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Bends - Flying / Altitude

Dr Jules Eden, dive medicine specialist and founder of e-med, answers divers' questions - as published in Sport Diver magazine:
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Q - We are going to the Maldives soon and I would appreciate your advice on inter-island seaplane flights after diving.

We are having six days on an island and had assumed that we could dive only on the first five days because of needing a dry day before taking the morning seaplane flight off the island, followed by an afternoon international flight back to the UK.

However, we have been advised by the diving travel agent that the seaplane flight after diving is not an issue, because those seaplanes do not rise sufficiently high during the 40 minute transfer flight to cause a problem. Is that right in your opinion?

How high do the seaplanes fly? I have no idea, but it is clearly a lot less than the international flights. How high is critical? I have no idea. The flying is all visual reference and the cabins are not pressurised.

A - I’ve recently joined the merry throng of lunatics that are ‘married men’, so thought it was about time we visited the Maldives. Hence this exact issue reared its head whilst we were contemplating our navels on a paradise beach in the Indian Ocean. I’d kept an eye on the altimeter in the cockpit on the way in, which seemed to register 3500- 4000ft during the transfer. Chatting to the pilot, it turns out they sometimes need to fly above or around thunderstorms, and hence can reach altitudes of 15000ft. And as you say, the cabins are not pressurised.

Now the US Navy Diving Manual states transfer of a DCS patient by air should be at 1 atmosphere if possible, ie. at sea level; if not, it should be by “unpressurised aircraft flown as low as safely possible (no more than 1,000 feet is preferable)”. So clearly they are concerned about the risk of bubble expansion at altitudes higher than this. To my mind, we therefore need to be conservative about the no-fly time after diving, and I would suggest a minimum of 24 hours, with the final days’ diving conservative (a single, shallow, no-stop dive if possible).

So even a single dive on day 6 might be pushing it. I’d advise an entire day off to off- gas fully; indulge yourself in ridiculously large meals and beach time, with some nocturnal Bacchanalian boozing thrown in. Having just done this, I can highly recommend salt and pepper squid served with chillies and copious quantities of chilled white – guaranteed to eradicate any gut bugs that might be trying to hitch a ride home with you.

Q - On a recent trip to an unidentified exotic location (I'd rather not say where we were to prevent embarrassment!), we had the opportunity to do some sightseeing from a non-pressurised plane, which we took full advantage of. We then hopped over to another island and dived in a lake which was high up in the hills, I think at about 3000 feet or so. It didn't even occur to us during the transit but once there we realised we'd probably violated lots of rules by going up to altitude and then diving. We had no symptoms and were fine throughout. Have problems been caused by this sort of behaviour before, and are there ways to minimise the risk? Thanks for your advice.

A - One oft-overlooked precipitant of symptomatic bubble expansion is the trip home over the hills or a mountain range. For some reason such a land-based ascent is frequently neglected, but the pressure reduction can be similar to that of a flight and hence equally capable of bringing on a bend. When diving at altitude, adjustments do need to be made to compensate for the reduced atmospheric pressure – basically, more decompression is required than an identical dive at sea level. You can use specific altitude tables, or apply a cross-correction technique that rejigs the depth and stops to their sea level equivalents (see the trusty US Navy Diving Manual for specifics). Dives at altitudes of up to 300 feet are generally feasible without correction, but as altitude increases, progressively greater fudge factors are used. Another interesting facet that sometimes comes into play (and may have done in your case) is acclimatisation. If you dive within 12 hours of arriving at altitude, you will still have residual onboard nitrogen from your point of departure (assuming it was at sea level, or at least at a lower altitude), which needs to be factored in as well. In effect, the ascent to altitude is counted as a dive, and therefore you’ll start your first actual wet dive in a repetitive group. So there are a few modifications to familiarise yourself with, but nothing that a bit of time with a dive planner won’t clarify. I’m dying to know where your dives were now!

