Gas exchange and cardiovascular responses during breath-holding in divers

To check whether the evolution of alveolar pressures of O₂ (P_AO₂) and CO₂ (P_ACO₂) explains the cardiovascular responses to apnoea, eight divers performed resting apnoeas of increasing duration in air and in O₂. We measured heart rate (f_H), arterial pressure (AP), and peripheral resistances (TPR) beat-by-beat, P_AO₂ and P_ACO₂ at the end of each apnoea. The three phases of the cardiovascular response to apnoea were observed. In O₂, TPR increase (9 ± 4 mmHg min l^-1) and f_H decrease (-11 ± 8 bpm) were lower than in air (15 ± 5 mmHg min l^-1 and -28 ± 13 bpm, respectively). At end of maximal apnoeas in air, P_AO₂ and P_ACO₂ were 50 ± 9 and 48 ± 5 mmHg, respectively; corresponding values in O₂ were 653 ± 8 mmHg and 55 ± 5 mmHg. At end of phase II, P_AO₂ and P_ACO₂ in air were 90 ± 13 mmHg and 42 ± 4 mmHg respectively; corresponding values in O₂ were 669 ± 7 mmHg and 47 ± 6 mmHg. The P_ACO₂ increase may trigger the AP rise in phase III.