Page 295 - Clinical Application of Mechanical Ventilation

Monitoring in Mechanical Ventilation 261

Capnography

Capnography is a measurement of the partial pressure of carbon dioxide in a
gas sample. When the sample is collected at the end of expiration, it is called
end-tidal partial pressure of carbon dioxide (PetCO ). PetCO monitoring pro-
2
2
vides real-time, noninvasive analysis of a patient’s expired CO trend during
2
mechanical ventilation. Ventilators have built-in end-tidal CO monitoring ca-
2
pabilities.
The exhaled CO from the patient ventilator circuit is collected and measured by
2
the infrared absorption technique (Hess, 1990). A mainstream sensor is placed di-
rectly onto the ventilator circuit, usually attached to an adaptor on the endotracheal
tube. A sidestream sensor aspirates a sample of gas via a small tube connected to the
endotracheal tube adaptor. Figure 9-12 illustrates the mainstream and sidestream
capnography sensors.
The major advantage of mainstream analysis is the fast response time between
actual CO sampling and the display update. The disadvantage of the mainstream
2
adaptor is its excessive weight on the endotracheal tube as well as the additional
deadspace in the ventilator/patient circuit. With mainstream sampling, water con-
densation does not affect analysis; however, secretion buildup on the cell windows
can affect the accuracy. A mainstream analyzer also tends to be more frequently
handled than the sidestream sensor because the clinician must disconnect it manu-
ally to suction the patient (Shelley, 1989).
A sidestream analyzer (aspirating analyzer) places the analyzing mechanism
safely within the monitor and draws a sample via a tube connected at the pa-
tient’s airway (e.g., endotracheal tube). The major advantage with sidestream
analysis is the ease of handling, and the analyzer can be attached to other pa-
tient devices (e.g., cannula, mask). The major disadvantage is that with peri-
odic aspiration of air samples, secretions and water can be drawn into the sam-
pling tube and cause an occlusion. Lag time for CO display is slightly longer
2
(a few tenths of a second) than the mainstream analyzer but it is negligible.
Equipment contamination may be a problem with the sidestream analyzer
(Shelley, 1989).

Capnography Waveforms and
Clinical Application

A capnogram (Figure 9-13) shows the changes in P CO during a complete respi-
2
E
ratory cycle. The P CO is at zero before exhalation. At the beginning of exhala-
2
E
tion, the P CO remains at zero as anatomic deadspace volume exits the airways
E
2
(phase I). The P CO then increases dramatically as alveolar gas begins mixing
2
E
with deadspace gas (phase II). Then the curve plateaus, reflecting the exhalation
of alveolar gas (phase III). The end of the “alveolar plateau” is called the end-tidal
The PetCO 2 may be used
to estimate the PaCO 2 . PCO (PetCO ). Since the PetCO approximates the alveolar PCO , this value may
2
2
2
2
be used to estimate the PaCO .
2
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