Which statement correctly describes Phase I of the normal capnogram?

Which statement correctly describes Phase I of the normal capnogram?

• A. Phase I represents the respiratory baseline consisting of CO2-free gas from anatomical dead space
• B. Phase II is the inspiratory upstroke
• C. Phase III is the alveolar plateau
• D. Phase IV is the exhalation of alveolar gas

Answer: (A)

Understanding capnography phases helps you read ventilation patterns. Phase I is the initial portion of exhaled gas that comes from the anatomical dead space—the conducting airways. This gas is CO2–free, so the capnogram baseline is essentially zero at this moment. As expiration continues, CO2-rich gas from the alveoli mixes in, causing the characteristic rapid upstroke of Phase II. Phase III is the alveolar plateau, where CO2 concentration remains relatively stable as mainly alveolar gas is exhaled, and the end-tidal CO2 is taken at the end of this phase. Phase IV, when it appears, typically corresponds to the start of the next inhalation, with CO2 levels dropping toward zero; the exhalation of alveolar gas happens during Phase III, not Phase IV. So the statement that Phase I represents CO2-free gas from anatomical dead space correctly describes this phase.

Understanding capnography phases helps you read ventilation patterns. Phase I is the initial portion of exhaled gas that comes from the anatomical dead space—the conducting airways. This gas is CO2–free, so the capnogram baseline is essentially zero at this moment. As expiration continues, CO2-rich gas from the alveoli mixes in, causing the characteristic rapid upstroke of Phase II. Phase III is the alveolar plateau, where CO2 concentration remains relatively stable as mainly alveolar gas is exhaled, and the end-tidal CO2 is taken at the end of this phase. Phase IV, when it appears, typically corresponds to the start of the next inhalation, with CO2 levels dropping toward zero; the exhalation of alveolar gas happens during Phase III, not Phase IV. So the statement that Phase I represents CO2-free gas from anatomical dead space correctly describes this phase.