In this column we will discuss hypothermia. Most of you would probably consider hypothermia a risk only for people exposed to extremely cold conditions. Actually, most cases of hypothermia occur at temperatures between 30-50 degrees. At these temperatures, victims underestimate the danger and are unprepared for the conditions they encounter. Because of this they get into trouble. The circumstances of the exposure, and the health of the victim, play major roles in who suffers from hypothermia. With that in mind lets get started.
Hypothermia is defined as a core temperature of less than 95 degrees Fahrenheit( 35 Celsius). Your body is a machine which is designed to function over a very narrow temperature range. Body heat is gained and lost through conduction, convection, radiation and evaporation.
Conduction is the transfer of heat by direct contact. The heat flows from the warmer object( your body) to the colder object(your treestand seat). A more serious example of conductive heat loss is when your body is submersed in cold water. Water conducts heat much more efficiently than air and will rapidly cool your body. This can be quickly life threatening. The story at the beginning of this article is an example of how wet clothes can quickly increase conductive heat loss. I was wearing plenty of warm clothes and was prepared to stay on stand all day. Wet clothing caused hypothermia to begin within an hour. A late season hunt involving open water is always dangerous. Submersion in cold water for even a brief period of time can quickly become serious. For example in 32 degree water death will occur in less than 15 minutes. Even in water 40-50 degrees exhaustion will occur in less than 1 hour and death within 3 hours.
Convection is the loss of heat by wind. Windy conditions disrupt the layer of warm air surrounding your body and accelerate heat loss. The wind chill factor takes the wind velocity into account. All hunters know that it feels much colder on a windy day.
Radiation is the loss of heat from exposed body surfaces such as your head or face.
Evaporation of water and sweat also consumes body heat. Body heat is used to dry your wet skin after you have been sweating and body heat is also used to warm and humidify the cold dry air that you are breathing.
Your body has developed mechanisms to combat heat loss and keep your temperature normal. Within your brain you have a thermostat which attempts to regulate your temperature. As you begin to get cold your thermostat tells your body to constrict the blood vessels in your arms and legs to conserve heat. This helps to keep the warmer blood around your vital organs such as your heart, lungs and kidneys. Colder blood is trapped in your extremities. It also begins the process of shivering. This produces body heat through muscle work. Most importantly, the thermostat makes you aware that you are getting cold and to do something about it. Typically you will head inside to warmer conditions or add another sweater , build a fire etc. If your response is to ignore your thermostat for any reason (the whitetail rut in late November) you are setting yourself up for trouble. Shivering is the first sign of hypothermia and should not be ignored. Your body is telling you it needs to warm up.
Hypothermia is divided into different stages depending on your body temperature. From 90-95 degrees Fahrenheit is classified as mild hypothermia and is characterized by shivering and mild incoordination (that must be why I missed that late season buck in Pennsylvania!). As your body temperature drops below 90 degrees your body functions begin to slow down. Your heart rate , breathing and blood pressure will all drop. You will become confused and very sleepy. Shivering will also stop in this temperature range. Your body is beginning to shut down and without some quick intervention you will die. Often at this stage because of your confusion and lethargy you will need assistance for survival. If you are alone, allowing yourself to go to sleep will lead to certain death. Below 86 degrees your heart may begin to beat irregularly and you will drift into a coma.
Treatment for hypothermia depends on the temperature of the patient and the equipment available. Ideally we need to stop heat loss and actively rewarm the patient. For mild hypothermia, getting the victim to a warmer environment and into dry clothing will usually be sufficient. In the field, a fire and a dry warm sleeping bag can be life saving. If a fire is not possible then use your body heat to warm the victim by hugging the victim or getting into the sleeping bag with them. If you do not have a sleeping bag, build a shelter to break the wind and contain the heat from your fire. As noted above, as body temperature drops, the victim will become uncoordinated and confused. If you are alone you will have great difficulty building a shelter or even taking simple survival measures as your body temperature decreases. Shivering is a early warning mechanism to take action! As the temperature of the victim gets lower more aggressive warming measures will be necessary. Warming the patient becomes more tricky at this point. As discussed above during cold conditions your body protects it's vital organs by shunting blood away from the extremities. If you apply external heat to the extremities of a victim who is moderately hypothermic the cold blood which has been trapped in the extremities could quickly return to the central circulation and actually make the patient worse.
Below 90 degrees the victim will likely need medical attention and more invasive warming techniques. With moderate hypothermia the victim needs to have his body core rewarmed actively but the extremities should be allowed to warm more gradually. You should get the victim into dry clothes and quickly transport them to a hospital. At these temperatures hot water bottles applied to the neck, chest and groin are useful but the extremities should not be actively warmed. All unconscious patients need to see a physician. The victim should be handled and transported very carefully because as the body temperature drops, the heart becomes irritable and rough handling could cause cardiac arrest. Survival has been reported even after 4.5 hours of cardiac arrest due to hypothermia so all victims should be transported to a hospital for an attempt at rewarming if at all feasible. In the hospital they will use techniques to quickly warm the patient's body core. These methods are beyond the scope of field first aid. One final note, in most first aid classes, you are taught to elevate a patients legs for suspected shock. In hypothermic patients lifting the legs should not be done as the legs are storing a large amount of cold blood which will be returned to the heart when the legs are lifted.
Prevention of hypothermia should be a part of every hunt plan. You never know when you may become stranded or have to spend a night in the field. Fifty degrees Fahrenheit can be deadly if you are wet and the wind is blowing.
Clothing should be picked with consideration for a wide range of possible conditions. Layering is the standard for cold weather conditions for good reason. Layers can be added or removed as temperature and exertion dictate keeping your body warm and dry in a wide range of conditions.
- Typically cold weather enthusiasts will wear a wicking layer next to the skin to move perspiration away from the body.
- Next comes an insulating layer such as polar fleece or down. This traps a layer of warm air next to your body.
- The outer layer should provide protection from the elements such as wind, snow and rain. Wool is the old standby which is favored by most hunters for good reason. Newer fabrics such as Gore-tex and Windstopper can add a protective layer which can significantly help to keep you warm and dry.
The worst thing you can do is sweat heavily on the way to your stand and then sit in the cold wind for 6 hours. Your sweaty wet clothing will quickly conduct heat away from your body. By wearing a light layer on the way to stand and taking your time you can keep your clothes dry and warm. As you get settled in you can add layers as needed to keep you comfortable. Shivering should not be ignored. Your body is warning you that your temperature is dropping and you need to take action of some type to warm it up. If you wait too long you will become confused and uncoordinated and be unable to take the necessary action. If you are alone do not go to sleep if you are very cold or shivering. Your body temperature will drift lower and you will slip into a coma. Hopefully by dressing appropriately and carrying simple survival gear with you at ALL times you will never be faced with a case of serious hypothermia. If you are faced with such a situation remember that a two pronged approach of stopping further heat loss and providing a warm dry environment can be life saving.
For More Information on staying warm - See our Feature Article on dressing for Very Cold Weather Bowhunting.