Q - As well as being a keen diver I also have a passion for hang-gliding. I suspect I already know the answer to my question, but I'm going to ask it anyway - would it be safe for me to go hang-gliding after a dive? I'm guessing "no", but I wondered whether there are any tables or formulas to give me some idea on how much altitude I can gain before the risk of DCS becomes too high. Surely our venerable Royal Navy or Air Force would have looked into this, but maybe the information is top secret hush-hush stuff. Can you help?

A - I can’t imagine why you’d want to jump off a cliff and catch thermals immediately after a dive (there are many easier ways to dry off – a towel, maybe?) but each to their own I suppose. You are correct – from the point of view of DCS risk, this would be an eminently unsafe thing to do, for reasons already outlined. That said, there are occasions when people have no choice but to ascend to altitude, for example in military situations (as you’ve alluded to), or when no pressurised mode of air transport can be found. Far from being top secret, the US Navy guidelines are publically available – commendably full and open transparency from the same country that set up the First Earth Battalion and believed they could kill goats by staring at them. There are tables therein which enable you to calculate the surface interval required before flying, dependent on the planned increase in altitude and the highest repetitive group designator obtained in the previous 24 hours. If you absolutely must fly, the current generic recommendations are to do it “as low as safely possible, preferably less than 1000 feet”. Although I’ve never tried hang-gliding, I would imagine that it’s easily possible to exceed this altitude, and moreover find it very difficult to control the speed of ascent if you end up in a thermal. Might I suggest a slightly less provocative post-dive pastime – macramé perhaps, or beekeeping?

Q - I've heard a lot about the mandatory wait before flying home after a dive trip, but is there any advice or issue with the reverse situation, ie. diving after flying? My thinking is that if I'm going on a dive holiday for a week, and know I can't dive on the last day, then in order to maximise my dives can I jump in as soon as the plane has landed?

A - Flying leads to numerous assaults on our usually jovial demeanour, turning us from polite, compliant citizens into rabid, slavering Visigoths. These arise as a result of the reduced air pressure and oxygen content in the cabin, the motion of the plane, the general stress of travelling and the major irritation of fellow passengers, who always seem to be enormous smelly social outcasts of dubious genetic stock who spill over into your seat and have bladders the size of dried peas. Add in a heady mix of dehydration, fatigue and immobility, together with a large splash of pre-holiday alcohol, and your recipe for a ragged, strung-out holidaymaker on arrival is complete. There’s no scientific reason that you should be more susceptible to a problem if you dive soon after a flight, but with these factors swirling about I’d always recommend a bit of a chill-out on touching down, so you’re in the right frame of mind for your first dive.

Q - I was returning from an overseas business trip recently and heard over the tannoy one of the stewardesses asking for a doctor. It turned out that someone was having an angina attack and they wanted some medical help. It all passed off without incident but as a diver, it got me thinking: what would I do if I get symptoms of DCS on a plane? If it comes on soon after take-off and it's a long-haul flight, there's potentially a good few hours of suffering ahead before getting to a chamber. Is there anything that I should or shouldn't do in the meantime?

A - A few basics first: the atmospheric pressure at sea level is defined as 1 atmosphere absolute (1 ATA). Aircraft cabins are usually pressurised to around 0.8 ATA (the equivalent of 7500 feet). This doesn’t sound much less, but it’s enough to allow bubbles to expand and start causing symptoms. If this happens, then you need to alert the cabin crew immediately, get yourself on oxygen, and stay as well hydrated as possible: basically the same steps as you would take with a bend in any other circumstance. Obviously there’ll be limited resources available for DCS treatment on a plane, so the sooner it descends the better. There is some limited evidence that tenoxicam, an anti-inflammatory drug, can reduce the number of recompression treatments required to achieve recovery, so if you happen to have any of these in your medical kit then there wouldn’t be any harm in taking 20mg. (To my knowledge there have been no big trials of other anti-inflammatories as yet.) Otherwise, it’s get thee to a chamber as soon as you land, and try to resist knocking back the Bloody Mary’s en route.

