eFigure 25-24.

Normal capnogram.

During inspiration, Pco₂ is zero. At the start of exhalation, Pco₂ remains zero as gas from the anatomic dead space leaves the airway (comparable to phase I in the single-breath nitrogen washout test). Next, Pco₂ rises rapidly as alveolar gas mixes with gas from the dead space (comparable to phase II in the single-breath nitrogen washout test), and then the Pco₂ level stabilizes as gas from the dead space decreases and all the gas comes from alveoli containing carbon dioxide. The Pco₂ at the end of the “alveolar plateau” is called the end-tidal Pco₂ (comparable to phase III in the single-breath nitrogen washout test).