Seadragon VI: a 7-day dry saturation dive at 31 ATA. II. Characteristics of diuresis and nocturia.

Four male divers were exposed to a dry, 31 ATA, He-O2 environment for 7 d (Seadragon VI). Urine was collected diurnally (0700-2200 h) and nocturnally (2200-0700 h) before (predive 1 ATA air), during, and after (decompression and postdive 1 ATA air) exposure to 31 ATA. A typical hyperbaric diuresis associated with a reduction in urine osmolality was observed in the face of a constant creatinine clearance. A significant increase (P less than 0.05) in osmolal clearance (COSM) was observed with concomitant decrease (P less than 0.05) in negative free water clearance, indicating that the diuresis has both osmotic and free water components. Although urine flow increased at pressure during both day and night, its magnitude was twofold greater at night as compared to daytime. Moreover, the diurnal diuresis was entirely due to an increase in free water excretion, whereas the nocturnal diuresis was largely due to an increased COSM. These results indicate that hyperbaric diuresis is induced by inhibition of tubular reabsorption of free water during daytime and of certain solutes during the night. Approximately 80% of the nocturnal increase in the excretion of osmotic substances was accounted for by Na, K, Cl, and urea.