By Dr. Susan Northrup, FAA Federal Air Surgeon

Since the topic of this issue is weather, a refresher on hypothermia (body temperature too low) and hyperthermia (body temperature too high) seems appropriate. Although the relative risk of one versus the other certainly varies with the season, both are potential year-round concerns.

First, a review of the basics. In simplest terms, body temperature reflects the balance between heat produced from metabolic processes and heat lost to the environment. We lose heat through evaporation (sweat and respiration), radiation, conduction, and convection. The latter three can also result in a net gain depending on the environment. The brain’s hypothalamus regulates our body temperature. We shiver and constrict peripheral blood vessels (vasoconstriction) when cold. When it is too warm, we sweat and dilate blood vessels (vasodilation), which can cause us to feel faint or actually faint (syncope). Illness, medications (including many that the FAA authorizes for flight duties), and illicit drugs can interfere with thermoregulation, predisposing to either hypothermia or hyperthermia. As the subject of taking precautions with medication for hypothermia and hyperthermia is not typically part of a routine pilot-physician discussion, be sure to ask your doctor about any medications that you might be on and pay attention to your own response to them.

Even mild hypothermia can compromise safety of flight. Initially, the only manifestation is shivering without impact on cognition or functioning. As the core body temperature drops, you may be impaired even if you still feel alert. The key here is to address the problem early, even making a precautionary landing if the problem persists. While most general aviation aircraft have cabin heat available, its effectiveness varies, and the risk of carbon monoxide (CO) inhalation, while low, is always present.

Let’s highlight a couple of precautions for using a pulse oximeter (O2 monitor). When you are cold, vasoconstriction is common. As a result, blood flow to the extremities is reduced, and an O2 monitor can show relative hypoxia even though there is adequate oxygenation of central tissues (like the brain). The treatment is to warm up; of course, adding supplemental oxygen won’t hurt, but it doesn’t address the fundamental problem. In addition, the risk of CO poisoning increases with the use of cabin heat. Not only poisonous, CO also imparts a red tint to the skin that can “fool” a pulse oximeter into a normal reading even when you are hypoxic. If you see readings that look too good (such as 100% at 10,000 feet MSL), be skeptical.

The other side of the coin is hyperthermia. It is not uncommon to see exertional heat illness in athletes, military personnel, and others who engage in strenuous activities. On a hot day, moving an aircraft with a towbar, multiple trips to the FBO (or many flights for instructors), a lack of physical fitness, and/or being overweight can all contribute to hyperthermia and a heat-related illness. Many GA aircraft cabins readily heat up in the summer sun, and most lack air conditioning; as a result, heat exposure can continue until an aircraft reaches altitude. Furthermore, the lack of convenient bathroom access often prompts many pilots to limit their fluid intake, resulting in dehydration. This leads to irritation and poor judgment and increases susceptibility to heat-related illnesses. As noted earlier, illnesses (even with mild symptoms) and various medications can reduce heat tolerance.

So, how is hyperthermia most likely to present itself? Symptoms include disorientation, irritation, headaches, and difficulty concentrating. If these symptoms are present, step back to cool down and rehydrate. Fly during a cooler time of day if possible. I also encourage avoiding multiple flights without a break. One of the challenges of hyperthermia is that even mild cognitive impairment can hinder a person's ability to recognize their own impairment. Prevention and early intervention are key to a safe flight.

Finally, while most flights are local and relatively short, in many parts of the country, relatively hostile terrain can be close. Carry at least some survival equipment appropriate for the season. Don’t survive the crash just to succumb to exposure!

Dr. Susan Northrup received a bachelor's degree in chemistry, a medical degree from The Ohio State University, and a master’s degree in public health from the University of Texas. She is double board-certified by the American Board of Preventive Medicine in Aerospace Medicine and Occupational Medicine. She is a retired U.S. Air Force colonel and a former regional medical director for Delta Air Lines. She is also an active private pilot and aircraft
owner.

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