Alert Diver Article
Flying After Diving Studies at DAN Are Still in FlightNovember / December 2007 Issue
By: Richard D. Vann, Ph.D., Neal W. Pollock, Ph.D., Petar J. Denoble, M.D., J. Jake Freiberger, M.D., M.
Flying at low barometric pressure after diving increases a diver's risk of decompression sickness (DCS) unless enough time is allowed at or near sea level for excess nitrogen to be washed out of the body. The time a diver should wait after a given dive to avoid unacceptable DCS risk has been a vexing question for 45 years.
The first reports of apparent DCS due to flying after diving appeared in 1961, when the pilot and copilot of a commercial aircraft reported symptoms.
They were flying at a cabin altitude of 8,000-10,000 feet (2,438-3,048 meters) and became incapacitated after diving not deeper than 30 feet of sea water (fsw)/9 meters of sea water (msw) less than four hours earlier. The flight engineer, who had been diving about 12 hours earlier, was less affected and landed the aircraft safely.
A 1967 U.S. Navy study using dogs found that 90 percent developed DCS after a one-hour surface interval but none after 12 hours. In 1972 the Navy recommended a 12-hour wait before flying after decompression dives and in 1985 specified a minimum two-hour delay after no-decompression dives.
The no-decompression recommendation was based on 39 human trials. The trials showed two DCS incidents in 39 exposures at 8,000 ft. (2,438 m) after a five-minute surface interval and nine incidents at 16,000 ft. (4,877 m) after surface intervals of five minutes to two hours.
In 1990 the U.S. Air Force required a 24-hour surface interval after any diving, and in 1991 Divers Alert Network(r) recommended at least 12 hours before flying and longer than 12 hours after repetitive multiday or decompression dives. Based on available data, none of these guidelines could be accepted with confidence.
PHASE I To develop information that might assist in establishing more rational flying after diving guidelines, DAN(r) began human trials of flying after diving in 1992 at the Duke Hyperbaric Center. Two phases have been completed, and a third has begun.
The Exposures
The objective of Phase I was to find the preflight surface intervals that had low DCS risk after long no-decompression dives.1 The experimental subjects were seated at rest in a dry hyperbaric chamber during both dives and flights (Figure 1). The dive depths were 40, 60 and 100 fsw (12, 18 and 30 msw).
Researchers tested four single-dive profiles and five repetitive-dive profiles (Table 1). The surface interval between repetitive dives was one hour. The flight altitude was 8,000 feet (2,438 m), the maximum commercial airline cabin altitude allowed by the Federal Aviation Administration.
Table 1. Flying after diving dive profiles tested by DAN
The Results
In Phase I, 40 subjects out of 802 (5 percent) were classified as experiencing DCS as a result of flying. The minimum safe preflight surface interval for a dive was indicated by the surface interval at which the DCS incidence rose sharply as the duration of the surface interval was reduced. Single dives generally needed surface intervals of 11-12 hours for a low DCS risk, while repetitive dives needed up to 17 hours. A surface interval of 17 hours or more appeared to ensure a low DCS risk during subsequent flights for most no-decompression dives with dry, resting subjects.
The Discussion
The Phase I results were the basis of the guidelines for flying after recreational diving2 (see bottom, next page).
PHASE II The U.S. Navy used the Phase I trials as the basis for flying after diving guidelines3 that were published in the 1999 U.S. Navy Diving Manual4 and the 2001 NOAA Diving Manual5. The Navy guidelines were much more flexible than the DAN guidelines, but many were untested, and the Navy asked DAN to validate several of these. This work was done as Phase II at the Duke Hyperbaric Center6.
The Exposures
Phase II tested a short no-decompression dive and a decompression dive, both to depths of 60 fsw (18 msw), to allow comparison with the earlier 60-fsw experiments (Table 1).
The no-decompression dive was for 40 minutes, and the decompression dive was for 120 minutes with a 26-minute stop at 10 fsw (3 msw). The Navy and NOAA dive manuals required a preflight surface interval of 12 hours, five minutes for the 40-minute dive and a 22-hour, 46-minute surface interval for the 120-minute dive. The same dry, resting conditions were used as in Phase I.
The Results
There were 12 DCS cases (2.1 percent) in 562 Phase II trials. For the single 55-minute dive in Phase I, the low DCS-risk surface interval had been 10-11 hours. For the 40-minute dive, the surface interval was progressively reduced to five minutes with no indication of increasing DCS incidence.
