Flying After Diving

Should the recommendations be reviewed?

A group of authors associated with DAN Europe published the results of a new flying-after-diving study in March.¹ The results are intriguing and may lead some divers to wonder if it is time to revisit the flying-after-diving guidelines.

The researchers conducted echocardiographic postdive monitoring of gas bubbles in venous blood (venous gas emboli, or VGE). The study was conducted over six days with 56 volunteer recreational divers. All the divers were apparently healthy and had no history of decompression sickness (DCS). Each diver did roughly 13 dives for a total of 726 dives in the study.

The researchers monitored VGE after each dive at 30, 60 and 90 minutes after surfacing. Of the 56 divers, 23 almost never developed detectable bubbles (the NB group), 17 divers bubbled occasionally (the OB group), and 16 divers produced bubbles every day after almost every dive (the B group). The dive profiles were monitored by dive computers; the average maximum depth was around 99 feet of seawater, the average dive duration was about 49 minutes, and the ascent rates ranged from 30 to 60 feet per minute.

Immediately before takeoff, 24 hours after the last dive ended, VGE were not detected in any subject. After takeoff, however, bubbles were detected in eight subjects from the regular bubblers group and in none from other two groups. At 90 minutes, all subjects were bubble free.

This was the first in-flight study of real-life dive exposures, and there are two important findings from it that divers should appreciate. First, there was great variability in postdive bubble degree, and it is important to note that the same divers seemed to consistently exhibit either low or high bubble production. While one can safely assume that the risk of DCS is negligible without demonstrable venous gas bubbles, the risk of divers who do have bubbles is not exactly linearly proportional to the bubble grade. At a low bubble grade the risk may not be significantly different than for nonbubblers. In this study, although there were 16 divers who bubbled regularly, nobody developed any postdive symptoms (nor did any of these divers have a history of DCS). It’s possible that with deeper and longer dives the distinction between bubblers and nonbubblers might disappear.

The second important finding of this study is that flying in a commercial aircraft even after a 24-hour surface interval can produce bubbles in divers’ blood. The current guidelines for the kind of diving these volunteers did (multiple days of no-decompression diving) recommend a minimum preflight surface interval of 18 hours.^2,3

The question of whether the current recommendations need review naturally arises. Consider these factors: VGE may lead to DCS, but the risk is of concern only if the bubble grade is high. In this case, the highest level of bubbles detected during flight was grade three (on a scale of zero to five), and this was in only one diver. The Defence and Civil Institute of Environmental Medicine (DCIEM) decompression tables, which were based on bubble monitoring and are considered reasonably conservative and safe, consider a dive profile safe if the postdive bubble level did not exceed grade two in more than 50 percent of tested dives. Based on that guideline there is no reason to change the current recommendations (this study found VGE greater than grade two in only one out of 56 postdive man-flights). It is important to note that the DCIEM tables were tested using Doppler VGE detection (which uses a grading scale of 0-4), while this study used echocardiography (with a grading scale of 0-5). Grade three on this scale is not necessarily greater than Doppler grade two.

Current guidelines recommend minimum preflight surface intervals before flying on commercial aircraft. To be on the safe side, it is always better to wait longer. This study has shown that a 24-hour interval is probably safe, but the 18-hour interval may deserve another look. We hope this group will continue their research and provide more data to increase our confidence in answering these important questions.

References
1. Cialoni D, Pieri M, Balestra C, Marroni A. Flying after diving: should recommendations be reviewed? In-flight echocardiographic study in bubble-prone and bubble-resistant divers. Diving Hyperb Med. 2015; 45(1):10-15.

2. Sheffield P, Vann R (eds.). Flying After Recreational Diving Workshop Proceedings. Durham, N.C.: Divers Alert Network, 2004.

3. Vann RD, Gerth WA, Denoble PJ, Pieper CF, Thalmann ED. Experimental trials to assess the risks of decompression sickness in flying after diving. Undersea Hyperb Med. 2004; 31(4): 431-444.

4. Nishi RY, Tikuisis P (1996). Current Trends in Decompression Development: Statistics and Data Analysis. Defence R&D Canada Technical Report. DCIEM-96-R-65. Retrieved 01-10-2012.