Chapter 8 in Introduction to Biomedical Equipment Technology: Electrocardiography
By Joseph Carr and John Brown
Biomedical Instrumentation I
Schematic Representation of
Electro-Conduction System
• SA Node• AV Node• Bundle of
His• Bundle
Branches• Purkinjie
Fibers
From Berne and Levy Physiology 3rd Edition Figure 23-25
Pathway of Electro-Conduction System of the calf heart starting at AV Node
• AV Node• Bundle of
His• Bundle
Branches• Purkinjie
Fibers
From Berne and Levy Physiology 3rd Edition Figure 23-28
Electrocardiograph (ECG)• Components:
– P wave = Atrial Contraction
– QRS Complex = Ventricular Systole
– T Wave = Refractory Period
• Typical measurement from right arm to left arm
• Also see 1 mV Calibration pulse
Carr and Brown Figure 8-1
Different Segments of ECGP wave: the sequential activation (depolarization) of the right and left atria
QRS complex: right and left ventricular depolarization (normally the ventricles are activated simultaneously)
ST-T wave: ventricular repolarization
U wave: origin for this wave is not clear - but probably represents "afterdepolarizations" in the ventricles
PR interval: time interval from onset of atrial depolarization (P wave) to onset of ventricular depolarization (QRS complex)
QRS duration: duration of ventricular muscle depolarization
QT interval: duration of ventricular depolarization and repolarization
RR interval: duration of ventricular cardiac cycle (an indicator of ventricular rate)
PP interval: duration of atrial cycle (an indicator or atrial rate
Typical LeadsRA = right armLA = Left armLL = left legRL = right legC = Chest
Different leads result in different waveform shapes and amplitudes due to different view and are called leads
Cardiac Axis by different Leads
Carr and Brown Figure 8-2
Types of LeadsBipolar Limb Leads: are those designated by
Lead I, II, III which form Einthoven Triangle:– Lead I = LA connection to noninverting amp. input
And RA connecting to inverting amp. Input
– Lead II = LL connection to amp. Noninverting input RA connect to inverting input and LA shorted to RL
– Lead III = LL connected to noninverting input LA connected to inverting input
LL LL LL
Einthoven Triangle:
Note potential difference for each
lead of triangle
Carr and Brown Figure 8-3
Each lead gives a slightly different representation of electrical activity of heart
•
Unipolar Limb Leads= augmented limb leads: leads that look at composite potential from 3 limbs simultaneously where signal from 2 limbs are summed in a resistor network and then applied to an inverting amplifier input and the remaining limb electrode is applied to the non-inverting input
Lead aVR = RA connected to non-inverting input while LA and LL are summed at inverting input
augmented (amplified) Voltage for Right arm (aVR)
Lead aVL = LA connected to non-inverting input while RA and LL are summed at inverting input
augmented (amplified) Voltage for Left arm (aVL)
Lead aVF = LL connected to non-inverting input while RA and LA are summed at inverting input
augmented (amplified) Voltage for Foot (aVF)
Types of Leads
LL LL LL
Types of LeadsUnipolar Chest Leads: measured with signals from
certain specified locations on the chest applied to amplifiers non-inverting input while RA LA, and LL are summed in a resistor Wilson network at amplifier inverting inputs
LL
Wilson’s Central Terminal
• Configuration used with Unipolar Chest Leads where RA LA and LL are summed in resistor network and this is sent to the inverting input of an amplifier
Electrocardiograph Traces from different leads
Normal ECG with RA, LA, LL connected
Artrial Tachycardia with RA, LA, LL connected
Ventricular Tachycardia with RA, LA, LL connected
Variations in Chest Leads C with RA and LA connected C1
C2
C3
1st Degree block RA LA LL connected
Normal
PR wave is prolonged >0.2 sec have a prolongation of delay between atrial and ventricle depolarization
2nd Degree Block Intermittent failure of AV conduction, such that not every P wave
is followed by QRS complex
Normal
3rd Degree Block
Complete failure of conduction between atria nd ventricles. Common cause is AMI (Acute Myocardial Infarction
Normal
R Bundle Branch Block
Normal
Widened QRS complex abnormalities in R S as well as T wave Q is not as affected because the left bundle branch initiates depolarization
Other ECG Signals
• Interdigital ECG: Signal taken between 2 fingers usually for home monitoring
• Esophageal ECG: electrode placed in esophagus close to heart typically used to record atrial activity where P and R wave are used to determine position
• Toilet Seat ECG: used to detect cardiac arrhythmias that can occur during defecation
Block Diagram of ECG
ECG Pre-Amplifier
• High Impedance input of bioelectric amplifier
• Lead selector switch• 1mV calibration source• Means of protecting amplifier from high
voltage discharge such as a defibrillator used on a patient
• Amplifier will have instrumentation amplifier as well as isolation amplifier
Isolation Amplifier
• Needed for safety! Want to isolate patient from high voltages and currents to prevent electric shock where there is specifically a barrier between passage of current from the power line to the patient.
