Hypothermia in the Athlete

The body loses heat by radiation, conduction, convection and evaporation, and normally balances this against the heat produced by metabolism and muscle activity, defending the core temperature by shivering and by narrowing the skin vessels to reduce loss. Hypothermia develops when heat loss outstrips heat production for long enough that these defences are overwhelmed. Wet clothing and wind greatly increase loss, and water conducts heat away far faster than air, which is why immersion cools a person so quickly. Exhaustion, injury, being unable to move, low energy stores and alcohol all reduce heat production or increase loss and make it more likely.

As the core temperature falls, the picture changes in stages. Early on the body shivers hard and the heart rate rises, but as cooling continues shivering eventually stops, the level of consciousness falls, and the heart and breathing slow. The cold heart becomes prone to dangerous rhythms, particularly ventricular fibrillation (VF, a chaotic non-pumping rhythm), and rough movement or handling can set this off. Cold water immersion adds an early cold shock response, an involuntary gasp and rapid breathing on entry that carries a real drowning risk, before hypothermia itself takes hold.

Presentation runs by severity. In mild hypothermia the athlete is cold, pale and shivering, with a fast pulse, and although still conscious may become clumsy and show the 'umbles', that is stumbles, mumbles, fumbles and grumbles, as coordination and thinking are affected. In moderate hypothermia the shivering may stop, the athlete becomes increasingly confused, drowsy and slurred, coordination worsens and the pulse slows. In severe hypothermia the person is unconscious and may appear lifeless, with a very slow or undetectable pulse and breathing, rigid muscles and a high risk of a dangerous heart rhythm. A striking feature in severe cases is paradoxical undressing, where the confused, cold person removes their clothing. Where a reliable core temperature is not available, severity can be judged clinically using the Swiss Staging System, which grades the casualty by their consciousness and whether they are still shivering.

The core temperature is the key measurement, and it must be taken with a validated low-reading core thermometer, because standard clinical thermometers do not read far enough below normal and will underestimate the problem. A rectal, oesophageal or appropriate tympanic core measurement may be used, but ordinary infrared ear thermometers are unreliable in this setting. Because the pulse can be very slow and hard to feel, an unconscious hypothermic casualty should have their pulse and breathing checked for up to a minute before cardiac arrest is diagnosed and chest compressions are begun. An electrocardiogram (ECG, a tracing of the heart) is useful, since hypothermia slows the rhythm and can produce a characteristic extra deflection after the main complex, the J or Osborn wave, and it allows dangerous rhythms to be picked up. In hospital, blood tests including glucose and electrolytes guide care, though the field priority is clinical recognition and core temperature measurement.

The first step is to stop further heat loss: move the athlete out of the cold, wind and wet into shelter, remove wet clothing by cutting rather than pulling where possible, and insulate them well including the head, ideally with a windproof and waterproof outer layer. Throughout, handle the casualty gently and keep them horizontal, because sudden movement of a cold, irritable heart can precipitate ventricular fibrillation.

Rewarming is matched to severity. Mild hypothermia in a fully conscious athlete is treated with passive and active external rewarming, that is insulation, a warm environment and warmth applied to the trunk, along with warm sweet drinks if they can safely swallow. Moderate and severe hypothermia need active rewarming with gentle handling, cardiac monitoring and transfer to hospital, where the most severe cases may need rewarming of the blood outside the body, for example with extracorporeal membrane oxygenation (ECMO). Warmed intravenous (IV) fluids and warm humidified oxygen are useful adjuncts that mainly prevent further cooling rather than achieving adequate rewarming on their own, and warmth is applied to the trunk rather than the limbs, to avoid pushing cold blood back to the core.

In cardiac arrest with hypothermia, resuscitation is modified. After three unsuccessful shocks, further defibrillation is delayed until the core temperature is above 30 degrees Celsius, and adrenaline is withheld below 30 degrees Celsius and given at longer intervals between 30 and 35 degrees Celsius. Cardiopulmonary resuscitation (CPR) is continued and rewarming pursued before any decision to stop, since recovery is possible from very low temperatures. That is the practical meaning of not dead until warm and dead, which holds unless there are clearly fatal injuries, a chest too frozen to compress, or another established reason to withhold resuscitation. After rescue the core temperature can keep falling, an afterdrop that reflects redistribution of blood and continued conductive heat transfer, which is a further reason to keep handling gentle and horizontal and to focus active warming on the trunk.

Prevention is about limiting exposure and maintaining heat. Athletes should dress for the conditions in layers, with windproof and waterproof outer clothing, keep dry, and stay fuelled, since energy is needed to produce heat. Planning for the weather, acclimatising, using wetsuits or appropriate protection for cold water, working in pairs, and recognising the early signs all help, as does stopping before exhaustion sets in, because a tired athlete makes less heat. Getting out of the wind and wet quickly when cold prevents progression.

Return to activity after mild hypothermia follows complete resolution of symptoms, a return to a normal core temperature, and normal thinking and coordination, once the athlete has taken on food and fluid. After a moderate or severe episode, return is individualised and follows recovery, medical clearance, and assessment of the cause and of any cold injury, with attention to what went wrong so that future exposure is managed better.

• •Wilderness and mountain medicine guidance on hypothermia sets out grading, gentle handling, rewarming and the approach to the apparently lifeless cold casualty.
• •Resuscitation guidance addresses cardiac arrest in hypothermia, including the low-temperature modifications to defibrillation and drugs and prolonged resuscitation before stopping.
• •Water safety guidance covers cold water immersion, the cold shock response and safe rescue.
• •UK sports, expedition and event medical resources support planning for cold conditions.

• •Hypothermia is a core temperature below 35 degrees Celsius: mild 35 to 32, moderate 32 to 28, severe below 28.
• •Wet and wind greatly increase heat loss, and cold water cools a person very quickly.
• •Handle a cold casualty gently and keep them horizontal, since the cold heart is prone to ventricular fibrillation.
• •Measure with a validated low-reading core thermometer, since ordinary ear thermometers are unreliable.
• •Rewarm the trunk rather than the limbs, and expect a possible afterdrop after rescue.
• •Not dead until warm and dead, unless there are clearly fatal injuries or another reason to withhold resuscitation.