Cardiovascular, respiratory and splenic responses to rebreathing and apnoea during exercise

We investigated integrative physiological responses to eupnoeic exercise (EX), rebreathing exercise (RB), dynamic apnoea (DA) and dynamic apnoea with cold-water face immersion (DAFI) in 20 healthy participants. Trials involved non-steady-state cycle exercise at 60 W for an average duration of 66 s. With increases in heart rate and stroke volume, EX and RB increased cardiac output compared with baseline (mean [SD] EX +47 [13]%, RB +43 [15]%). During DA and DAFI, the increase in cardiac output was attenuated (DA +26 [23]%, DAFI +14 [21]%). EX and RB elicited reductions in total peripheral resistance (EX -37 [7]%, RB -23 [15]%). This reduction was absent during apnoeas (DA +3 [31]%, DAFI +15 [40]%). Pulmonary oxygen uptake was the lowest during DAFI. At the end of hypoxic trials, end-tidal partial pressures of O₂ were RB 50.3 [11.9], DA 57.9 [14.0] and DAFI 61.4 [13.6] mmHg, indicating a preservation of the central oxygen store during DA and DAFI. At the same time, peripheral tissue oxygen saturation, measured in the working rectus femoris muscle, declined the most during DA and DAFI (RB -1.4 [3.5]%, DA -4.7 [3.3]%, DAFI -5.6 [4.4]%). Splenic volume increased during EX (+8.4 [5.8]%) but decreased during RB (-10.5 [10.2]%), DA (-6.4 [10.8]) and DAFI (-13.3 [11.1]%) when compared with EX, suggesting erythrocyte mobilization in the threat of hypoxia. The non-steady-state apnoea interventions of the present study evoke a progressive shift from exercise-induced cardiovascular responses towards a diving response, including cardiac, vascular and splenic responses. These responses are amplified to some extent by cold-water face immersion. Apnoea-induced responses lead to central oxygen preservation and a decrease in peripheral oxygen stores.