Capnography

(see also Weaning Parameters)

Waveform capnography measures the partial pressure of carbon dioxide (CO₂) detected at the end of exhalation. Known as EtCO₂ (end-tidal CO₂), this value provides important information about the status of the patient, especially respiratory distress, cardiac arrest or shock.

Capnography devices are configured as either sidestream or mainstream:

In a sidestream configuration, the CO₂ sensor is located in the monitoring device, which is at a distance from the patient. The exhaled CO₂ is diverted from the airway into the device via a sampling tube of 6 to 8 feet in length, which is attached to a breathing circuit fitted to the patient. A sidestream device can monitor both intubated and spontaneously breathing patients.
In a mainstream configuration, the CO₂ sensor and a sampling cell are integrated into a small device that connects directly at the airway, between the breathing circuit and the endotracheal tube. This system is more costly and tends to be reserved for use in ventilated patients.

Key Point

These devices should not be confused with a colorimetric CO₂ detector, which serves as an adjunct to confirm correct endotracheal tube placement. These devices respond quickly to exhaled CO₂ with a simple color change (device specific) usually from purple/blue to yellow/gold. (“Yellow is mellow” or “Gold is good.”) They are widely available on hospital crash carts because of their ease of use and convenience, but are qualitative in nature, in that they only present a range of the EtCO₂ rather than yielding a specific number or waveform.

CO₂ is present during each state of respiration. Normal EtCO₂ levels are 35 to 45 mm Hg, and the waveform has a rectangular shape. Alterations in waveform morphology and values can be interpreted to provide clues to the clinical situation of the patient. (As a rule of thumb, EtCO₂ values should be just slightly lower than the PaCO₂ levels.)

Figure 3.8

Measuring EtCO₂ enables many types of pulmonary assessments to be made:

Monitoring patients during mechanical ventilation weaning from the ventilator
Predicting survival during cardiopulmonary resuscitation
Disconnection of the patient from the ventilator
Identifying the patient's attempts to breathe while receiving paralytic agents
Detecting a potential diagnosis of pulmonary embolism

Abnormal capnograms occur for a variety of reasons, as identified on Table 3.4.

Table 3.4

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