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End-Tidal Carbon Dioxide

Monitoring

11

M. Kabir Abubakar

Capnography

Capnography or end-tidal carbon dioxide (PetCO2) monitoring is the continuous and noninvasive measurement of

CO2 in exhaled respiratory gas. 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, 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. Confirmation of endotracheal tube placement (3)

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 two types of

analyzers: Mainstream and sidestream (4–7)

a. With a mainstream analyzer, the sensor is attached

directly to an optical adapter that is in line with the

endotracheal tube (Fig. 11.1).

b. With a sidestream analyzer, a low–dead-space

adapter is placed in line with the endotracheal tube

and gas is aspirated continuously to the analyzer for

measurement (Fig. 11.2).

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) (4–10).

2. Monitoring the severity of pulmonary disease and evaluating response to therapy, particularly therapy intended to

change the ratio of dead space to tidal volume (11) or to

improve the matching of ventilation to perfusion (V/Q) (12)

3. Accurate and continuous graphic reflection of CO2

elimination when weaning ventilator support (4,13)

4. Continued monitoring of the integrity of the ventilatory

circuit (14)

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 (5).

6. Verifying that tracheal rather than esophageal intubation has taken place (3,15,16)

E. Contraindications

There are no absolute contraindications to capnography in

the mechanically ventilated infant, but consideration

should be given to the amount of dead space and weight

that will be added to the breathing circuit by these devices.

76 Section II ■ Physiologic Monitoring

F. Limitations (4,17,18)

1. The composition of the respiratory gas mixture may

affect the capnogram; the infrared spectrum of CO2 has

some similarities to the spectra for both oxygen and

nitrous oxide (most available capnographs have a correction factor already incorporated into the calibration).

2. Rapid changes in respiratory rate and tidal volume may