Q - I bought a holiday home in Italy many years ago and have visited every summer for the last 10 years. It's at about 1000m above sea level and has amazing views over the Adriatic. I always wondered what diving was like so a few years back I took a trip down and found a local dive school. After a couple of try dives, I got absolutely hooked (I suspect like many others) and every summer since I've been diving for a week or so whilst staying there. Now I retired last year and am going to live there for pretty much the whole year I've decided, but someone mentioned to me recently that there might be a problem with driving back to my villa after the diving, because of the height gain. I didn't think it would be a problem but then I remembered buying one of those "salads in a bag" in a shop after the diving (at sea level), I left it on the passenger seat. On the way home the bag expanded so much that it burst and showered the whole car with bits of lettuce! Apart from nearly crashing I thought the whole incident rather amusing but it suddenly occurred to me that this might be happening to my lungs on the way up. So to cut a long story short, is it safe for me to continue my diving in this way, ie. driving down to the sea for a couple of dives then going back home to 1000m every day? And if not, what do you suggest to make my diving safer? I'd hate to give up my new passion especially as I now have all the time in the world to indulge in it!

A - Exploding bags of salad are a somewhat unusual diving hazard but I imagine you must have nearly rolled your vehicle with your windscreen suddenly being splattered with foliage. It’s a very good illustration of expanding gases and Boyle’s Law though. We at the Chamber have been considering this long and hard, and have done some digging around as it is a bit of an unusual one. However there don’t seem to be any specific tables or hard and fast recommendations for this scenario. In essence what you are doing here is the equivalent of flying after diving, and as such you really ought to be leaving a good 24 hours after your final dive before driving home. In naval rescue situations the usual ceiling on flying immediately after diving is 300m, so you are well in excess of this, and as such this is somewhat risky. The potential for bubble expansion precipitating a bend is well known even with moderate altitude increases. If you are going to be doing this regularly, then the best advice would be to stay at sea level for the duration of your dive trip and then leave 24 hours before driving home – maybe you can get to know your greengrocer and eat some salads fresh instead of bagged. If you really must go home each day then maximise your safety margins and minimise your nitrogen load, by eg. using Nitrox on air tables, leaving it as long as possible after diving before ascending, allowing regular no-diving days to allow yourself to off-gas, and keeping your depths and times well within the tables. I have to say I could not possibly recommend this course of action as it does pose a significant risk of getting bent – if you can afford it I would almost consider getting your own recompression chamber built in a bespoke annexe so that you can leap in as soon as you get home!

Q - I have recently qualified as a Padi Open Water diver but my main sport is flying hang gliders. There will be occasions when it would be nice to enjoy both on the same holiday, perhaps occasionally on the same day.
Both my dive computer and Padi OW manuals suggest not flying for a minimum of 12 hours after diving. The Padi manual goes on to say that this is so that you should be sympton free from decompression sickness at a cabin pressure equivalent to 8000 ft.

In the (coastal) areas where I might envisage carrying out both activities within a period of less than 12 hours I would not expect to get much higher than 2000 ft and could easily remain lower. I haven't seen any warnings about changing altitude by 2000 ft during a drive home from a dive and I imagine this often happens. Are you able to offer any guidelines about what should be considered an acceptable change in altitude in these circumstances? I note that for diving at altitude my dive computer manual recommends adjustments be made to settings when diving at more than 700m (2300 ft) altitude. Would it be sensible to assume that varying altitude within this limit after diving should also be OK?

I find the variety of questions addressed in your regular spot in Sport Diver interesting and any help you can give me would be much appreciated.

A - I think you need to use good old common sense here. Sure the manuals and tables show that at this height you would not precipitate a DCS. But we have all heard stories of driving over from Dahab to Sharm where the road takes you this height and it bringing on a bend.

It would be sensible to glide in the morning and dive after that. But I suppose there aren't any good warm thermals in the dawn of a day.

My gut feeling it do not risk it, but I am sure if you dove on Nitrox, to a reasonable depth and used air tables, then a glide to 2000ft would be OK a few hours later.

(other dive medical questions)


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