• Can be done using light (photo emitter and photo detector) or a transformer (set of inductors that are used in a step up / step down configuration)
Isolation of Signal of Patient from Power needed for safety
Typical Representation of an Isolation Amplifier
Common Mode Rejection
• Until now we assumed Amplifiers were ideal such that the signal into each terminal would completely cancel lead to complete common mode rejection
• However with practical Op Amp there is not perfect cancellation thus you are interested in what common mode rejection is.
Simplistic Example of ECG Circuit
Would like to analyze what type of common mode voltage (CMV) can be derived
Common Mode Voltage (CMV)• If 2 inputs are hooked together into a differential
amplifier driven by a common source with respect to ground the common mode voltage should be the same and the ideal output should be zero however practically you will see a voltage.
• CMV is composed of 2 parts:– DC electrode offset potential– 60Hz AC induced interference caused by magnetic and
electric fields from power lines and transformers• This noise is a current from in signal, common and ground wires• Capacitively coupled into circuit• (Other markets that work at 220-240 V will experience 50Hz noise)
Analysis to reduce noise in ECG
• Common Mode Rejection: – Instrumentation amplifier
(EX. INA128) using a differential amplifier which will cancel much of the 60 Hz and common DC offset currents to each input
– If each signal is carrying similar noise then the some of the noise will subtract out with a differential amplifier
Analysis to reduce noise in ECG
• Right leg driver circuit is used in a feedback configuration to reduce 60 Hz noise and drive noise on patient to a lower level.
Use of Feedback to reduce Noise
• Thus Vn is reduced by Gain G1• Note Book forgot V in equation 5-35
G1 G2ΣΣ
Β
Vin VoV1 V1G1
Vn = Noise
V2 V2G2
B Vo+
+ +
1211
21
1*
211
21*
211
21
211
2
1
1
211
21
211
221
221211
22121
22121
21
22
12
112
1
G
VV
GG
GGV
G
Vn
GG
GGVin
GG
GGV
GG
VG
G
G
GG
VGGV
GG
VGVGGV
VGVGGGGV
VGVGGVGGV
VGVGGVGGV
GVGVVV
GVV
VGVVV
VGVV
VVV
nino
o
nino
nino
nino
ninoo
noino
noino
o
noin
n
oin
Derivation:
• Isolation Amplifier also will attenuate noise
• Shielding of cables further reduce noise
Analysis to reduce noise in ECG
Review of Five ways to reduce Noise in ECG
• Common Mode Rejection (differential Amplifier)
• Right Leg Drive (feedback loop to decrease noise)
• Shielding of wires
• Isolation amplifier
• Notch filter to reduce 60 Hz noise
How to overcome offset voltage
1025
251
5.5
)25(21
2)3(
K
K
K
K
Rin
Rf
Rin
Rf
Vin
AVout
diff
diff
ngnoninverti
ngnoninverti
Instrumentation Amplifier Gain (A1,A2,A3) = Non-Inverting Amplifier A4
501510
251
)4(
K
Rin
Rf
Vin
AVout
• If you had 300 mV of DC offset sent through two gains of 10 and then 50 you would have an offset of (300mV)(10)(50) = 150V thus you would saturate your amplifiers and not see any of your signal
• 3V offset after first set of noninverting amplifiers goes through differential amplifier A3 which reduces the offset voltage.
