American Board of Pediatric Dentistry (ABPD) Oral Boards Practice Exam 2026 - Free Pediatric Dentistry Practice Questions and Study Guide

During exhalation, the capnography trace goes through a few distinct parts. The initial portion reflects dead-space gas with very low CO2, followed by a rapid rise as a mix of dead-space and alveolar gas leaves the lungs. After dead space is flushed, the remaining exhaled gas is mainly from the alveoli, producing a relatively flat, steady plateau. This plateau represents exhaled alveolar gas and is where the end-tidal CO2 is measured. The steep rise is not alveolar gas, and the final return to zero happens during the next inhalation. Therefore, the segment that corresponds to exhalation from alveoli is the plateau portion of expiration—the middle part of the expiratory phase.