements for penetration. In general, systems use the highest frequency possible. For a small organ like the thyroid, a higher frequency can be used than is possible with a large organ like the liver. The second option is limited by the fact that stronger focusing (i.e. lower f#) will result in a reduced “depth-of-field”. This means that lateral resolution is rapidly degraded at depths out of the focal plane. Doppler ultrasound The Doppler shift is a familiar phenomenon at audible frequencies. The same thing happens when ultrasound is scattered by a moving object such as a red blood cell.

The factor of two appears in the equation because the moving object “hears” a Doppler shift, as does the observer of the scattered ultrasound. For typical blood velocities and



ultrasound frequencies, the Doppler shift is in the kHz regime. In simple systems, this can be presented to the operator as an audible signal.

The Doppler signal must be found in the “clutter” of signals from stationary objects. This is one case where a narrow-band or CW system is advantageous. One problem with such a system is that it provides no depth resolution so it may be difficult to localize the blood flow information. This is overcome in pulsed Doppler systems that determine the Doppler shift as a function of depth.

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