Problems of offset voltage and how to correct
Other Corrections for Offset• Feedback circuit where output of A4 goes
through HPF of A5 so only responses larger than cutoff frequency pass through thus the DC offset is attenuated
R and C should be switched because this is really a LPF
Affect of High Pass Filter of A5• Feedback through HPF has a
time constant of RC• 3 Modes:
– Diagnostic Mode (most time) where
RC = 1x10-6F*3.2x106Ώ = 3.2 sec
Cutoff Freq = 1/(2πRC) = 0.05Hz
– Monitor Mode (medium time) where
RC = 1x10-6F*318x103Ώ = 0.318 sec
Cutoff Freq = 1/(2πRC) = 0.5Hz
– Quick Restore (least time) where
RC = 1x10-6F*80x103Ώ = 0.08 sec
Cutoff Freq = 1/(2πRC) = 2Hz
With Feedback the DC offset is eliminated and thus can have a gain of 50 on the 2nd Non-inverting Amplifier Stage without Saturating the Circuit
Drawn IncorrectlyR and C should be switched
High Pass Active FiltersAttenuates High frequency where
cutoff frequency is 1/(2) =1/ 2RiCi
-+ Voutput
Vinput
RiA
Rf
Ci
RfRi Voutput0
IRfIi
VinputCi
CijRi
Rf
V
V
CijRi
V
Rf
V
IiI
CijRi
VIi
Rf
VI
input
output
inputoutput
Rf
input
output
fR
1
1
00
1
0
0
When frequencies (w) is large gain ~ -Rf/RiGain (1MHz, Ci=1mF)
Ri
Rf
j
xRi
Rf
xxjRi
Rf
Vin
Vout
4
36
1059.1)101)(101)(14.3(2
1
When frequencies (w) is small gain is reducedGain (1Hz, Ci =1mF)
numberRi
Rf
jRi
Rf
xjRi
Rf
Vin
Vout
159
)101)(1)(14.3(2
13
Low Pass Active Filters = IntegratorAttenuates High frequency where cutoff
frequency is 1/2=1/2RfCf
-+ Voutput
Vinput
RiA
Rf
Cf
RfRi Voutput0
IRfIi
Vinput
Cf
ICf
1
1
11
1
00
1
0
0
0
1
0
CRfjRi
Rf
V
V
Ri
V
Rf
CRfjV
Rf
CjV
Ri
V
Rf
V
Cj
V
IiIRfIcfRi
VIi
Rf
VIRf
Cj
VIcf
input
output
inputoutputoutput
inputoutputoutput
input
output
output
When frequencies (w) are low gain ~ -Rf/RiGain (0 Hz, C=1mF)
Ri
Rf
Ri
Rf
CRfjRi
Rf
V
V
input
output
10
1
1
1
When frequencies (w) are high gain is reducedGain (1M Hz, C=1mF)
numberRi
Rf
RfmMjRi
Rf
CRfjRi
Rf
V
V
input
output
1*1*1
1
1
1
Defribillator
• A Defribillator = a high voltage electrical heart stimulator used to resuscitate heart attack victims
• When a physician applies this high voltage the high voltages and currents can cause damage to medical equipment BUT physician still needs to view ECG of the patient
• How do you protect your medical equipment from excessively voltages and currents?
