Capnography phases order?

• A. Respiratory baseline, Expiratory upstroke, Alveolar plateau, Inspiratory downstroke
• B. Expiratory upstroke, Respiratory baseline, Alveolar plateau, Inspiratory downstroke
• C. Inspiratory downstroke, Respiratory baseline, Expiratory upstroke, Alveolar plateau
• D. Alveolar plateau, Inspiratory downstroke, Respiratory baseline, Expiratory upstroke

Answer: (A)

Capnography traces the progression of a single breath through four phases, showing how CO2 moves from inspired gas to exhaled air and back to inspiration. The sequence starts with a respiratory baseline, which is CO2-free inspired gas from the airway or ventilator circuit. As exhalation begins, CO2-rich alveolar gas mixes with the dead-space gas, producing the expiratory upstroke. Once alveolar gas dominates, the trace reaches the alveolar plateau, reflecting a steady alveolar CO2 concentration. At the end of expiration, the next inhalation causes the inspiratory downstroke as fresh gas washes CO2 out of the airway, and the trace returns toward zero until the next breath. End-tidal CO2 is read at the end of the alveolar plateau. Other sequences don’t match the actual flow of air and CO2 during a breath, so they’re not correct.