Altitude Exposure after Diving

INTRODUCTION

It is generally recognized that altitude exposure after diving poses some increased risk of decompression sickness (DCS). Studies underway at the Divers Alert Network and elsewhere have now confirmed this increased risk. Lesser degrees of altitude exposure still pose a risk but have not been as well studied. Guidelines for altitude exposure after diving have been proposed by many different agencies. Among those considered in the formulation of this document are NOAA, USN, USAF, DAN, DCIEM, UHMS and the Australian Standard for Occupational Diving. Any proposed standard must be flexible enough to be modified as new data becomes available.

BACKGROUND

Scuba Diving is generally considered to be safe when practiced in accordance with accepted limitations of depth and time. Nevertheless, it is also known that decompression sickness is a "probabilistic" event. It can occur at any time, and seemingly for "no reason" (i.e., within the no-decompression limits of the table used). As medical science advances, we learn more of previously unknown risk factors that increase the likelihood of getting DCS. The state of the body that describes the probability of dissolved nitrogen coming out of solution to produce DCS is referred to as "decompression stress." It is thought by many researchers that disordering of the microcirculation can occur in some situations such as multiple or deep dives, thus interfering with predictable off-gassing of nitrogen. This increased risk and deviation from predictability could be referred to as a state of increasing "decompression stress." Altitude exposure after a dive also increases your bodies "decompression stress."

ALTITUDE EXPOSURE RECOMMENDATIONS (waiting time in hours)

Maximum Altitude
After Diving
Minimum Delay
Single, Low Stress Dive
Minimum Delay
Multiple or Deco Dives
1000 Ft. (300 m.) none none (low stress)
6 hrs. (mod stress)
2000 Ft. (600 m.) none 2 hrs. (low stress)
8 hrs. (mod stress)
4000 Ft. (1200 m.) 8 hrs. 12 hrs.
6000 Ft. (1800 m.) 12 hrs. (F.A.D.) 24 hrs. (F.A.D.)

Table Definitions

F.A.D.: "Flying After Diving" surface wait recommendations prior to air travel.
Low Stress: A dive performed well within the no-decompression limits, without significant risks such as high exertion, extremes of temperature, extreme fatigue etc.
Moderate Stress: Any dive performed to the no-decompression limit, decompression diving or dives involving one of the known risk factors for DCS.
High Stress: Extreme dive profiles, missed decompression stops or multiple risk factors. High Stress = High Risk. No predictions are possible due to the inherent alterations in off-gassing.
After Recompression, Status Post DCS: The highest risk of all is after recompression Hyperbaric Oxygen Therapy for even a mild case of decompression sickness. Many cases are on record documenting return of DCS symptoms with exposure to altitude. Recommendations are hopelessly inadequate except to suggest that an injured diver wait as long as is realistically possible before flying home, or driving over a high mountain pass to get home. One week is not too long to wait.

F.A.D. (Flying After Diving)

Studies presented to the Undersea and Hyperbaric Medical Society at their Annual Meeting show that DCS risk while flying after a single, low risk dive reaches a low level after approximately 10 hours. Allowing for the small number of divers studied it would seem prudent to add a safety margin. When considering multiple or decompression diving, DCS risk seems to reach a minimum at 18 hours, but again the "n" (number of subjects studied) is small. The following are F.A.D. recommendations, supported by experimental results and include a small safety margin.

Flying After Diving Recommendations

Type of Diving Minimum Time to Wait
   
Single, Low Stress *12 Hrs.
(Repetitive Factor = 1.0)
Multiple or Deco 24 Hrs.

*DCIEM recommendations are "your RF must be 1.0 before you fly." I no longer recommend using the USN/NOAA "Group D" rule. Recent experimental evidence has identified DCS risk while flying with this level of decompression stress.

ACKNOWLEDGMENTS

Ron Nishi, M.A.Sc., P.Eng., Senior Scientist at DCIEM has more than any other, contributed greatly to my understanding of DCS. His input was invaluable in formulating the above recommendations. Ron informs me that the original concept of a multi-tiered table based on levels of decompression stress was first proposed at DCIEM in unpublished work from the early 1980's. I strongly support efforts to revisit this approach if divers can be convinced to think in terms of variable risk for DCS.

CONCLUSION

These altitude exposure recommendations are submitted as guidelines and not dogma. It is recognized that no two divers will have the same degree of decompression stress after the same dive. Careful decompression diving will often be less stressful than haphazard no-deco diving. Therefore, relative decompression risk will vary in a manner commensurate with overall decompression stress. A diver should always travel after diving in the most conservative way possible as it is impossible to design a totally risk free table or altitude exposure guide which always works for everyone, all the time. These guidelines are an attempt to introduce a surface interval prior to altitude exposure in order to minimize decompression stress without an unnecessary delay in travel.

FEEDBACK is encouraged and gratefully accepted.

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Copyright 2000 Edmond Kay, M.D.
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Last modified March 24, 2000