[Hyperthermia and hypothermia. Effects on the cardiovascular system].

Heat and cold are environmental factors which severely affect the cardiovascular system. An increase in the body core temperature (hyperthermia) from approximately 36.5 to 39 degrees C causes a doubling of the cardiac output. In connection with vasoconstriction in the splanchnic circulation and in skeletal muscle this results in large increases of skin blood flow. The underlying vasodilatation is evoked by reflex regulation of the efferent sympathetic system. While there is a reduction of alpha-adrenergic vasoconstriction, there is also evidence for active sympathetic cholinergic and nitric oxide-dependent vasodilatation. In the presence of risk factors, e.g. age and diabetes, the circulatory adaptation to heat stress may be compromised. During a reduction of the core temperature (hypothermia) there is a reflex adrenergic vasoconstriction (noradrenalin) of the skin. Cardiac output falls below a core temperature of 34 degrees C due to increasing bradycardia. The reflex vasoconstriction following cold exposure may be aggravated at higher ages, which may cause steeper increases of arterial blood pressure. Due to the reflex nature, the regulatory processes are severely compromised during anaesthesia.