Protection Devices in ECGs: Glow Lamps
• Glow Lamps are pair of electrodes mounted in a glass envelope in a atmosphere of lower pressure neon gain or a mix of inert gases
• Typically impedance across electrodes is high but if voltage across electrodes exceeds ionization potential of gas then impedance drops so you create a short to ground so vast majority of current goes safely to ground and avoids your amplifiers
Protection Devices in ECGs: Zener Diodes
• Diode: device that conducts electricity in one direction only
• Zener Diode: “Turns-On” when a minimum voltage is reached so in this configuration if a large voltage is applied (ie defibrillator) the zener diode will allow current to flow and shunts it to grounds thus current goes to ground and not to the amplifiers
Protection Devices in ECGs: Current-
Limiting Diodes
• Diode: device that conducts electricity in one direction only
• Diode acts as a resistor as long as current level remains below limiting point. It current rises above the limit, the resistance will change and the current will become clamped
• Can also use a varistor (variable resistor) which functions like a surge protector that clips spikes in voltages
Types of Defibrallitor Damage
• Defibrillator is 6X greater than normal working voltage so damage will eventually occur
• Two forms of Damage:– Both Amplifier inputs are blown thus readout is a flat
line– One amplifier input is blown so the ECG appears
distorted• Cause is from zener diodes becoming open or
from glow lamps becoming defective from an air leak, or recombination or absorption of gases
• Recommended that lamps are changed every 1-2 years or sooner if ECG is in Emergency Room
Effect of Voltage Transient on ECG
• Sometime a high voltage transient is applied to the patient (defibrillator) which cause magnitudes much greater than biopotential signal (ECG) which saturates the amplifier
• Once the voltage transient signal is removed the ECG signal takes time to recover
Example of bandwidth and magnitude of various biopotentials
ECG is approximately 1 mV and spans from DC to 500 HzBook assumes Diagnostic mode is 0.05 Hz to 100 Hz
Electro-Surgery Unit (ESU) Filtering
• While a surgeon is conducting surgery he/she will want to see their patient’s ECG
• ESU can introduce frequencies into the ECG of 100KHz to 100 MHz and with magnitudes up to kVolts which can distort the ECG
• ESU introduces:– DC offsets– Obscures the signal
• ESU needs to be of diagnostic quality thus you must eliminate higher frequencies which are noise
Correct for high frequency noise using LPF so ECG can function with ESU
RC Filters
• Low Pass Filters will pass frequencies lower than cutoff frequency of FH =1/2RC
Vs
Frequency
• High Pass Filters will pass frequencies greater than cutoff frequency of FL =1/2RC
FH
Vs
FL
Schematic of Multi-channel Physiological Monitoring System
• Figure 8-11
• Instrumentation Inputs:– Up to 12 leads to ECG– Lead 13 is for RL driver (feedback to patient and then machine needed to reduce common
mode voltage– Blood pressure– Body Temperature– Blood gases
• Buffers which are noninverting amplifiers to give high input impedance or large resistor • Wilson Network: series of resistors • Digitization of Signal• Serial data output to display
Instrumentation Amplifier using OPA621
5124
249*21
21
Rg
RfG
Differential resistors are the same thus this stage of the circuit has a gain of 1
CMR OPA621
Vin = V1 = V2
Vin
VoutVV
Vout
nsignalionofcommomagnificat
nalrentialsigionofdiffemagnificat
Acomm
AdiffCMRR
12
Frequency has an effect on CMR!
Circuit Schematic of an example of ECG
•Lead I (LA – RA) means LA is going to the noninverting input and RA is going to inverting input•Precordial are the chest leads
Block diagram of Entire ECG Circuit
Digitization of Signal
DC Offset severely affect the resolution of your signal and if DC offset is too highYou may not see your ECG SignalMore bits to A/D board (10, 12, 19, 22) the more resolution to your signal because you Can represent you signal with better resolution
Homework
• Read Chapter 9• Derive the gain equation for an instrumentation
amplifier.• What resistor values could be used to produce a
gain of 10 for an instrumentation amplifier?• Why do you use non-inverting Op Amps in the
first stage of an instrumentation amplifier?• Prove that feedback used for the right leg driver
can decrease the overall noise in your circuit.• Problem 1 Chapter 8
Schedule
• Home Ch8 due 4/4• Exam 2 on 4/11
– Material on Exam 2 Chapters 7, 2, 8, 9, studio exercises, labs, homework, class notes
• ECG design labs due 5/2• ECG team presentations 5/5 (Dr. Alvarez presenting at
conference, Florence Chua and class will grade presentations)
• Final Exam 5/13 from 2:30 to 5:00 in Colton 327 (same room that we meet)– Cumulative, anything discussed during the semester will
be on the final, more emphasis on the material not covered on Exam 1 & 2
ECG Example
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