CAPNOMETRY AND PULSE OXIMETRY
CAPNOGRAPHIC DEVICES
Infrared Absorption Photometry Molecular Correlation Spectrography Colorimetric Devices Mass Spectrometry Raman Scattering
INFRARED
First developed in 1859 Based on the Beer-Lambert law, which describes
the absorption of infrared light by CO2 The higher the CO2 concentration, the higher the
absorption N2O, H2O, and CO can also absorb infrared light at
the wavelength used Two types: mainstream and side stream More compact and less expensive than the other
types of capnometers Requires sampling gas flow of ~150ml/min thru the
unit
SIDE STREAM
Gas is sampled through a small tube that pulls it out of the main gas stream
Analysis is performed in a separate chamber Very reliable Time delay of 1-60 seconds Less accurate at high rates Sampling tube is prone to plugging by water/secretions Ambient air leaks affect reading Connector is lightweight and doesn’t pull on airway Easy to use when patient is in an unusual position, such as
prone
MAINSTREAM
Sensor is located in the airway Response time as quick as 40 msec Very accurate Difficult to calibrate without disconnecting Reading more prone to being affected by moisture Larger and heavier than sidestream…can kink the
ETT Adds deadspace to the airway Bigger chance of being damaged by mishandling Sensor window can be clogged with secretions Difficult to use in unusual positions, such as prone
Molecular Correlation Spectrography Uses an infrared emission that precisely
matches the absorption spectrum of CO2 Allows for the use of very small samples at
very low flow rates Samples are measured every 25 msecs and
uses a flowrate of 50 ml/min
COLORIMETRIC
Contains a pH sensitive dye which undergoes a color change in the presence of CO2
The dye is usually metacresol purple and it changes to yellow in the presence of CO2
Portable and lightweight Low false positive rate…higher false negative rate Acidic solutions (eg-lidocaine, epi, atropine) will
permanently change the color Deadspace high for a neonate – can’t use for long
MASS SPECTROMETRY
Separates and counts ionized molecules to determine the concentration of gas
A gas sample is aspirated into a vacuum chamber when an electron beam ionizes and fragments the components of the sample
The ions are accelerated into a final chamber which has a magnetic field that allows for determination of the components of the gas and the concentration of each component
Very expensive and bulky, but have the advantage of being able to monitor multiple patients at a time (eg-OR)
RAMAN SCATTERING
Raman scattering occurs when light hits a molecule and it scatters the light…most of the scattered light is the same wavelength as the laser source, but a small amount of light scattered is at a different wavelength
The different wavelength produced gives information about the molecule
An argon laser is shone through a gas sample and the CO2 in the sample will scatter it…the amount of scattering is related to the CO2 level
NORMAL CAPNOGRAM
Phase I: the beginning of exhalation…CO2 level is zero
Phase II: alveolar gas begins to mix with the deadspace gas and the CO2 rises rapidly
Phase III: elimination of CO2 from the alveoli…usually has a slight upward slope
Phase IV: end exhalation Phase 0: inspiration
THE NORMAL CAPNOGRAM
ABNORMALITIES
Increased Phase 3 slope: Obstructive lung dx
Phase 3 dip: Spont resp Curare cleft
Horizontal Phase 3 with large ET-art gradient: Pulm. Embolism Decreased CO hypovolemia
Sudden decrease to 0 Ventilator malfunction ETT disconnect ET obstruction Extubation
Sudden decrease Partial obstruction Air leak
Exponential decrease Severe hyperventilation CP event
ABNORMALITIES
Gradual decrease Hyperventilation Decreased T Gradual decrease in
volume Sudden increase
Bicarb administration Release of limb
tourniquet
Gradual increase Fever Hypoventilation
Increased baseline Rebreathing CO2 Exhaused CO2 absorber
PaCO2-PetCO2 GRADIENT
Usually <6 mm Hg PetCO2 is usually less than arterial Difference depends on the number of
