Dead space in the breathing apparatus; interaction with ventilation

Dead space in breathing apparatus may cause increased ventilation and/or CO 2 retention. Interactions between ventilation and dead space were tested in the breathing apparatus of three divers: a full face mask with an oro-nasal cup (AGA), a full face mask without an oro-nasal cup (EXO-26) but designed to minimize dead space, and one mouthpiece. Experiments were performed at three depths; 0, 30 and 45 m seawater (msw). The breathing gas was air except at 30 msw where it was 36 O 2 in N 2 . Five certified SCUBA divers were exercised at three levels (0, 50 and 100 W), Ventilation and gas exchange were measured. The dead space in the AGA mask was not influenced by either depth or exercise (mean 0·201). The mean dead space of the EXO-26 was 0·341, but it increased with exercise (p