Cardiovascular and metabolic responses at rest and to exercise during 48 hours of head-out immersion: a case report

Head-out immersion in water (HIW) leads to several cardiovascular adjustments. Cardiac output increases by increasing stroke volume with no substantial changes in heart rate (HR) at rest. Therefore, total peripheral resistance to blood flow declines, leading to elevated O₂ delivery. During exercise, cardiac output increases for a given workload while the HR response differs in relation to exercise intensity: it does not change during moderate exercise, but decreases during intense exercise. Oxygen uptake does not change either at rest or during exercise. In this case report, we report data on the cardiovascular and metabolic responses in a 29-year-old man at rest and during constant-load exercise during 48 h of HIW. The subject performed constant-load exercise on cycloergometer at 50% VO_2max in the laboratory and during 48 h of HIW. Pulmonary gas exchange, VO₂ kinetics, HR and rate of perceived exertion were determined. At rest, HR was similar but $dot VO_2 $ increased during HIW relative to the laboratory. During exercise, the metabolic cost of exercise was similar and HR was the same during HIW and in the laboratory. $dot VO_2 $ kinetics were faster during HIW. The cardiovascular and metabolic responses at rest and during moderate exercise to a few hours of HIW are not different from those during 48 h of HIW.