End-Tidal Carbon Dioxide Monitoring
M. Kabir Abubakar
CAPNOGRAPHY
Capnography or end-tidal carbon dioxide (PetCO2) monitoring is the continuous and noninvasive measurement of CO2 in exhaled respiratory gas. Capnography has become an increasingly valuable tool in airway and ventilation monitoring during intensive care and anesthesia. It is a useful adjunct tool in the management of ventilated infants, providing information about CO2 production, pulmonary perfusion, alveolar ventilation, respiratory patterns, and the elimination of CO2 from the lungs. If ventilation and perfusion are well matched, with no alveolar disease, PetCO2 will approximate PaCO2.
A. Definitions
1. Capnography is the continuous analysis and graphical representation over time of CO2 concentrations in exhaled respiratory gases. A capnograph is the measuring instrument that displays the waveform or the capnogram.
2. Capnometry refers to numerical measurement or analysis of CO2 concentrations. A capnometer is a device that measures and displays the breath-to-breath numeric values of CO2.
B. Purpose
1. Noninvasive continuous analysis and recording of CO2 during tidal breathing (1)
2. PetCO2 monitoring (2)
3. Additional confirmation of endotracheal tube placement (3)
4. To monitor the quality of cardiopulmonary resuscitation and indicate return of spontaneous circulation (4)
C. Background
1. CO2 may be measured in a gas sample by several techniques. Infrared and colorimetric technology are the most commonly used methods in clinical practice:
a. Infrared technology: The most commonly used technique in capnography. CO2 absorbs specific wavelengths of infrared light. The amount of CO2 in a gas sample can be determined by comparing the measured absorbance of infrared light by that gas with the absorbance of a known standard.
b. Colorimetry: Used primarily for small disposable PetCO2 detectors for verification of endotracheal tube placement. A pH-sensitive nontoxic chemical indicator strip is housed in a clear dome; the strip changes color from purple to yellow in the presence of exhaled CO2; the color change is reversible and changes from purple to yellow with each exhaled breath in correctly intubated patients.
c. Molecular correlation spectrography.
d. Raman spectrography.
e. Mass spectrography.
f. Photoacoustic spectrography.
2. Capnographic devices incorporate one of the two types of analyzers: mainstream and sidestream (5, 6, 7, 8).
a. With a mainstream analyzer, the sensor is attached directly to an optical adapter that is in line with the endotracheal tube.
b. With a sidestream analyzer, a low-deadspace adapter is placed in line with the endotracheal tube and gas is aspirated continuously to the analyzer for measurement.
D. Indications
1. Evaluation of the exhaled CO2, specifically PetCO2, which is the maximum partial pressure of CO2 exhaled
during a tidal breath just prior to the beginning of inspiration (designated PetCO2) (5, 6, 7, 8, 9, 10, 11).
during a tidal breath just prior to the beginning of inspiration (designated PetCO2) (5, 6, 7, 8, 9, 10, 11).
2. Monitoring the severity of pulmonary disease and evaluating response to therapy, particularly therapy intended to change the ratio of deadspace to tidal volume (12) or to improve the matching of ventilation to perfusion (V/Q) (13).
3. Accurate and continuous graphic reflection of CO2 elimination when weaning ventilator support (12, 14).
4. Continued monitoring of the integrity of the ventilatory circuit (15).
5. Use of capnography in combination with pulse oximetry can allow for additional monitoring to detect airway obstruction or subclinical degrees of respiratory depression in the sedated patient (16).
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