Interpretation of capnography.

The anesthetist will get the most information out of a capnograph if it is examined systematically. First, the anesthetist must determine whether exhaled CO2 (i.e., a waveform) is present. The differential diagnosis of absent CO2 includes esophageal intubation, accidental tracheal extubation, disconnection of the breathing circuit, complete obstruction of the endotracheal tube or conducting system (kink, inspissated blood or secretions, extremely severe bronchospasm) or of the breathing circuit, apnea, and cardiac arrest. Second, the shape of the waveform must be analyzed systematically by looking at, and in sequence, phase I (inspiratory baseline, which should be zero); phase II (expiratory upstroke, which should be nearly perpendicular to the inspiratory baseline); phase III (expiratory or alveolar plateau, which should be a straight, nearly horizontal, line); and phase IV (inspiratory downstroke, which should be nearly perpendicular to the inspiratory baseline). This discussion will follow this systematic approach but will emphasize diagnosis that can be obtained from the phase III alveolar plateau.