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Winter is here! The cold weather has arrived with a vengeance and with it many of us will stow the dive gear for a few months or be fortunate enough to sneak off to the tropics for a spell. But what about those crazy few who will continue to plunge into the chilly seas off our coast? From my observations the water temp can lurk around 13 degrees or even less in some areas. In these temperatures our approach to diving must change to stay warm and safe, and the possibility of hypothermia must be recognised.
Diving in cold water means more than just feeling chilly, it often means significant changes to gear configuration, weather conditions and safety margins. Some of us will don dry suits for 3 or 4 months which we are less familiar with. These suits are often more bulky, constrictive, claustrophobic and tiring to use. They require different skills, weight adjustment and carry the risk of poor buoyancy control and the dreaded inverted uncontrolled ascent. A session in the pool to get your weights right and check the functions of the suit may be wise before your first dive. Once mastered however, the drysuit makes winter diving a pleasure (especially watching your mates freeze their t…s off in the boat between dives!).
What about hypothermia? We have all heard of it but how dangerous is it in temperate waters. The survival time of an unclad human in 13 degree water is 5 hours or less. The better quality of wetsuit one wears then the greater time this is increased. The more you resemble a Buluga whale or walrus the longer you will survive (this is the only advantage of being overweight!). Water conducts heat away from your body 25 times more rapidly than air. Heat is lost from exposed surfaces especially those with high blood flow like the head.
The physiological effects of immersion in cold water are summarised as follows:
Cardiovascular Blood flow to the peripheries reduces which means that more blood flow is directed centrally to the heart and lungs etc. This conserves heat by keeping blood flow away from the exposed skin and muscle, but can have adverse effects on the heart in susceptible individuals eg abnormal heart rhythms or heart failure. Sudden cardiac deaths have occurred following immersion.
Respiratory Initial gasping may be followed by hyperventilation. This uncontrollable breathing may cause the swimmer to inhale water. The cold exposure may trigger bronchospasm in those with asthmatic tendencies.
Once these initial responses are survived, then a gradual decrease in core temperature begins. Shivering starts early in an attempt to generate heat by muscle work. As temperature falls to around 35 degrees the subject loses interest in their surroundings (certainly is not focusing on the dive tasks and thinks only of getting out of the water - something all of you will have experienced). The pooling of blood to the central organs triggers a response to offload excess volume, so increased urine is formed and excreted. This produces dehydration, an undesirable feature in any diver.
As temperature falls to 30-33 degrees, the diver will become drowsy then unconcious. Heart arrythmias again become a problem. Shivering will cease and may be replaced by rigid muscles. Severe hypothermia less than 30 degrees is a lethal condition and is difficult to treat even in a modern hospital emergency room.
Treatment
Accurate measurement of the core temperature can be quite difficult without sophisticated equipment. For mild hypothermia which many of us will have suffered, get the diver out of the water, dried and into warm clothes ASAP, then putting them in a warm room or car with a warm drink will usually suffice. For the diver who looks unwell, is incoherent or uncoordinated, do all of the above but do not give anything by mouth in case swallowing is impaired. Medical advice should be urgently sought in these cases. Gradual rewarming is the key for all victims, with rapid heating eg a hot bath to be avoided due to risk of suddenly dropping core temperature as the blood flows back to the cold peripheries. The use of a bath with a gradual increase in temperature may be acceptable if the patient is fully concious.
There is an old saying in medicine that you can't be cold and dead, only warm and dead. In very cold patients it can be difficult to feel a pulse or detect much breathing so death may be incorrectly diagnosed! Resuscitation efforts should generally not be ceased until expert medical attention has been sought. Stories of prolonged submersions/drownings in freezing water with a good outcome after resuscitation only apply to truly icy water where the immersion generates the "diving reflex" of breath holding and a very slow pulse rate. Such immersions in our temperate waters are very unlikely to survive.
Keep warm! |
