Heart rate and blood pressure time courses during prolonged dry apnoea in breath-hold divers

To define the dynamics of cardiovascular adjustments to apnoea, beat-to-beat heart rate (HR) and blood pressure and arterial oxygen saturation (SaO₂) were recorded during prolonged breath-holding in air in 20 divers. Apnoea had a mean duration of 210 ± 70 s. In all subjects, HR attained a value 14 beats min⁻¹ lower than control within the initial 30 s (phase I). HR did not change for the following 2–2.5 min (phase II). Then, nine subjects interrupted the apnoea (group A), whereas 11 subjects (group B) could prolong the breath-holding for about 100 s, during which HR continuously decreased (phase III). In both groups, mean blood pressure was 8 mmHg above control at the end of phase I; it then further increased by additional 12 mmHg at the end of the apnoea. In both groups, SaO₂ did not change in the initial 100–140 s of apnoea; then, it decreased to 95% at the end of phase II. In group B, SaO₂ further diminished to 84% at the end of phase III. A typical pattern of cardiovascular readjustments was identified during dry apnoea. This pattern was not compatible with a role for baroreflexes in phase I and phase II. Further readjustment in group B may imply a role for both baroreflexes and chemoreflexes. Hypothesis has been made that the end of phase II corresponds to physiological breakpoint.