CHAPTER 24 Capnography
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.
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:
