Hyperbaric diuresis at a thermoneutral 31 ATA He-O2 environment

The basic pattern of body water exchange was studied in four Japanese male divers during exposure to a thermoneutral 31 ATA (He-O2) environment for 3 d (Seadragon V). The hyperbaric chamber temperature was raised from 25 degrees C +/- 0.5 degrees C at 1 ATA (air) predive to 31.5 degrees C +/- 0.3 degrees C at 31 ATA. Both rectal and mean skin temperatures were measured every hour (including during sleep) and were maintained at the same level at both pressures. The exposure to 31 ATA induced an increase in the daily urine flow and a corresponding reduction in the insensible (and evaporative) water loss without changing the total daily water output. However, the daily fluid intake decreased by 600 ml at 31 ATA, and hence the divers developed a state of negative fluid balance, as reflected by a reduction in body weight and an increase in hematocrit. All changes in the pattern of body water exchange observed at 31 ATA were gradually reversed during subsequent decompression. As observed in a previous dive to 31 ATA (Seadragon IV) in which there was a subtle cold stress (as indicated by the 1 degree C reduction in mean skin temperature at 31 ATA), the increase in daily urine flow at pressure was almost entirely due to the increase in overnight urine flow. However, the hyperbaric nocturia observed in the present dive was a water diuresis in nature whereas that in the previous dive was an osmotic diuresis. These results indicate that the hyperbaric diuresis at 31 ATA is due to an increase in overnight urine flow and that the hyperbaric nocturia is not in any way related to the subtle cold stress attendant in many hyperbaric environments.