Cardiovascular responses to dry resting apnoeas in elite divers while breathing pure oxygen.

PURPOSE We hypothesized that the third dynamic phase (ϕ3) of the cardiovascular response to apnoea requires attainment of the physiological breaking point, so that the duration of the second steady phase (ϕ2) of the classical cardiovascular response to apnoea, though appearing in both air and oxygen, is longer in oxygen. METHODS Nineteen divers performed maximal apnoeas in air and oxygen. We measured beat-by-beat arterial pressure, heart rate (fH), stroke volume (SV), cardiac output (Q˙). RESULTS The fH, SV and Q˙ changes during apnoea followed the same patterns in oxygen as in air. Duration of steady ϕ2 was 105 ± 37 and 185 ± 36 s, in air and oxygen (p<0.05), respectively. At end of apnoea, arterial oxygen saturation was 1.00 ± 0.00 in oxygen and 0.75 ± 0.10 in air. CONCLUSIONS The results support the tested hypothesis. Lack of hypoxaemia during oxygen apnoeas suggests that, if chemoreflexes determine ϕ3, the increase in CO2 stores might play a central role in eliciting their activation.