Monday 23 March 2009
Prolonged exposure to low ambient temperature leads to hypothermia, a condition seen all too frequently in homeless persons. Lowering of body temperature is hastened by high humidity in cold, wet clothing and dilation of superficial blood vessels as a result of the ingestion of alcohol. At about 90°F, loss of consciousness occurs, followed by bradycardia and atrial fibrillation at lower core temperatures.
Chilling or freezing of cells and tissues causes injury in two ways:
Direct effects are probably mediated by physical disruption of organelles within cells and high salt concentrations incident to the crystallization of the intracellular and extracellular water.
Indirect effects are exerted by circulatory changes. Depending on the rate at which the temperature drops and the duration of the drop, slowly developing chilling may induce vasoconstriction and increased permeability, leading to edematous changes. Such changes are typical of "trench foot."
Atrophy and fibrosis may follow. Alternatively, with sudden sharp drops in temperature that are persistent, the vasoconstriction and increased viscosity of the blood in the local area may cause ischemic injury and degenerative changes in peripheral nerves.
In this situation, only after the temperature begins to return toward normal do the vascular injury and increased permeability with exudation become evident. However, during the period of ischemia, hypoxic changes and infarction of the affected tissues may develop (e.g., gangrene of toes or feet).