By Dr Simon Mitchell
I am often asked how long one should wait before flying after diving. This is a terribly difficult question to answer. Indeed, "how long is a piece of string" is probably the most valid answer. In this article I will outline the difficulties in prescribing a safe pre-flight interval after diving, and summarize the current thinking on the issue.
On completion of a compressed air dive our bodies inevitably contain residual dissolved nitrogen in an amount that is determined by factors such as the time and depths of the dives we have completed, and the ascent protocols used. In addition, it is likely that we will have formed at least some nitrogen bubbles in the blood and / or various body tissues. Over a period of time after the dive the excess nitrogen is gradually eliminated, and the bubbles gradually resolve. The accurate tracking of this process has proved to be an elusive goal, and this complicates the determination of a safe minimum interval prior to flying.
Part 1If a diver whose blood and tissues are carrying a load of dissolved inert gas ascends to altitude, then as the ambient pressure falls the solubility of the dissolved inert gas also falls, and this might precipitate bubble formation. In addition, if there are already inert gas bubbles present, then any further decline in solubility of dissolved inert gas will promote its diffusion into the bubble, causing bubble growth. Bubbles will also grow in accordance with Boyle's law as ambient pressure falls. Such events can precipitate DCI, and this has been demonstrated many times in divers leaving dive sites by road over hills, and in divers returning home on airliners after diving holidays. Indeed, although something of a digression, it is worth noting that rapid ascent to extreme altitude in an unpressurised aircraft can precipitate DCI in aviators with no residual inert gas load at all (other than that we all carry running around at sea level breathing air).
The two common situations in which we encounter an ascent to altitude after diving are driving overland and flying. The prescription of safe parameters for driving and flying is a vexing issue, and one that is by no means fully resolved.
FLYING AFTER DIVING
Divers are traveling to dive more and more frequently. Although airliners are "pressurized", they are not pressurized to atmospheric pressure. Traveling in a passenger jet usually exposes the traveler to an equivalent altitude exposure of about 8000' (2400m). Consequently, the possibility of inducing DCI by altitude exposure when flying home after a diving program has become an important issue.
How can this risk be minimized? If we think about it carefully, the parameters that we can regulate in order to minimize the risk of any ascent to altitude after diving are:
- The dive itself. The more conservative the dive(s), the less the residual nitrogen load, and the less the risk of any ascent to altitude.
- The delay prior to ascending to altitude. The longer we wait prior to the altitude exposure, the greater the elimination of both residual nitrogen and bubbles, and the lower the risk.
- The altitude. The less altitude exposure, the lower the provocation for further bubble formation.
In practice, all of these parameters are manipulated to some degree. For example, it is fairly common practice to make the final dive(s) in a pre-travel diving sequence a little more conservative. However, it is the second strategy (imposing a delay) which receives the most attention. The problem though, is how to be sure we have waited long enough?
Various strategies have guided our thinking over the years. Prior to the advent of diving computers, there were rules in place for use with the various commonly used dive tables. Perhaps the best known of these was a recommendation for use with the US Navy table which prescribed a minimum interval sufficient to get the diver into repetitive group D. This frequently resulted in very short pre-flight surface intervals and ultimately proved to not be conservative enough.
Other than the inherent lack of conservatism in the "D-group rule", the main problem shared by all of these table-based systems in the contemporary setting is that hardly anyone dives using tables any more. Most recreational diving is performed using computers and it is difficult or impossible to adapt these rules to the use of diving computers. Indeed, almost all of these computers have built-in flying after diving guide functions. The derivation of the flying after diving advice provided by the various diving computers is unknown, but based on my understanding of the research that has been done (or not done) done in this area I would suggest that of all the data provided by a dive computer, the flying after diving recommendations are likely to be the most speculative. Many computers still allow flying after very short intervals, and recent data to come out of DAN in the USA suggests that these may not be long enough more often than we would be comfortable with. Indeed, I recommend that if the recommendations provided by your computer are less conservative than those promulgated by DAN (see below) then you should use the DAN guidelines by preference.
