In wilderness medicine, we often speak of the dangers of hypothermia. Hypothermia refers to a drop in core body temperature below 35° C, so that the body does not have enough heat to support normal cellular metabolism. Hypothermia not only poses a significant health risk on its own, but can greatly aggravate other illnesses or injuries in your patients.
Causes of Hypothermia
Sometimes hypothermia or hypothermia risk goes unnoticed, even in people who are obviously having a difficult time dealing with their environment. This is because we tend to think hypothermia as caused strictly by cold temperatures. However, environmental conditions that are easily managed by one person might overcome another, especially when other factors are involved. Causes of hypothermia not related to temperature include,
- Inadequate clothing, or other protection.
- Age (very old and very young are both at increased risk).
- Illness or injury.
- Previous or ongoing exposure to cold or wet conditions.
- Alcohol or drug use (including prescription medications.)
These are useful guidelines to help you identify who might be at greater risk, but there is a larger lesson: Just because you are doing okay, don’t assume that others around you can handle the environment.
Methods of Heat Loss
When treating (or, better, acting to prevent) hypothermia, you must consider all four methods by which the body loses heat.
- Conduction: transfer of heat through direct contact with a colder object. Wet clothing and sitting or lying on the ground or snow are two significant conductive heat loss culprits in backcountry environments.
- Convection: transfer of heat to moving currents, either wind or water. The rate of convective heat transfer increases exponentially with the speed of the wind or water current, so that an 8 km/hr wind speed causes four times the rate of heat loss that a 4 km/hr wind speed does.
- Radiation: loss of heat to cooler objects in the surrounding environment. Because radiative heat loss is poorly understood, and not immediately obvious, it is more likely to go untreated - however, it can account for the bulk of heat loss in a hypothermic patient.
- Evaporation: the evaporation of liquids from the surface of the skin if a process that consumes the body’s heat. This is, of course, the body’s main form of cooling - it’s why we sweat. It is also why wearing wet clothing in a windy environment is so unpleasant - heat loss in this situation is more than the sum of convective and conductive cooling, it is further aggravated by evaporative heat loss as the wind dries your clothing.
By considering all these methods of heat transfer, and keeping an eye out for additional risk factors, you can more quickly identify and protect those at risk of hypothermia, and more efficiently treat people who are already hypothermic.
Assessing Hypothermic Patients
Recognizing hypothermic patients and accurately assessing their degree of hypothermia are key to establishing the need for treatment, and initiating the proper treatments. There are three levels of hypothermia:
Mild Hypothermia: Patients with a core temperature between 35° C and 32° C are considered to be mildly hypothermic. Look for increased heart rate, rapid breathing, shivering, and unclear speech. These patients are still fully oriented and rational, but will have difficulty with tasks requiring fine motor control (writing, doing up buttons, etc…). Look for a set of signs known as the “-umbles”: fumbles, stumbles and tumbles (which represent declining motor function) as well as mumbles and grumbles (which signify early stages of cognitive dysfunction).
Moderate Hypothermia: Moderately hypothermic patients have a core temperature between 32° C and 28° C, and are undergoing profound deficiencies in coordination and mental acuity. Look for a marked decrease in gross motor control, confused, incoherent speech, and ineffective or absent shivering. Moderately hypothermic patients often don’t have the coordination necessary to help themselves - they are unable to put on and fasten layers of clothing, or start a fire - and may exhibit dangerously confused behaviours, such as removing their own clothing.
Severe Hypothermia: Severely hypothermic patients are unresponsive, very cold, with flaccid muscles. Their heart rates and breathing may be almost undetectable slow, or stopped altogether.
In all cases, simply warming a hypothermic patient is not enough. It is important to recognize why they became hypothermic in the first place, and to prevent it from recurring. Sometimes this is easy - for example, if your patient fell in a lake - but always consider the risk factors above, especially if the reasons for your patient’s condition aren’t obvious.
- Treating Mild Hypothermia: Mildly hypothermic patients still have functioning thermoregulatory processes, and treatment should focus on supporting and supplementing these processes. Ensure the patient is clothed in dry, warm clothing and layers sufficient to insulate them from the environment. Apply external heat sources if practical and safe (heat packs under the arms and at the head and neck, and nice cozy fire nearby are medically indicated!) and support shivering with high calorie drinks (warmed for comfort, if practical).
- Moderate to Severe Hypothermia: Moderately and severely hypothermic patients need more aggressive protection from heat loss, and almost certainly require additional heat sources. The Hypowrap is the tool of choice - as described in my last post. These patients must be monitored very carefully, treated very gently, and kept in a horizontal position. Evacuation without delay is, of course, a top priority.
Special Considerations in Treating Hypothermic Patients
- Afterdrop: As the body cools below normothermic temperatures, it tries to conserve heat for the vital organs by shutting down circulation to the extremities. As a result, the arms and legs will be much cooler than the core - perhaps as cold as the ambient environmental temperature. As your hypothermic patient warms in response to your treatment, that peripheral circulation will be restored - resulting, unfortunately, in warmer blood losing heat through convection and conduction to the still-cold arms and legs, before returning to the core. This leads to a secondary drop in temperature, known as afterdrop, which can cause patients who have recovered consciousness and started shivering to decline again. All moderately and severely hypothermic patients must be monitored for afterdrop, and heat sources reapplied to the hypowrap if necessary.
- Moving Hypothermic Patients: Moderately and severely hypothermic patients are at significant risk of having their heart thrown into a fatal dysrythmia if moved too quickly or too roughly, leading to cardiac arrest. All moderately or severely hypothermic patients must be kept horizontal, moved only when absolutely necessary, and then moved as gently as possible.
- Cardiac Arrest in Hypothermia: No one is dead until they are warm and dead. A hypothermic individual is in a "metabolic icebox" where their oxygen and nutrient needs are extremely low, and they can sometimes survive surprisingly long periods of time without breathing or without effective circulation. Do not begin CPR without checking for breathing and a pulse for a full minute - both may be very slow. Before beginning compressions assist ventilations at a rate of 1 breath every 5-6 seconds for 3 minutes, and then rechecking for pulse and spontaneous breathing for another full minute. Do not stop CPR after 30 minutes with no signs of life, as is typical in remote environments - if you begin CPR on a hypothermic patient, be prepared to continue until taken over by rescue personnel, no matter how long that may take. Hypothermic patients have had full recoveries even after several hours of continuous CPR - but if you stop CPR, they will certainly not be able to be resuscitated.
Recognizing and treating hypothermia and other cold injuries is an evolving field of wilderness medicine that goes far beyond the equipment and training available to first aid practitioners. These guidelines form the basis of the hypothermic response you learn in our Wilderness First Aid course, but if you want to know more, I cannot recommend Gordon Giesbrecht and James Wilkerson's "Hypothermia, Frostbite & Other Cold Injuries" highly enough. Dr. Giesbrecht is professor of thermophysiology at the University of Manitoba, in Winnipeg, so you know he has some familiarity with cold emergencies!
With summer over, I am back on a normal person's schedule - in two weeks, we will talk about wound care, including cuts, scrapes, amputations, impalements, burns and pain control. Please share your thoughts on this article, and remember - I take requests! Let me know if there are any Wilderness Medicine topics you would like to see discussed in the future.
Stay safe out there,