Which phase of capnography is the alveolar plateau where CO2 reaches its expiration peak?

• A. Baseline
• B. Expiratory upstroke
• C. Alveolar plateau
• D. Inspiratory downstroke

Answer: (C)

The alveolar plateau is the portion of the capnogram that occurs during expiration after the rapid upstroke, and it represents the exhalation of alveolar gas. During this phase the CO2 level remains relatively steady as alveolar air is expelled, and the peak of expiration—the end-tidal CO2 value—occurs at the end of this plateau. This end-tidal point reflects the CO2 concentration of the alveolar gas and is used to assess ventilation. In contrast, the baseline is the CO2-free dead space air, the expiratory upstroke is the rapid rise as CO2-rich alveolar air mixes with dead space air, and the inspiratory downstroke marks the start of inhalation when CO2 levels fall toward zero.