With the advent of summer, typically comes heat. Since wilderness adventures take place during the summer and heat stroke and hyponatremia can be fatal, heat exhaustion and dehydration debilitating, and heat rash and photosensitive reactions uncomfortable, prevention is the key to playing happily outside in the summer heat and sun.
Most serious heat illnesses occur when the heat challenge overwhelms the human body's ability to cool itself. While it can happen at any time, we are especially susceptible to life-threatening heat illnesses when the temperature changes abruptly. The heat challenge is a combination of metabolic and environmental conditions; it increases as temperature increases, wind decreases, humidity increases, and exercise levels increase. Fever, shivering, tremors, convulsions, hyperthyroidism, sepsis, stimulants, and other drugs and conditions can increase heat production, increase body temperature, and predispose people to heat illnesses.
The human body responds to an increasing heat challenge through systemic vasodilation to increase radiant, conductive, and convective cooling and by sweating to increase evaporative cooling. Your ability to effectively dump heat is variable and depends upon your health and your level of acclimatization. In fit, healthy individuals, initial heat acclimatization occurs within 3-5 days, with greater than 80% occurring within 2-3 weeks, and 100% weeks to months later. Heat acclimatization is slower in persons with poor health (heart disease, diabetes, disease, etc.).
Your rate of acclimatization (and that of your friends and family) should have a profound affect on how you plan your summer trips and how you go about your day. If you routinely exercise and the environment during your wilderness trip is similar to your home environment, you will have little or no heat related problems. If, as is often the case during summer trips, the trip environment is significantly warmer than your home environment, you are unlikely to reach complete acclimatization (or even 80% acclimatization) during a trip lasting less than a few weeks. Armed with this information, it's important to take it easy during the first week in a hot environment; critical if you have pre-existing health issues. Due to the varying rates of acclimatization between people, the differences in overall health and fitness, and the length of time spent in the hot environment, wilderness guides typically reach full acclimatization by mid-season while the majority of their clients have difficult reaching 80% acclimatization on any given trip. If you are a guide, this means you need to take it easy on your clients or risk dealing with their heat related illnesses (and poor tips).
As the body acclimates to the heat, major changes occur in the thermoregulatory centers of the brain, within peripheral vessels, and within the heart. Sweating, and therefore evaporative cooling, is increased and begun at a lower core temperature while electrolyte loss from both sweat and urine is minimized. Metabolic efficiency is significantly increased; essentially more usable energy is produced with less heat. Peripheral blood flow increases and starts earlier, increasing radiant, conductive, and convective cooling. The redistribution of blood flow to the peripheral vessels combined with a high loss of fluids and electrolytes places a tremendous burden on the heart; this means wilderness travelers with preexisting cardiac problems are predisposed to arrest. Once acclimated, the combination of increased stroke volume and decreased pulse effectively reduces the workload on the heart and the chance of arrest. Again, go easy.
Maintaining your hydration and electrolyte balances are critical to your ability to dump excess heat. It's vitally important to increase your sodium intake during the acclimatization period to avoid an potentially life-threatening imbalance (hyponatremia). Urine, not the amount of fluid intake, is the primary evaluative tool when assessing water balance; it should be clear or pale yellow. That said, if your urine is clear, you are peeing like a race horse, drinking lots of water, not eating, have some muscle cramps, and feel sick, you are suffering from hyponatremia. Limit your water intake, eat a few salty pretzels (or anything else with salt and carbs; no fat or protein), and you should start to feel better in 4-6 hours. Mental status is the primary evaluative tool when assessing severe heat injuries. Any change from a normal mental status (alert and cooperative to alert and irritable, alert and anxious, dizziness, lethargic, etc.) is a significant finding. If vasodilation and sweating do not serve to maintain a normal mental status, heat injury has occurred; and, if left untreated, death will follow rapidly.
At 108º F proteins—including the enzymes responsible for all chemical reactions within the human body—break down. This rapidly leads to irreversible multiple organ failure and death and is known as heat stroke. Water stores heat; our bodies are roughly 65% water. Death secondary to heat stroke tends to occur during the second day of exposure in unacclimated people as stored heat from the previous day carries over into to present; this is especially true if nighttime temperatures are also high. Exertional heat stroke occurs when the heat challenge is too BIG and overwhelms your body's cooling ability. Classical heat stroke occurs when dehydration leads to a loss of cooling mechanisms. With a 3% loss of water during an exercise-related heat challenge, vasodilatation decreases more than 50%. With a 7% loss, the rate of sweating decreases by 25% and the onset is delayed. Most deaths due to heat stroke are combination of the classical and exertional heat stroke. Heat exhaustion is often a precursor to heat stroke and occurs when a person becomes dehydrated in a heat challenge and their core temperature rises but remains below 105º F.
Most, if not all, the serious heat related illnesses are preventable. Following a few simple guidelines can make your summer trip pleasurable rather than miserable, or even epic:
- DO NOT PUSH YOURSELF during a new heat challenge.
- While you are acclimatizing to a new heat challenge, plan to exercise (hike, climb, bike, etc.) during early morning and evening when it's cooler.
- Seek shade and rest during the hottest part of the day; preferably near water.
- Stay hydrated and increase your daily electrolyte (especially sodium) intake during the first week in a new heat challenge.
- Have an effective cooling system in place if you or a member of your party become uncomfortably hot at any time.
- Plan alternate "bail out" routes and execute one before the heat challenge exceeds your ability to deal with it.
- AVOID sunburn. Learn how sunburn leads to dehydration and, in turn, predisposes people to heat problems in a previous blog article.
Learn about heat and sun-related rashes.
Interested in learning more about caring for your self and other in the outdoors? Take one of our wilderness medicine courses. Guides and trip-leaders should consider attending our Wilderness First Responder course.