The severity of hypothermia cases have been categorised in various ways, often into ‘mild’, ‘moderate’ and ‘severe’ hypothermia. One way that is often seen is a categorisation based on core temperature. For the outdoors person, a pitfall with a categorisation based on body core temperature is that it’s not possible to monitor internal body temperatures in the field (even if you are very good friends, using a rectal thermometer while on the slopes of Ben MacDui is not practical). Further, using temperature alone as a diagnostic tool has the potential to be misleading. A drop in core temperature will affect every part of your body’s system so we must look for the signs and symptoms of these changes:
An early sign of being too cold is shivering. Shivering increases your metabolic heat production to a level between two- and six-times that of your normal, resting rate. This has a short term warming benefit on your body but it reduces energy stores in your body. In the long term, this energy depletion adds to your deterioration by contributing to eventual exhaustion. When your body detects it is starting to cool down below its normal temperature, it fairly quickly starts to shiver so it is a good early indicator of potential hypothermia. Maximal shivering is reported to occur at a body core temperature of 35oC (95.0oF). If you or someone else is shivering, do something about it!
Another relatively early sign can be cold, stiff and/or white hands or feet. This is due to vasoconstriction, a reduction in blood supply to the area. This is one of your body’s reactions to cold, intended to keep heat in your core. If your hands are white, cold or stiff it is a warning sign that your whole body is too cold. Again, do something about it! As hypothermia takes greater hold, there will likely be a significant reduction in manual dexterity which, if on your own, could mean you are no longer able to help yourself.
Hypothermia progressively reduces mental function, the early signs of which are often missed. The first signs are mood changes, irritability and/or social WITHDRAWAL. This is really important but hard to spot in a group – you have to notice who isn’t noticeable; spot the one who has disappeared amongst the group. Weather conditions can often blunt your ability to observe your companions and you yourself may be battling to cope with the elements and keep warm.
Typical conditions under which you have to be vigilant for signs of hypothermia. Photo: Paul Kirtley.
Even mild hypothermia can reduce your ability to look after yourself or your companions. Studies have shown even mild core cooling to temperatures in the range 34oC (93oF) to 36oC (97oF) will impair mental function. And as you get colder, things get worse; for each one degree Celsius drop in core temperature from 35oC (95oF) to 25oC (77oF) your brain metabolism drops between 6% and 10%. Decreased mental function translates into poor decision making, indecision, irrational behaviour, confusion and forgetfulness – all dangerous failings in the great outdoors.
Heart rate and blood pressure will likely have increased at this stage. It won’t be possible to detect this in others, particularly since exertion and fear promote similar responses. Nor is it likely that you will be able to discern this in yourself since it will be tangled up with other feelings and concerns; people often describe a “sense of anxiety” at this stage.
As the victim’s body core temperature continues to drop, their mental function and physical coordination will be further reduced. Weakness, stumbling and repeated falling are typical. They may be disorientated. They may be forgetful or irrational. The may appear drunk. Their behaviour may be bizarre.
A good way of grouping some of the increased clumsiness and uncoordinated behaviour that a hypothermia victim can go through is remembering them as “the umbles”, characterised by
Shivering may still be present although there have been serious cases where the victim has seemingly progressed to stumbling, repeated falling and stupor without their companions having noticed any shivering. This could be due to lack of observation on the part of the companions or it could be related to the victim’s hypothermia being precipitated by exhaustion. Maybe they didn’t have the energy to shiver? Certainly experiments have shown that once blood sugar drops below a certain level, shivering ceases.
As they continue to cool, the hypothermia victim will range from being lethargic to losing consciousness to comatose. Their lips and possibly fingers will be blue (cyanosis). Shivering is likely to have ceased as they will almost certainly be exhausted. Their temperature will plummet as the heat generated from shivering ceases. Muscles become stiff. As their temperature drops their pulse will slow and weaken, blood pressure will drop and their breathing rate will slow. It may be impossible to feel a pulse.
The casualty may appear dead and indeed they might be – cardiac arrest often occurs with a body core temperature between 28oC (82.4oF) and 25oC (77oF).
Even if the casualty appears dead, however, it may be because vital signs are impossible to discern in the field. The lowest recorded core temperature from which someone has been successfully resuscitated is 13.7oC (56.7oF). There is an old adage “a person is not dead until they are warm and dead.”