During which capnography phase does EtCO2 rise on expiration?

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

Answer: (B)

Capnography tracks CO2 across the breathing cycle, with a distinct upstroke that marks the start of expiration. The rise in EtCO2 during expiration happens in the expiratory upstroke: as you begin to exhale, the first gas is dead-space air with little CO2, but soon alveolar gas rich in CO2 mixes into the airway and pushes the CO2 level up rapidly toward the plateau. The end-tidal CO2 value is reached at the end of this rise, during the alveolar plateau when the CO2 concentration stabilizes. The inspiratory downstroke shows the CO2 level falling as you inhale, and the baseline is the near-zero portion between breaths. So the phase where EtCO2 rises on expiration is the expiratory upstroke.