Gas exchange and cardiovascular responses during breath-holding in divers.

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