The Impact of Temperature Regulation on Diving Safety and Hypothermia - Diving Medicine

Dr. Lin Zhengyan reply Diving Medicine

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Hello, Chinmao: Hypothermia is caused by the body being exposed to an environment where the heat lost exceeds the heat produced.
If this condition persists, it can lead to a decrease in the core body temperature of the individual, ultimately resulting in hypothermia.
Hypothermia is defined as a core temperature below 35°C.
The lethal hypothermia threshold for humans is a rectal temperature of 23°C to 25°C.
The severity of hypothermia depends on (1) environmental factors and (2) physiological conditions.
Environmental factors include (1) water temperature; (2) duration of immersion; (3) insulating materials (such as wetsuits, fat, and oils); and (4) the composition of mixed gases.
Physiological conditions include (1) physical fitness; (2) activity level while exposed to water; (3) sensitivity to cold; and (4) the use of drugs or alcohol that cause vasodilation.
For individuals sensitive to cold, sudden immersion in cold water can lead to water inhalation and stimulation of the vagus nerve receptors in the throat, resulting in reflex bradycardia, decreased cardiac output, and peripheral vasoconstriction, which in severe cases can lead to cardiac arrest and death.
Exposure to cold water can cause increased respiration leading to respiratory alkalosis; subsequently, when the temperature drops below 35°C, respiration slows down, causing carbon dioxide accumulation and respiratory acidosis.
Exposure to cold environments can trigger reflexive increased respiration, but when the body temperature becomes too low, it can suppress the respiratory center, leading to respiratory acidosis and hypoxia.
When the rectal temperature is below 35°C, individuals may experience difficulty speaking, cognitive impairment, slowed movements, and altered consciousness, with dizziness and delirium also possible.
At a rectal temperature below 33°C, only half of the individuals remain conscious, while temperatures between 29°C and 31°C can lead to coma.
At temperatures below 30°C, blood pressure becomes difficult to measure.
Hypothermia can reduce intestinal motility and slow bacterial destruction.
The impact of hypothermia on motor function can lead to poor coordination and muscle weakness, increasing the difficulty of physical tasks and fatigue.
Skin freezing occurs at temperatures below -0.53°C, while seawater freezes at -1.9°C.
Skin freezing can occur upon immersion at these temperatures.
Immersed skin can become soft and swollen, a phenomenon known as "washerwoman's skin," commonly seen in the fingers and toes.
Individuals sensitive to cold may experience histamine release leading to urticaria.
To prevent hypothermia: (1) wear adequate clothing and take measures to shield against the cold; (2) prevent heat loss from the core due to vomiting; (3) limit exposure time to cold environments; (4) minimize the exposed surface area of the body; adopting a fetal position in water can increase survival rates by 50%; (5) wear cold-resistant, insulated wetsuits; (6) reduce movement in water to prevent heat loss; (7) individuals who fall into the water should huddle together unless very close to shore, avoiding excessive swimming.
The principles for on-site management of hypothermia are: (1) remove the individual from the environment causing hypothermia; (2) minimize movement of the patient to avoid inducing ventricular fibrillation; (3) avoid exposure to windy environments; (4) cover the patient with clothing or blankets to prevent continued heat loss; (5) transport the patient in a head-down, feet-up position to prevent complications from low blood pressure; (6) if the patient experiences cardiac arrest, perform cardiopulmonary resuscitation.

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Reply Date: 2002/03/27