Hypervolemia and plasma vasopressin response during water immersion in men

Immersion studies were performed on seven mildly dehydrated male subjects to examine the effect of suppression of plasma vasopressin (PVP) on diuresis in water immersion. The water was kept at close to 34.5 C and the subjects remained in the water for 4 hr after sitting for 2 hr. Na and K levels in the serum and urine were analyzed, as were osmolality, red blood cell count, renin activity, total protein, albumin amounts, hematocrit, and hemoglobin. Plasma volume was monitored from samples drawn at specified intervals during immersion. The plasma volume increased significantly 30 min after immersion, but no PVP was observed. The dehydration induced elevated serum osmotic concentrations. It is concluded that the hydration condition before immersion and the volume of fluid intake during immersion affects the hemodilution during immersion.