Cardiorespiratory, thermal, and renal responses to a 30-min head-out immersion in 15 degree C water were studied at 1-ATA air and 11-ATA helium-oxygne environments in four male subjects wearing dry suits. Cardiorespiratory responses to immersion (reductions in heart rate, expiratory reserve volume, vital capacity, and thoracic impedance; and increases in stroke volume, cardiac output, and inspiratory capacity) were comparable at both pressures. However, thermal responses to immersion (a reduction in mean skin temperature and increases in skin heat flux and suit conductance) were significantly greater at 11 ATA compared to those at 1 ATA. The rate of urinary excretion of norepinephrine increased significantly during and after immersion at 11 ATA but not at 1 ATA. In contrast, the urinary excretion of epinephrine was not altered by pressure or immersion. The immersion diuresis was greater and lasted longer at 11 ATA than at 1 ATA although there was no difference in the endogenous creatinine excretion . This diuresis was accompanied by a significant natriuresis which was more marked at 1 ATA than at 11 ATA. At 1 ATA, the urinary excretion of both aldosterone and antidiuretic hormone (ADH) decreased during immersion. At 11 ATA, the rate of excretion of these hormones before immersion was lower compared to that at 1 ATA and did not change significantly during immersion. These results indicate that immersion in a hyperbaric helium-oxygen environment presents a greater cold stress than at 1-ATA air, and also that immersion diuresis and natriuresis at high pressure may be induced by a factor other than inhibition of aldosterone and ADH.

Matsuda, Nakayama, Arita, Morlock, Claybaugh, Smith, Hong, , (1978). Physiological responses to head-out immersion in water at 11 ATA. Undersea biomedical research, 1978 Mar;5(1):37-52. https://www.ncbi.nlm.nih.gov/pubmed/636073