The capnogram

The CO2 concentration of exhaled gas consists of 4 phases (see figure). The presence of significant concentrations of CO2 in phase 1 implies rebreathing of exhaled gas. Failure of an expiratory valve to open is the most likely cause of rebreathing during manual ventilation, although an inadequate flow of fresh gas into a rebreathing bag is a common cause. The slope of phase 3 is dependent on the rate of alveolar gas exchange. A steep slope may indicate ventilation-perfusion mismatch since alveoli that are poorly ventilated but well perfused discharge late in the respiratory cycle. A steep slope is seen in patients with significant auto-PEEP.

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