Alveolar gas composition during maximal and interrupted apnoeas in ambient air and pure oxygen

We tested the hypothesis that the alveolar gas composition at the transition between the steady phase II (φ2) and the dynamic phase III (φ3) of the cardiovascular response to apnoea may lay on the physiological breaking point curve (Lin et al., 1974).

Twelve elite divers performed maximal and φ2-interrupted apnoeas, in air and pure oxygen. We recorded beat-by-beat arterial blood pressure and heart rate; we measured alveolar oxygen and carbon dioxide pressures (P_AO₂ and P_ACO₂, respectively) before and after apnoeas; we calculated the P_ACO₂ difference between the end and the beginning of apnoeas (ΔP_ACO₂).

Cardiovascular responses to apnoea were similar compared to previous studies. P_AO₂ and P_ACO₂ at the end of φ2-interrupted apnoeas, corresponded to those reported at the physiological breaking point. For maximal apnoeas, P_ACO₂ was less than reported by Lin et al. (1974). ΔP_ACO₂ was higher in oxygen than in air.

The transition between φ2 and φ3 corresponds indeed to the physiological breaking point. We attribute this transition to ΔP_ACO₂, rather than the absolute P_ACO₂ values, both in air and oxygen apnoeas.