Decompression during ascent from SCUBA dives has been shown to generate nitrogen bubbles (venous gas emboli, VGE) even in the absence of decompression sickness. These bubbles are filtered out of circulation by the lung, potentially altering ventilation perfusion ration (VA/Q) distribution. A study following recreational dives showed changes in VA/Q, but it was unclear whether these resulted from VGE. We measured changes in VA/Q following a hyperbaric exposure known to cause bubbles, in the absence of the confounding effects of immersion and exercise present in recreational dives. Twelve volunteers were seated and pressurized in a dry hyperbaric chamber to 60′ sea water (2.77 ATA) within five minutes, kept at 2.77ATA for the remainder of 60 minutes from the time of initial compression, and then decompressed to atmospheric pressure in 4 minutes. This hyperbaric exposure was at the limit of U.S. Navy guidelines for no-decompression-stop recreational dives, and was a time-depth profile known to produce VGE. Tests of pulmonary function were conducted before, 1 hr and 24 hrs following hyperbaric exposure and included intra-breath VA/Q slope (iV/Q) (a measure of VA/Q inequality), rebreathing diffusing capacity (DLCO), and forced vital capacity (FVC). Subjects refrained from heavy exercise during the study and were limited to minimal physical activity between the start of the exposure and 1 hr post-exposure pulmonary testing.
There were no significant changes in iV/Q, DLCO or FVC following the hyperbaric exposure. (Table)
While a previous study showed alterations in VA/Q following recreational dives, the hyperbaric exposure we used, comparable in nitrogen load and decompression profile, showed no change, suggesting that alterations in lung function found following recreational dives may have been caused by immersion in water, exercise, or other factors as opposed to the effect of VGE on the pulmonary circulation.
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