What phase of a normal capnogram corresponds to alveolar volume?

The phase of a normal capnogram that corresponds to alveolar volume is indeed the third phase, which is characterized by a plateau in the waveform of the capnogram. This phase represents the exhalation of gas from the alveoli, where the concentration of carbon dioxide is relatively constant and reflects the gas exchange taking place in the lungs.

During Phase III, the gas that is being exhaled is coming primarily from the alveolar regions of the lungs, which have the highest concentration of carbon dioxide. As the tidal volume comes from the alveolar spaces, the flat part of this phase shows a stable level of CO2, indicating that the alveolar gas is being effectively eliminated.

This phase is significant in clinical assessments as a reliable measurement of alveolar ventilation and overall lung function. Proper interpretation of Phase III can provide insights into issues such as ventilation-perfusion (V/Q) mismatch or the effectiveness of alveolar ventilation in a patient.