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

INTRODUCTION: 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).

METHODS: 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 (PA O2 and PA CO2 , respectively) before and after apnoeas; we calculated the PA CO2 difference between the end and the beginning of apnoeas (ΔPA CO2 ).

RESULTS: Cardiovascular responses to apnoea were similar compared to previous studies. PA O2 and PA CO2 at the end of φ2-interrupted apnoeas, corresponded to those reported at the physiological breaking point. For maximal apnoeas, PA CO2 was less than reported by Lin et al. (1974). ΔPA CO2 was higher in oxygen than in air.

CONCLUSIONS: The transition between φ2 and φ3 corresponds indeed to the physiological breaking point. We attribute this transition to ΔPA CO2 , rather than the absolute PA CO2 values, both in air and oxygen apnoeas.