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Table 2: Phase I Flying After Diving Guidelines
The guidelines apply to air dives followed by flights at cabin altitudes of 2,000 to 8,000 feet (610 to 2,438 m) for divers who do not have DCS symptoms. The guidelines should reduce DCS risk during flying after diving but do not guarantee avoidance of DCS. Preflight surface intervals longer than the recommendations will reduce DCS risk further.
Guideline (a). Dives within the No-Decompression Limits
For a single no-decompression dive, a minimum preflight surface interval of 12 hours is suggested.
For multiple dives per day or multiple days of diving, a minimum preflight surface interval of 18 hours is suggested.
Guideline (b). Dives Requiring Decompression Stops
There is little experimental or published evidence on which to base a recommendation for decompression dives. A preflight surface interval substantially longer than 18 hours appears prudent.
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The Discussion
The bottom time at 60 fsw (18 msw) seemed to have a very strong influence on low-risk preflight surface intervals.
Repetitive diving and a decompression stop also appeared to strongly influence low-risk surface intervals. For the repetitive-dive profiles with 75 or 95 minutes total bottom times in Phase I, the low-risk surface interval was 15-16 hours. For the longer 120-minute decompression dive of Phase II, however, the DCS incidence remained low until a two-hour surface interval.
Two factors may have been responsible for the shorter low-risk surface intervals after the 120-minute dive. First, compared to single dives, the repetitive dives exposed subjects to multiple decompressions. These multiple ascents may have caused more bubble formation, which could reduce nitrogen elimination and thereby require longer surface intervals. Second, the 26-minute decompression stop at 10 fsw (3 msw) during the 120-minute dive may have avoided bubble formation and helped wash out dissolved nitrogen; this allowed shorter surface intervals.
The trials of the 120-minute decompression dive provide information that bears on Guideline (b) listed at left. Decompression dives may not necessarily require the long preflight surface intervals that Guideline (b) supposed. In fact, decompression stops or safety stops may be an effective means for reducing the DCS risk of flying after diving. This possibility warrants further investigation, as does oxygen breathing during preflight surface intervals.
PHASE IIIThe recommended flying after diving guidelines in the Navy and NOAA diving manuals were much longer than appeared necessary according to Phase II. For the 40-minute dive, direct ascent to 8,000 ft. (2,438 m) was possible with low DCS incidence, while the Navy guideline specified 12 hours, five minutes. For the 120-minute dive, a three-hour surface interval was achieved with low DCS incidence, while the Navy guideline specified 22 hours, 46 minutes.
Is this adequate information to justify shorter preflight guidelines for these dives and perhaps other dives as well? This might be true for dry, resting divers, but exercise, immersion and temperature can significantly affect nitrogen exchange and DCS risk.
The Phase III trials is testing divers who are immersed and exercising in warm water during a 55-minute dive to 60 fsw (18 msw). The results will be compared to the Phase I results for the same dive with dry, resting dives.
The reason that the Phases I and II divers were not tested with exercise and immersion was because only two wet divers can be exposed at the same time in the Duke chambers (Figure 2), while as many as 12 dry, resting divers can participate at once (Figure 1). We will need approximately two years to complete Phase III. The results will be combined with Phases I and II and used to calibrate a DCS probability model for recreational diving. The model will include both dive conditions and altitude exposure.
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References
1 Vann RD, Gerth WA, Denoble PJ, Pieper CF, Thalmann ED. "Experimental trials to assess the risks of decompression sickness in flying after diving." Undersea & Hyperbaric Medicine 2004; 31(3):431-444.
2 Sheffield PJ, Vann RD, eds. Flying After Recreational Diving. Durham, N.C.: Divers Alert Network, 2004.
3 Flynn E. "1999 U.S. Navy procedures for ascent to altitude after diving." In: Sheffield PJ, Vann RD, editors. Flying After Recreational Diving; Durham, N.C.: Divers Alert Network, 2004: 20-31.
4 "Ascent to altitude after diving/flying after diving." Arlington, Va.: U.S. Department of the Navy; 1999 April 1982. Report No.: SS521-Ag-PRO-010 / 0910-LP-708-8000, Revision 4.
5 NOAA Diving Manual: Diving for science and technology. 4 ed. Silver Spring, Md.: U.S. Department of Commerce, National Oceanic and Atmospheric Administration, 2001.
6 Vann RD, Pollock NW, Freiberger JJ, Natoli MJ, Denoble PJ, Pieper CF. "Influence of bottom time on preflight surface intervals before flying after diving." Undersea & Hyperbaric Medicine 2007; 34(3):211-220.
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(c) DAN - Alert Diver November / December 2007
