Hypothermia and Diving
With colder than average weather predicted for this winter, I thought this month should be about hypothermia. This is defined as a core body temperature below 35°C and the drop in core temperature may be rapid as in immersion in near-freezing water, or slow as in prolonged exposure to more temperate environments. The effects of hypothermia are proportional to the change in temperature, with metabolic rate reduced by about 10% for every 1°C fall in temperature. If heat retention and production are less than the cold challenge, then hypothermia will occur in time.
We lose heat thorough 4 methods of heat transfer: Conduction, convection, radiation and evaporation, but essentially heat only travels in 1 direction, from an area of higher temp to lower temp and can be reduced with warm dry clothing.
In more detail, there are 2 types of hypothermia: Primary and secondary.
Primary hypothermia is due to environmental exposure, with no underlying medical condition and secondary hypothermia is low body temperature resulting from a medical illness lowering the temperature set-point. These disorders include:
1. Decreased heat production. Eg hypothyroidism
2. Increased heat loss eg burns
3. Impaired thermoregulation, Eg strokes
4. Drug administration. Eg betablockers and alcohol
Risk factors for hypothermia include the following:
- Very old or very young people
- Trauma patients
- Chronically ill, especially with cardiovascular disease
- Malnourishment
- Underlying medical conditions, as above
Water absorbs heat very effectively and even the smallest current ensures heat is transferred away from the body. Therefore cooling will always occur in the sea with all wet suits and dry suits without active heating elements, given enough time. To minimise heat loss, adopt the foetal position and move as little as possible. Sharing body heat prevents the adoption this position and results in 2 colder people.
Presentation
Low-reading thermometers, preferably oesophageal, are required. Tympanic thermometers are unreliable in low temperature measurement. Check first for localised cold injury. Hypothermia usually occurs gradually and can be classified as mild, moderate or severe: The symptoms of hypothermia depend on how cold the environment is and how long you are exposed for.
Mild hypothermia (32-35°C): Lethargy, confusion, shivering, loss of fine motor co-ordination, feeling cold, low energy, or cold, pale skin. Although hypothermia is defined as occurring when the body temperature drops below 35°C (95°F), mild hypothermia can start at higher body temperatures.
Moderate hypothermia (28-32°C): Confusion, slowed reflexes violent, uncontrollable shivering, being unable to think or pay attention, confusion (some people don’t realise they are affected), loss of judgement and reasoning, difficulty moving around or stumbling (weakness), feeling afraid, memory loss, fumbling hands and loss of coordination, drowsiness, slurred speech, listlessness and indifference, or slow, shallow breathing and a weak pulse.
Severe hypothermia (below 28°C): Very cold skin, unresponsive, coma, difficulty breathing, abnormal heart rhythms, loss of control of hands, feet, and limbs,
uncontrollable shivering that suddenly stops, unconsciousness, shallow or no breathing, weak, irregular or no pulse, stiff muscles, and dilated pupils.
Babies with hypothermia may look healthy but their skin will feel cold. They may also be limp, unusually quiet and refuse to feed.
Severe hypothermia needs urgent medical treatment in hospital. Shivering is a good guide to how severe the condition is. If the person can stop shivering on their own, the hypothermia is mild, but if they cannot stop shivering, it is moderate or severe.
As divers, we need to be aware of the symptoms of hypothermia and minimize the risks by wearing adequate exposure protection, such as a wetsuit or drysuit. Even in warm water, hypothermia can occur in cases of accidental separation. Therefore wearing at least a wet suit is a safety issue, as well as a comfort issue. Once shivering begins during a dive, its time to finish the dive and make sure your symptoms do not get worse. If your dive buddy is shivering vigorously while underwater, make the decision for both of you.
Management of hypothermia
Mild hypothermia can be dealt with in the field. Prevent further heat loss by removing the patient from the cold environment and replace wet, cold clothing with warm blankets and supply hot food and drinks.
Severe hypothermia require immediate attention to airway, breathing and circulation and initiation of cardiopulmonary resuscitation may be required. Severely hypothermic people can appear dead, but don’t give up on them, especially as a rescuer. Administer oxygen via a bag reservoir device, or as high a concentration as possible. Establish intravenous access, prevent further heat loss, apply warm dry blankets/clothes and arrange transfer to hospital. If the patient can take hot/warm food and fluids without help, then it’s safe to provide these. Active rewarming in hospital requires careful patient monitoring and includes humidified oxygen, heated intravenous saline, warmed blankets and heat lamps.
Question 1:
What are the outcomes like for hypothermia?
This depends on the severity and nature of the cause. Most people tolerate mild hypothermia, which is not associated with significant morbidity or mortality.
Mortality is in the order of 20% in cases of moderate hypothermia.
Question 2:
How big a problem is hypothermia in the UK?
Age concern estimates 8,000 more elderly will die for every time the temperature drops 1°C below average. Nearly every year a party of schoolchildren is rescued from the moors, suffering from hypothermia. Even in summer, wet clothing increases heat loss by 5-10 times and wind can result in rapid loss of body heat – for the water to evaporate from the clothing it needs to absorb the latent heat of evaporation. (2.26 kJ/ml).
Question 3:
How can I reduce the risk of hypothermia?
Wear an appropriate exposure suit, hood and gloves to prevent body heat from escaping from your head, face and neck. Mittens are most effective because these keep fingers in closer contact with one another. Avoid activities that cause excessive sweating. Clothing made of tightly woven, water-repellent material is best for wind protection. Wool, silk or polypropylene inner layers hold more body heat than cotton. Stay as dry as possible, wherever possible.
Question 4:
Are there any conditions that mimic hypothermia?
The extensive list of medical conditions above can cause hypothermia, however some strokes (cerebrovascular accidents) and some drug overdoses can appear like hypothermia. Drugs including barbiturate, benzodiazepines and cocaine
These conditions need to be looked for in hospital if this is a possibility, based on the history.
Question 5:
Will pouring warm water in my dry suit help keep me warm?
With a dry suit, this would prevent the suit from doing what it does best! Heat is retained because you stay dry. However a few cups of warm water in the wet suit would help you stay warmer for a bit longer, as energy would not be expended warming up the water close to your skin. But be careful not to burn yourself!
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We would like to thank David Carty, for arranging for him and 7 fellow divers to come to the Divers Emergency Service HQ, based at Whipps Cross Hospital for a dry dive and look around the unit.
