A beat-by-beat analysis of cardiovascular responses to dry resting and exercise apnoeas in elite divers

Abstract

Purpose

Cardiovascular responses during resting apnoea include three phases: (1) a dynamic phase of rapid changes, lasting at most 30 s; (2) a subsequent steady phase; and (3) a further dynamic phase, with a continuous decrease in heart rate (HR) and an increase in blood pressure. The interpretation was that the end of the steady phase corresponds to the physiological apnoea breaking point. This being so, during exercise apnoeas, the steady phase would be shorter, and the rate of cardiovascular changes in the subsequent unsteady phase would be faster than at rest.

Methods

To test these hypotheses, we measured beat-by-beat systolic (SBP), diastolic, and mean blood pressures (MBP), HR, and stroke volume (SV) in six divers during dry resting (duration 239.4 ± 51.6 s) and exercise (30 W on cycle ergometer, duration 88.2 ± 20.9 s) maximal apnoeas, and we computed cardiac output ($\dot{Q}$) and total peripheral resistance (TPR).

Results

Compared to control, at the beginning of resting (R1) and exercising (E1) apnoeas, SBP and MBP decreased and HR increased. SV and $\dot{Q}$ fell, so that TPR remained unchanged. At rest, HR, SV, $\dot{Q}$, and SBP were stable during the subsequent phase; this steady phase was missing in exercise apnoeas. Subsequently, at rest (R3) and at exercise (E2), HR decreased and SBP increased continuously. SV returned to control values. Since $\dot{Q}$ remained unchanged, TPR grew.

Conclusions

The lack of steady phase during exercise apnoeas suggests that the conditions determining R3 were already attained at the end of E1. This being so, E2 would correspond to R3.