24: Capnography

Published on 06/02/2015 by admin

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CHAPTER 24 Capnography

James Duke, MD, MBA

1 What is the difference between capnometry and capnography? Which is better?

Capnometry is defined as the numeric measurement and display of the level of CO₂. It is not nearly as valuable as capnography, in which the expired CO₂ level is graphically displayed as a function of time and concentration.

2 Describe the most common method of gas sampling and the associated problems

Sidestream devices aspirate gas (typically 50 to 250 ml/min), usually from the Y-piece of the circuit, and transport the gas via small-bore tubing to the analyzer. Sampling can also be performed by nasal cannulas; but, because of room air entrainment and dilution of the CO₂ concentration, the Y-piece provides qualitatively and quantitatively a better sample than nasal cannulas. Problems with sidestream measurement include a finite delay until the results of the gas sample are displayed and possible clogging of the tubing with condensed water vapor or mucus. Infrared absorption is the most common method of CO₂ analysis.

3 What is the importance of measuring CO₂?

End-tidal carbon dioxide (ETCO₂) monitoring has been an important factor in reducing anesthesia-related mortality and morbidity. Short of bronchoscopy, carbon dioxide monitoring is considered the best method of verifying correct endotracheal tube (ETT) placement. In addition to its value as a safety monitor, evaluation of expiratory CO₂ provides valuable information about important physiologic factors, including ventilation, cardiac output, and metabolic activity, and proper ventilator functioning. ETCO₂ levels have also been used to predict outcome of resuscitation. A prospective observation study of 150 out-of-hospital cardiac arrests found that an ETCO₂ level less than 10 mm Hg after 20 minutes of standard advanced cardiac life support was 100% predictive of failure to resuscitate.

4 Describe the capnographic waveform

The important features include baseline level, the extent and rate of rise of CO₂, and the contour of the capnograph. Capnograms may be evaluated breath by breath, or trends may be assessed as valuable clues to a patient’s physiologic status. There are four distinct phases to a capnogram (Figure 24-1). The first phase (A–B) is the initial stage of exhalation, in which the gas sampled is dead space gas, free of CO₂. At point B, there is mixing of alveolar gas with dead space gas, and the CO₂ level abruptly rises. The expiratory or alveolar plateau is represented by phase C–D, and the gas sampled is essentially alveolar. Point D is the maximal CO₂ level, the best reflection of alveolar CO₂, and is known as ETCO₂. Fresh gas is entrained as the patient inspires (phase D–E), and the trace returns to the baseline level of CO₂, approximately zero.

Figure 24-1 The capnographic waveform. A–B: Exhalation of CO₂ free gas from dead space; B–C: combination of dead space and alveolar gas; C–D: exhalation of mostly alveolar gas; D: end-tidal point (alveolar plateau); D–E: inhalation of CO₂ free gas.

5 What may cause elevation of the baseline of the capnogram?

The baseline of the capnogram may not return to zero at high respiratory rates. However, if the baseline is elevated more than approximately 2 mm Hg CO₂, the patient is receiving CO₂ during inspiration, and this is often termed rebreathing (Figure 24-2). Possible causes of rebreathing include the following:

An exhausted CO₂ absorber

Channeling of the gas within the CO₂ absorber

An incompetent unidirectional inspiratory or expiratory valve

Accidental administration of CO₂ (perhaps from a CO₂ tank used for laparoscopy)

Inadequate fresh gas flow

Figure 24-2 Rebreathing of CO₂ as demonstrated by failure of the waveform to return to a zero baseline.

6 Does ETCO₂ correlate with PaCO₂?

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