>Heat energy is transferred in four ways: conduction, convection, evaporation and radiation.
>Understanding Heat Transfer
- In conductive heat transfer, heat energy moves from a warmer object to a cooler object or medium in direct contact with it. A warm body on a cold ground or boat deck is an example of conductive heat transfer in action.
- Convective heat loss also involves direct contact, but instead of contact between two solid objects, convection takes place between a solid object and a fluid. For the purposes of diving and backcountry travel, the relevant fluid medium is either water or air. As the body heats the water or air around it, this warm layer moves away to be replaced with cooler water or air that the body must also warm. This process is ongoing and part of the reason a person in water loses heat approximately 25 times faster than someone on land. It's also the reason a wind-breaking layer of clothing can do a great deal to preserve heat on a windy day.
- Evaporative cooling occurs whenever moisture is present on the skin, as long as the humidity is less than 100 percent. The physical reaction of a liquid turning into a gas is endothermic, which means it requires an input of heat energy. As liquid on the skin evaporates, the requisite heat is taken from the body.
- Radiative heat transfer is the process by which a warm body emanates heat energy into its environment.
>Consider the following scenario: After an early morning dive, you notice a young girl whose lips look slightly blue. She is huddled in a towel and still wearing her wetsuit. A breeze has picked up, and her hair is wet. You ask her if she's warm enough, and she just shrugs her shoulders. You mention that she might be warmer if she changes out of her wetsuit. "Yeah, I tried," she mumbles, "but I couldn't get the zipper."
>Thermal Challenge: Hypothermia
>Mild hypothermia is marked by shivering and a group of symptoms that can collectively be referred to as "the umbles." These include fumbling, mumbling and grumbling, which characterize a combination of lethargy, apathy and a decline in motor function. Fine motor skills are the first to go, and tasks like zipping up a jacket or tying boots become difficult. The patient may neglect to take steps to minimize further heat loss, such as removing a wet wetsuit. Hypothermia is particularly problematic in remote environments because of the way it can subtly steal one's ability to think clearly and make good decisions.
>To successfully prevent or treat hypothermia, attention should be given to each of the four mechanisms of heat transfer:
- Conductive heat loss can be minimized by insulating the patient from direct contact with anything colder than he is. Place a dry wetsuit, boat cushion or foam sleeping pad between the patient and the ground (or the boat deck) to reduce heat loss into it. Layers of clothing or blankets are useful for insulating a person against cold air. Sleeping bags and down jackets are excellent examples of this approach to insulation; they are essential equipment in cold environments because they have significant loft for their weight and create substantial space for trapping heat around the body.
- Convective heat loss can be minimized by creating a stable microclimate around the body. Clothing made of nylon, other synthetic materials or tightly knit fabrics limit air flow and effectively reduce convective heat loss. By keeping the same air near the body, the patient doesn't have to keep reheating the air around him. This is similar to the way a wetsuit reduces convective heat loss in water.
- Evaporative heat loss is often subtle. Even a small amount of moisture on the skin can make it very hard for a cold person to warm up. Replace damp underlayers, and dry the skin thoroughly before adding more layers.
- Some fabrics are specially designed to minimize radiative heat loss, including those used in the "space blankets" often found in first aid kits. A reflective blanket on its own, though, is inadequate for warmth in a cold environment since it offers little protection against conductive or convective heat loss.
>If cold stress becomes hypothermia and is not addressed in the early stages, the patient will begin to experience uncontrollable shivering and a decline in gross motor skills. These are signs that moderate hypothermia has begun. The patient's coordination will be impaired, ultimately manifesting as stumbling or an inability to walk. At this point, the need for intervention is critical. It may still be possible to rewarm this patient in the field; efforts should begin in earnest.
>A patient with severe hypothermia will have lost the ability to shiver. Many basic body functions will slow down due to diminished core temperature. This patient may be unresponsive, and the heart rate may not be palpable. Respirations may be slow and faint. The objective of care should be to prevent further heat loss. Wrap the patient in as many insulating layers as are available, and use a tarp or tent fly to ensure that the patient and the insulating layers remain dry. Initiate an evacuation immediately, but take care to handle the patient gently. Violent movement could send the patient's heart into a potentially fatal rhythm. This combined with the possible difficulty identifying a heartbeat means that great care must be taken when assessing signs of life in severely hypothermic people. Chest compressions are appropriate only when signs of life are truly absent. Rescue breaths can be given freely, if needed.
>Now consider another scenario: You're enjoying a wonderful day on a dive boat, although it is full. As you wait to make your way to the rear of the vessel, you strike up a conversation with another diver in the back of the line. For nearly 15 minutes the two of you stand in the sun in head-to-toe black neoprene. By the time you make it to the stern, the man you've been talking to says his stomach doesn't feel well. You notice that he is sweating, his face is pale, and when he goes to put on his fins, he almost collapses.
>Thermal Challenge: Heat Illness (Hyperthermia)
>Heat exhaustion is the result of a hot environment combined with insufficient hydration. A heat-exhausted person may be insufficiently hydrated due to sweating, breathing dry air, inadequate fluid intake, vomiting, diarrhea and, perhaps most often, a combination of several of these factors. Common symptoms include headache, nausea, dizziness, vomiting, profuse sweating, pale or flushed skin and disorientation.
>The primary problem for the heat-exhausted individual is fluid volume depletion, so rehydration is the cornerstone of treatment. In addition to encouraging the patient to hydrate, find a place for him to rest in the shade.
>As in treating hypothermia, consideration of the mechanisms of heat transfer can be helpful when treating heat illnesses (hyperthermia):
- Conductive heat loss can be encouraged by placing an ice pack or cold compress on a hot person. Clothing should be removed to reduce insulation. Make sure patients are moved off of asphalt or other hot surfaces; if you can find a cool surface on which they can rest, use it.
- Convective heat loss can be promoted by fanning the patient.
- Evaporative heat loss is best achieved by keeping the patient's skin moist.
- Radiative heat gain can be minimized by moving the patient out of direct sunlight. Radiative heat loss can be promoted by moving a patient to a cool environment if one is available.
>Since the brain is particularly sensitive to temperature changes, symptoms of heat stroke are not unlike those associated with a head injury and may include hallucinations, combative behavior, seizures and decreased mental status. The patient with these symptoms must be cooled aggressively. In the hospital, he may be immersed in a bath of ice water; but in the field, immersing a patient who cannot control his own airway is not recommended. Instead, strip the patient, soak him with water, and fan him to maximize the effects of evaporative and convective cooling. In the event that ice or cold packs are available, place these at the patient's neck or armpits. Direct the open valve of a scuba cylinder at the patient to increase convective cooling, if there is one nearby. Even if the patient seems to improve significantly, he must be monitored closely until he can be evaluated by a physician.
>Preparing for temperature extremes is important when traveling in remote environments. Proper clothing, adequate food and hydration, and a basic understanding of the mechanisms of heat transfer are essential for both personal safety and the ability to care for others in the field.
>© Alert Diver — Summer 2010