In the early phase of hypothermia, heart rate, cardiac output, central venous pressure, blood pressure, and systemic vascular resistance increase. This reflects an increased metabolic demand associated with shivering and enhanced sympathetic nervous activity and catecholamine release, elicited by cold skin, fear, and fight against the cold. As the body temperature decreases further, the heart rate slows and atrial fibrillation and ventricular ectopy occurs. Cardiac output decreases progressively, approaching 50 per cent at 28 °C. This is due to a fall in heart rate and stroke volume as a result of decreased enzymatic and cell function. Blood pressure is maintained by catecholamine release and sympathetic nervous activity, increased blood viscosity, and the direct effect of cold on the vessel walls. Below 28 °C, blood pressure falls and shock progressively develops. Systole increases, resulting in a flat pulse wave which is sometimes discordant with the almost normal blood pressure values measured invasively.
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