underperfused alveoli Tend to mirror each other if the slope of
Phase 3 is horizontal or minimal Decreased CO will increase the gradient
LIMITATIONS
Critically ill patients often have rapidly changing deadspace and V/Q mismatch
Higher rates and small Vt can increase the amount of deadspace ventilation
High mean airway pressures and PEEP restrict alveolar perfusion leading to falsely decreased readings
Low CO will decrease the reading
USES
Metabolic Assess energy expenditure
Cardiovascular Monitor trend in cardiac output Can use as an indirect Fick method Measure of effectiveness in CPR
Diagnosis of pulmonary embolism: measure the gradient
PULMONARY USES
Effectiveness of bronchodilator therapy Monitor gradient Worsening indicated by rising Phase 3 w/o
plateau Find optimal PEEP by following the gradient
…should be lowest at optimal PEEP level Can predict successful extubation…Vd/Vt >
0.6 predicts failure Limited pulm usefulness if CV unstable
CAPNOMETRY
Measures and displays a numerical value of the CO2 level 30-43 mm Hg 4.0-5.6%
ESOPHAGEAL INTUBATION/ DISCONNECTION FROM VENTILATOR/TOTALLY OBSTRUCTED ETT
Air Leak/Loose Connection between sampling tube and capnograph
Increasing Temperature/Metabolism
Hypothermia/Reduced Metabolism/ Hyperventilation/Decreased CO Cause a gradual decrese in end-tidal CO2
Cardiac Oscillations
Bronchospasm/COPD/obstructed ETT Slanting and prolonged phase 2 and
increased slope of phase 3 Sometimes there’s a reverse phase 3 slope
seen in patients with emphysema
Ventilator IMV breath during spontaneous ventilation
Sticking Inspiratory Valve
Hypoventilation
Leak/Partial Disconnect in Circuit/ETT too high
Pulmonary Embolism/Pneumonia/ Hypovolemia/ Hyperventilation
Curare Cleft
Spontaneous Breathing
Rebreathing of CO2
CAUSES OF INCREASED PetCO2 Increased CO2 production and delivery to the lungs
Fever Sepsis Bicarb administration Increased metabolic rate Seizures
Decreased alveolar ventilation Respiratory center depression Muscular paralysis Hypoventilation COPD
Equipment malfunction Rebreathing Exhausted CO2 absorber Leak in ventilator circuit
CAUSES OF DECREASED PetCO2 Decreased CO2 production and delivery to the lungs
Hypothermia Pulmonary hypoperfusion Cardiac arrest Hemorrhage Hypotension
Increased alveolar deadspace Decreased CO Pulmonary embolism High PEEP levels
Increased alveolar ventilation Hyperventilation
Equipment malfunction Ventilator disconnect Esophageal intubation Complete airway obstruction Poor sampling Leak around ETT cuff Water in sampling line Air entrainment into sampling line Inadequate tidal volume
CAUSES OF INCREASED P(a-et)CO2 Pulmonary hypoperfusion Pulmonary embolism Cardiac arrest Positive pressure ventilation, especially with
PEEP High rate/low tidal volume ventilation
PULSE OXIMETRY
Uses spectrophotometry based on the Beer-Lambert law
Differentiates oxy from deoxy Hb by the differences in absorption of light at 660 nm and 940 nm
Minimizes tissue interference by separating out the pulsatile signal
Estimates HR by measuring cyclic changes in light transmission
Estimates functional Hb by comparing amounts of oxy and deoxy Hb
SOURCES OF ERROR
Sensitive to motion Sats below 85% have increased error Calibration is performed by company on
normal patients breathing various gas mixtures, so cal is accurate only down to 80%
Low perfusion state increases error Ambient light interferes with reading Delay in reading of about 12 seconds
SOURCES OF ERROR
Skin pigmentation Darker color may make the reading more variable
due to optical shunting Dark nail polish has the same effect, especially
black, blue, and green…red is OK Hyperbilirubinemia has no effect
Methylene blue and indigo carmine (dyes) cause underestimation of the saturation
SOURCES OF ERROR
Dysfunctional hemoglobin Carboxyhemoglobin leads to overestimation of
sats because it absorbs at 660 nm like oxyHb does
MetHb can mask the true saturation because it absorbs at both wavelengths used…sats are overestimated
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