Part 2The evolution of Diver's Alert Network advice
During the late 1980s DAN recommended a 24 delay to flying after any diving. This paralleled the recommendations of the US Air force (which still recommends a delay of 24 hours). In 1989, the Undersea and Hyperbaric Medical Society (UHMS) conducted a workshop to address the issue. This workshop endorsed the DAN recommendation, but suggested that for more trivial exposures a wait of 12 hours would be sufficient. The workshop defined a "more trivial exposure" as a total dive time (in the previous 48 hours) of less than 2 hours (with no "decompression diving"). Recent work by Richard Vann and his colleagues at DAN suggests that this rule functions reasonably well for single dives, but is probably not conservative enough for multiple dives which may nevertheless come within the definition. For example, there were 2 cases of DCI out of 50 divers who completed dives to 18m for 55 min, 18m for 20 min, and 18m for 20 min with 1 hour surface intervals between dives, and then followed this with a 8000' altitude exposure some 14-15 hours after the last dive. Interestingly, there were no cases of DCI in 51 trials of the same dive protocol when the divers waited 16-17 hours before altitude exposure (although the difference between these respective series, that is, between 2 cases out of 50 dives and 0 cases out of 51 dives is not statistically significant and may be due to random chance.
In keeping with the findings of these very limited data, DAN has modified its advice. They recommend that a single no decompression dive should be followed by a 12 hour preflight surface interval. For repetitive dives, decompression dives, and multiday dives they make an open ended recommendation of "more than 12 hours", with the implication being "the more the better". Divers should be not interpret this as meaning that 1 minute more than 12 hours is "safe". DAN's ambiguity on the issue is meant to reflect the lack of relevant data, and the imprecision in the little data that does exist. They are recommending as long a pre-flight surface interval as is practical. I would advise at least 24 hours after the typical live-aboard diving trip. However, even this is no guarantee of safety. Obviously, 48 hours would be even safer. I have seen DCI arise in divers who claim to have been symptom free prior to flying some 36 to 48 hours after diving. Like most things in diving, leaving a generous pre-flight surface interval is an exercise in risk minimization rather than risk elimination.
TRAVEL BY ROAD AFTER DIVING
All roads (except perhaps in Holland) run above sea level to some degree, and this begs the fundamental question "what constitutes an altitude exposure?" There is no firm and clearly validated policy on this, but it is commonly held that if a road journey after diving does not exceed 300m altitude then problems are very unlikely, even with relatively short delays between diving and driving. Like all "rules" in life, there are cases that provide the exception, but the "300m rule" has generally proven fairly robust. Clearly, common sense must prevail, and a diver would be foolish indeed to exit the water from a provocative dive and immediately drive straight to 300m. In practice, this would be very unlikely to occur.
Where roads exceed 300m it is probably wise to impose some degree of delay between diving and making the journey. Unfortunately, there are no published guidelines to help with such decision-making. The flying after diving rules are almost certainly too conservative. In most cases, the natural delay involved in exiting the water, disassembling and packing equipment etc seems to be adequate to minimize risk. However, if there is an ascent to greater than 300m early in the road journey, or if the diving has been relatively provocative, then a sensible diver will impose a longer delay. Unfortunately, in the absence of any data that guides decision-making, it is impossible to provide any guidelines with an objective basis. Some dive operators have utilized the group D rule in this context, and given the lower provocation of a modest road ascent compared to flying, this might be appropriate. Unfortunately, as has been pointed out, divers using computers may have difficulty in applying the group D rule. Sometimes it comes down to "giving it as much time as is practically available".
One strategy that seems intuitively obvious, but which is invalid, is to plan your time and depth for a dive as though you are already at the altitude that will be reached during the drive. It has been reasoned that this will allow you to ascend to altitude immediately after the dive if you wish, and some have even advocated the use of this method for flying after diving. Unfortunately, although the dive will be somewhat more conservative than it otherwise would have been, it is still likely to result in some degree of bubble formation, and the behaviour of these bubbles during the subsequent ascent to altitude is highly unpredictable. Testing of a variant of this strategy was found to result in unacceptably high rates of DCI and it is not recommended.
We can never be exactly sure when it becomes "safe to fly" after a dive as it will depend on the degree of inert gas loading, the degree of bubble formation, and for how long it persists. All of these parameters will vary between individuals for the same diving exposure. There is a gradual reduction of risk with time. It must now be apparent to you that, just as with decompression tables, where no table is completely safe for all divers all of the time, no guidelines for flying after diving can be guaranteed completely safe at all times. Flying shortly after a single, short dive might prove to be quite different to flying after a series of repetitive dives and some of the guidelines do not consider this.
In the light of the sparse and conflicting evidence about flying after diving, some authorities such as the US Air Force suggest that flying be avoided for at least 24 hours after diving. This is a very sensible rule to follow, especially if repetitive dives have been done over a number of consecutive days, as is often done on diving holidays. Some people believe this rule to be too conservative, but there are more than a few unfortunate divers who have suffered DCI after flying earlier than this.