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

- The transition between the phase 2-3 of the cardiovascular response to apnoea may correspond to physiological breaking point.
- Alveolar gas composition was determined at the end of interrupted phase 2 apnoeas, in air and in oxygen.
- In interrupted apnoeas, on O2-CO2 diagram, PAO2 and PACO2 lied on the physiological breaking point curve.
- Differences in PCO2 during apnoea, rather than the absolute PCO2 values, determine the volitional breaking point of apnoea.

IntroductionWe 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).MethodsTwelve 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 (PAO2 and PACO2, respectively) before and after apnoeas; we calculated the PACO2 difference between the end and the beginning of apnoeas (ΔPACO2).ResultsCardiovascular responses to apnoea were similar compared to previous studies. PAO2 and PACO2 at the end of φ2-interrupted apnoeas, corresponded to those reported at the physiological breaking point. For maximal apnoeas, PACO2 was less than reported by Lin et al. (1974). ΔPACO2 was higher in oxygen than in air.ConclusionsThe transition between φ2 and φ3 corresponds indeed to the physiological breaking point. We attribute this transition to ΔPACO2, rather than the absolute PACO2 values, both in air and oxygen apnoeas.