Cardiovascular time courses during prolonged immersed static apnoea

To define the dynamics of cardiovascular adjustments to apnoea during immersion, beat-to-beat heart rate (HR) and systolic (SBP) and diastolic (DBP) blood pressures were recorded in six divers during and after prolonged apnoeas while resting fully immersed in 27°C water. Apnoeas lasted 215 ± 35 s. Compared to control values, HR decreased by 20 beats min⁻¹ and SBP and DBP increased by 23 and 17 mmHg, respectively, in the initial 20 ± 3 s (phase I). Both HR and BP remained stable during the following 92 ± 15 s (phase II). Subsequently, during the final 103 ± 29 s, SBP and DBP increased linearly to values about 60% higher than control, whereas HR remained unchanged (phase III). Cardiac output (Q′) decreased by 35% in phase I and did not further change in phases II and III. Compared to control, total peripheral resistances were twice and three times higher than control, respectively, at the end of phases I and III. After resumption of breathing, HR and BP returned to control values in 5 and 30 s, respectively. The time courses of cardiovascular adjustments to immersed breath-holding indicated that cardiac response took place only at the beginning of apnoea. In contrast, vascular responses showed two distinct adjustments. This pattern suggests that the chronotropic control via the baroreflex is modified during apnoea. These cardiovascular changes during immersed static apnoea are in agreement with those already reported for static dry apnoeas.