Ever shiver from the cold? At that moment, you were hypothermic. Your body temperature dropped below normal and triggered a heat generating response from the brain commanding the muscles to twitch, thus producing heat.
Face it, you are a homeotherm. And that’s a good thing; it means you are warm-blooded and capable of maintaining a relatively constant internal temperature regardless of the temperature that surrounds you ... up to a point.
The body’s response to cold
Command central for temperature regulation is the hypothalamus in the brain. Often called the body’s thermostat, it is actually a rheostat. Instead of turning on all the heat at once — like your home’s thermostat — the hypothalamus responds to changes in temperature like a dimmer switch for a light fixture. If you need more light, increase the flow of electricity to the bulb; need less, turn down the current. The electrical signals to the hypothalamus come from cold receptors wired into the skin and deeper organs. These receptors fire as they sense a decrease in temperature, hitting the skin first, then the deeper organs. The deeper the cold penetrates into the body, the more receptors fire to the brain. The more signals received by the “temperature rheostat,” the more intense the response.
A mild drop in body temperature initially triggers vasoconstriction when the hypothalamus contacts the sympathetic nervous system. This prevents heat loss from the skin (convection). As the hypothermia progresses, more signals arrive to the brain and heat production — or thermogenesis — begins with the onset of shivering. The sympathetic nervous system is again contacted by the hypothalamus to increase cardiac output (heart rate multiplied by stroke volume) to increase circulation of warm blood. And since the muscles and heart are working harder, we need more oxygen intake and carbon dioxide disposal, thus increasing respiratory rate and depth. Unfortunately, as the mild hypothermia (32-35 degree C/90-95 degree F) progresses to moderate hypothermia (28-30 C/82-90 F) we begin to lose the ability to compensate due to the onset of progressive bradycardia, hypoventilation and decreased shivering. Once we become severely hypothermic (below 28 C/82 F) all body functions are severely depressed. And if the process continues, all systems will cease to function.
Hypothermia fatalities
From 1999-2002, hypothermia contributed to 4,607 deaths nationwide. Of course, a whole lot more folks have survived their bouts with hypothermia. Although states like Alaska, Montana and Wyoming have a higher incidence of hypothermia, it doesn’t just occur in cold climates. You can become hypothermic even when the outside temperature is above freezing; it just has to be lower than your internal temperature. Certainly, you are more susceptible if you have a faulty hypothalamus, as is common in the elderly; are nutritionally challenged, as frequently seen in the substance abuse population or the chronically ill; have an impaired thought process, such as dementia or a severe mental health disorder; and/or suffer inadequate cardiovascular function from any cause.
Even if you are healthy you may have an increased risk for hypothermia. Higher risk populations include adolescents and young adults (increased risky behavior), winter sports enthusiasts, water sports enthusiasts, and workers or travelers in “cold country” (I am sure I missed someone).
Hypothermia and your patient
How do you treat hypothermia? For the field, stop heat loss and begin re-warming. Keep the ambulance compartment uncomfortably warm, remove wet clothing, place warm packs, cover the trunk with a blanket, infuse warmed saline if available, and watch those ABCs repeatedly. There is a potential for dumping cold peripheral blood into the core from external warming, thus causing further decline in core temperature (after-drop), but this potential problem should not delay external warming. Keep the patient supine to minimize any potential decrease in blood pressure and monitor for dysrhythmias. For the prehospital hypothermic patient in cardiac arrest, refer to the AHA recommendations.
Your care will be continued in the receiving facility, which may offer additional treatment modalities such as warming blankets, peritoneal or pleural lavage, cardiopulmonary bypass and extracorporeal (out-of-body) membrane oxygenation (ECMO).
For the severely hypothermic patient with a pulse, using a forced warm air blanket successfully provides non-invasive re-warming and in at least one case series without the after-drop phenomena. A more invasive method for the severely hypothermic patient with a pulse is a device used to induce therapeutic hypothermia in emergency departments and intensive care units. This “endovascular temperature control system” can be utilized to take the temperature the other direction, successfully warming a severely hypothermic patient. The system consists of a temperature control catheter inserted through the femoral vein and advanced to the inferior vena cava. It is then connected to a bedside console that circulates warm or cool fluid through the catheter allowing this “indwelling radiator” to warm or cool blood flowing past the catheter.
For the patient with severe hypothermia and cardiac arrest, cardiopulmonary bypass and ECMO take over the function of the heart and provide extracorporeal circulation of the blood where it can be re-warmed and oxygenated. ECMO has an advantage, as it does not require general anesthesia to gain vascular access to connect the patient to the system and it can be utilized for a longer period of time (days). Although ECMO may be the preferred treatment in this situation, it has one distinct disadvantage: availability is limited to larger hospitals and medical centers.
Hypothermia prevention for rescuers
Prevention, of course, is easier — keep that in mind. Many of us work in cold country. You can brave the cold, but be prepared. Eat right, get enough rest, carry enough clothing (you can always take it off, but you can’t put it on if you don’t have it with you), stay hydrated, and seek heat when you recognize the early warning signals that you are getting too cold.
References
- Pozos RS, Danzl, DE (2001). Human Physiological Responses to Cold Stress and Hypothermia. In RS Plozos, DE Danzl (Ed.), Medical Aspects of Harsh Environments, Volume 1 (pp. 351-382). Office of The Surgeon General, Department of the Army, United States of America.
- Connolly E, Worthley LI. Induced and Accidental Hypothermia. Critical Care Resuscitation, 2000;3:22-29.
- Hypothermia-Related Deaths — United States, 1999-2002 and 2005. Morbidity and Mortality Weekly Report. March 17, 2006/55(10);282-284. Retrieved December 5, 2007 from http://www.cdc.gov/mmwR/preview/mmwrhtml/mm5510a5.htm
- Kornberger E, Schwarz B, Linkner Kh, Mair P. Forced Air Surface Rewarming in Patients with Severe Accidental Hypothermia. Resuscitation, 1999;41:105-111
- Laniewicz M, Lyn-Kew K, Silbergleit R. Rapid Endovascular Warming for Profound Hypothermia Corrected Proof, 7 August 2007. Annals of Emergency Medicine.
- Ruttmann E, Weissenbacher A, Ulmer H, Muller L, Hofer D, Kilo J, Rabl W, Schwarz B, Laufer G, Antretter H, Mair P. Prolonged Extracorporeal Membrane Oxygenation-Assisted Support Provides Improved Survival in Hypothermic Patients with Cardiocirculatory Arrest. The Journal of Thoracic and Cardiovascular Surgery, 2007;134:594-600.
