Capnography provides a waveform that reflects ventilation by measuring CO2 during which part of the respiratory cycle?

• A. During inspiration
• B. During expiration
• C. At peak expiration
• D. At rest

Answer: (B)

Capnography measures CO2 in the air the patient exhales, so it reflects ventilation during the exhalation phase. As air leaves the lungs, CO2-rich alveolar gas is expelled, causing the waveform to rise and peaking at the end of expiration (the end-tidal CO2). When breathing in, fresh air with little to no CO2 is drawn into the lungs, so the reading falls toward zero and the waveform pauses between breaths. Therefore, the waveform effectively represents ventilation occurring during expiration.