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June 2008    Download the Entire Issue (PDF) Available to the Public Vol. 34, No. 6   RSS Feed for Undercurrent Issues
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Wait Times for Flying After Diving

from the June, 2008 issue of Undercurrent   Subscribe Now

How long on the ground is long enough for divers to beat decompression sickness in the air? Duke University Medical Center has published a number of studies on the effects of bottom time (BT) on preflight surface intervals (PFSIs). Simulated dives were carried out in a hyperbaric chamber and subjects were assessed for the presence of signs and symptoms of decompression sickness after diving and before, during and after flying at simulated altitude. The risk of decompression sickness was assessed for a variety of dive profiles, including repetitive mid-depth, no-stop dives and a mid-depth long duration deco dive.

These studies make it clear that the incidence of decompression sickness decreases as PFSI increases, and repetitive dives generally require longer PFSIs to reduce risk than do single dives. A striking finding of a Duke study on PFSIs published in 2007 was that low DCS risk wait times for a single 60-foot, 120-minute dive with a large deco obligation were nearly 12 hours shorter than for a pair of moderately long, mid-depth, no-stop dives. The reasons for that result are speculative. It may be that repetitive ascents create bubbles in tissues outside of blood vessels, or that decompression stops reduce bubble generation and promote non-problematic off-gassing.

Here are the Diver Alert Network’s current recommendations: “For a single no-decompression dive, the minimum preflight surface interval should be 12 hours. For multiple dives per day or multiple days of diving, a minimum preflight surface interval should be 18 hours. For dives requiring decompression stops, there is little evidence on which to base a recommendation; however, a preflight surface interval substantially longer than 18 hours appears prudent.”

While the multiple and complex issues of deep stops and PFSIs still need to be further clarified, divers can feel secure by following the current recommendations for reducing DCS. Short and shallow profiles, slow ascents, long safety stops and surface intervals, use of EAN-to-air tables, midweek breaks on extended trips, proper hydration, and conservative delays between the last dive and flying combine to decrease the occurrence of DCS to a minimal level.

- - Doc Vikingo

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