COLD WATER IMMERSION
At the end of 2016, diver training agencies including BSAC launched Sea Survival training courses developed in conjunction with the RNLI. Despite the common view of the RNLI as being the provision of lifeboats and crew, there is much more that they are involved in from a safety at sea perspective, with particular foci on fishing industry accidents, Swim Safe training courses and safety advice for all water users. It’s astounding how many fishermen don’t wear lifejackets, especially local pot-boat skippers who often work alone.
To try to educate fishermen the RNLI brought a dozen of them from around the UK down to their training base in Poole. All the skippers had previously attended the mandatory Personal Survival Techniques course (and it’s predecessors) which are run in swimming pools around the country. The RNLI trainers asked about lifejackets and got the usual story, the fishermen had them but rarely wore them. The general feedback was that as strong swimmers they were confident that should they fall in the sea, they would be able to swim back to their boat, climb up the tyres on the side and self-rescue. Interestingly, qualified divers and anyone who swam in the sea was excluded from the test group. Repeated attendance at sea survival training had led each fisherman to conclude that their lifejackets weren’t necessary. The RNLI sought to challenge that belief.
The night before the training course, the trainers opened the doors around the training pool to let out the heat. Overnight the water temperature dropped to 15 degrees. If you are a diver around the British Isle I am sure there are days where you dream of 15 degree water! At the first attempt the fishermen were asked to wear their normal deck attire and jump in to deep water to simulate falling off their boat. With no life jackets on, the impact of cold water shock was immediate. None of the 12 fishermen lasted longer than 5 minutes before a rescuer intervened. Post dip interviews revealed their shock and surprise at how debilitating the cold water was, definitely nothing like their sea survival training course.
Cold water shock is an immediate short-lived response to immersion in water less than 15 degrees. Blood vessels at the skin contract rapidly, increasing blood pressure and the heart rate. An initial gasp for air can be followed by a breathing rate that is 6-10x higher than normal. It is likely that cold water shock accounts for most deaths when people have unexpectedly entered the water. If you are not wearing flotation during this phase, keeping your head above water becomes the biggest problem. Over the next 10 minutes, cold incapacitation reduces blood supply to the muscles, making it difficult to swim or self-rescue. A crew member throwing a life ring to you during this time will be frustrated that you can’t actually hold onto it or kick towards the safety of the vessel.
The following day the exercise was repeated but this time with lifejackets being worn. The same cold water shock reaction was initiated, but the fishermen didn’t have to work so hard to keep their airway out of the water, the cold incapacitation stage took longer therefore improving their chances of getting back to the ladder on their boat. You can see the videos from this exercise on the RNLI website.
This started me thinking about why divers were excluded from the test group. I’ve realised I still brace myself for the cold water after decades of diving. OK, I’m wearing a drysuit and the cold water shock reaction is pretty much limited to my head and hands. But how many of us drop beneath the surface in anticipation of that brain freeze moment? As the blood vessels rapidly contract they stimulate the trigeminal nerve sending pain signals to your brain. It hurts for a few moments until you become acclimatised. The fishermen in the RNLI training exercise couldn’t get past that brain freeze feeling.
I think we sometimes underestimate the impact that cold water immersion has on new divers. I can recognise it enough now, but when I think back to learning in a wetsuit I can remember the feeling of panic, rising heart rate and accelerated breathing rate as I used to get into the water. Although we will all recognise increased air consumption by trainee divers, perhaps part of this is their reaction to cold water immersion? I’m sure that with experience comes the anticipation, the forced control of breathing rate for the first few seconds, but until our new divers have developed their response, maybe we should keep a close eye on them for those first couple of minutes? If your trainees are hoofing through their air and their buoyancy is being disrupted by their rapid breathing rate, maybe it’s something to consider?
Michelle has been scuba diving for nearly 30 years. Drawing on her science background she tackles some bits of marine science. and sometimes has a sideways glance at the people and events that she encounters in the diving world.