Hypothermia

Excerpt

Hypothermia is defined as a drop in body temperature below 35 °C. The condition is common in cold geographic areas and during cooler months, though it can also develop in locations with milder climates. Hypothermia affects all organ systems. Presenting symptoms depend on the severity of the condition.

The hypothalamus regulates body temperature through autonomic mechanisms. This region of the brain receives input from central and peripheral thermal receptors. Muscle tone and basal metabolic rate (BMR) increase initially in response to cold stress. Heat production can double through these mechanisms. Shivering also enhances heat production, increasing metabolism 2 to 5 times the baseline BMR.

Newborns lack the shivering mechanism due to incomplete development of the nervous system. However, brown fat helps generate heat in newborns' bodies. Brown fat's thermogenin increases mitochondrial membrane permeability and disrupts the electron transport chain (ETC) enzymes. ETC disruption and subsequent hydrogen ion leakage block ATP production and generate body heat.

Thyroid, catecholamine, and adrenal hormones also increase in response to cold stress. Cold-induced, sympathetically mediated peripheral vasoconstriction reduces heat loss. Behavioral changes like adding more clothing, seeking shelter, starting a fire, and exercising help retain or produce body heat.

Patients with mild hypothermia have a core body temperature ranging from 32 to 35 °C (90-95 °F). The core temperature for moderate hypothermia is 28 to 32 °C (82-90°F). The core body temperature is less than 28 °C (82 °F) for severe or profound hypothermia. Durrer et al use a hypothermia staging scheme for rescue work (see under "Staging" below) to determine which patients can benefit from resuscitation. Worsening degrees of hypothermia result in great morbidity and mortality.