Are some divers prone — or resistant — to gas bubbles after diving?
Decompression sickness (DCS), which may occur in divers after decompression from a dive, is dependent on the combined dose of gas saturation during the dive and the rate and magnitude of decompression. However, there is a great variability of outcomes in subjects exposed to the same dive profiles. The variability decreases as the severity of exposure increases.
DCS is correlated with the degree of venous gas emboli (VGE), or “bubbles”, in circulation after a dive. Generally, the higher the VGE grade (more bubbles) the greater the probability of DCS, and vice versa. Similar to DCS, there is a great variance in the probability of VGE appearing postdive. Some researchers who practice VGE detection have hinted that some divers bubble after most dives and may exhibit a high bubble grade (HBG) and others tend not to bubble at all or rarely exhibit HBG. The former are often labeled as bubblers (or high bubblers), while the latter are labeled as nonbubblers (or low bubblers).
Given that risk of DCS in the absence of VGE is very low, nonbubblers could theoretically make provocative dives and be safe; on the other hand, the bubblers would be at greater risk of DCS and should dive more conservatively. A test that could distinguish bubblers from nonbubblers could help divers choose the proper dive exposure severity and help limit their chances of experiencing DCS.
Unfortunately, the existence of bubblers and nonbubblers has not been confirmed — and especially not that this distinction persists over the entire range of dive conditions and dive profiles. While it seems likely that normal biological variability may provide for such phenomenon, the distribution of proneness to VGE is more likely normal than dichotomous, meaning that most divers may be in between the two polar values — bubblers and nonbubblers who represent a small part of the population. Nonetheless, the concept is appealing and worth exploring because some divers could benefit from this knowledge.
The variability of decompression outcomes has been studied throughout the history of diving medical research. There was hope 30 years ago that a cause of that variability could be soon discovered. A study showed that complement — a group of proteins in the blood serum that helps the immune system remove harmful pathogens — appeared to be activated by VGE in some subjects and not in others. It seemed that this could be established by a simple blood test.
In one study 15 subjects were tested in vitro for susceptibility to complement activation in their plasma mixed with air bubbles.1 Seven of the subjects tested susceptible and eight were not susceptible. Susceptible divers were exposed to dives with a maximum depth range of 36-54 msw and a duration of 50-30 minutes respectively. After all dive profiles, the subjects manifested VGE grades between 1 and 3.7 (average 2.8). Out of 11 dives, five dives made by three divers resulted in DCS.
The eight nonsusceptible divers made 14 dives at a depth range of 36-72 msw and a duration of 50-40 minutes respectively, which was likely to produce two times more DCS than the previous dive series, according to the U.S. Navy decompression risk calculator. Divers bubbled on all dives, and the mean bubble grade was 3.1 (2-4). None developed DCS.
In a flurry of studies that followed, these clear-cut results were never repeated. It became evident that complement response to VGE in vivo varies like any other physiological parameter and that there is no correlation between its postdive changes and DCS.
Several points from this study are useful for the discussion of individual susceptibility to DCS:
- All divers in this study developed significant VGE load postdive. There were no nonbubblers or low bubblers.
- Despite a 100 percent incidence of VGE and high bubble grade, only a few divers developed DCS.
- The dives tested were severe enough to cause DCS, but the number of subjects and exposures were small enough to have a random occurrence of a cluster of DCS in just one group of divers.
- Other studies with a large enough number of tested subjects have shown a greater variability of complement response and no correlation with bubble grade or DCS outcome.
It is still hoped that at a less severe exposure typical of recreational diving, the distribution of high and low bubblers may still be of practical consequences for dive safety. However, the development of reliable tests for bubbling proneness and a demonstration that the classification holds for the given set of dive conditions would be a long and tedious scientific endeavor with no promises of success.
Reference
- Ward, CA, Weathersby PK, McCullough D, Fraser WD. Identification of individuals susceptible to decompression sickness. In: Bove AA, Bachrach AJ, Greenbaum LJ, edis. The Ninth International Symposium on Underwater and Hyperbaric Physiology. (Bethesda, Md.: Undersea and Hyperbaric Medical Society, 1987; pp. 239-247).