Cardiovascular System
The cardiovascular system has received the most attention in clinical studies of hypothermia because various surgical techniques, such as cardiac bypass surgery, have successfully employed low body temperatures. The reversibility of cold-induced ventricular fibrillation (cardiac arrest, or standstill) is one of the major determinants of survivability from hypothermia. A drop in core temperature will induce ionic alterations in cardiac muscle, such as hyperkalemia, which may induce cardiac standstill or fibrillation.
Cold stress induces sympathetically mediated peripheral vasoconstriction, an increase in cardiac afterload on the heart, and elevated myocardial oxygen consumption. These changes are often associated with an initial tachycardia. As the core temperature continues to fall, bradycardia and myocardial depression occur, resulting in a decreased cardiac output and hypotension. In mild hypothermia, the variability in circadian heart rate is greater than normothermia, possibly due to an imbalance between the parasympathetic and sympathetic nervous systems A in heart rate by 50% can be recorded from individuals with core temperatures near