Historical facts do not always suggest hypothermia. When exposure to cold is obvious by history, the diagnosis is simple. Measurement of core body temperature is important in diagnosis of less overt presentations. Core temperature is best measured by using a cold-recording rectal probe thermometer capable of measuring temperatures as low as 25°C (77°F). Clinical manifestations differ with the severity of hypothermia, which is classified based on core temperature as mild (35°C to 32.2°C, 95°F
to 90°F), moderate (<32.2°C to 28°C, <90°F to 82°F), and severe (<28°C, <82°F).2,3,5,7,12,13 This classification has implications for management because appropriate treatment depends on severity of the disorder, as described below. The onset of hypothermia is often insidious. Initial symptoms may be vague and include hunger, nausea, dizziness, chills, pruritus, or dyspnea. Extremity stiffness, weakness, and shivering may also be prominent. As core body temperature decreases, many patients no longer complain of cold, shivering disappears at temperatures below 32°C (<90°F), and muscles become rigid.5,14 At this point, the level of consciousness becomes markedly altered and systemic manifestations are readily evident. A severely hypothermic victim has a markedly decreased metabolic rate. As a consequence, the cerebral ischemic tolerance during cardiocirculatory arrest is considerably longer in contrast with the normothermic state.5,15,16 Therefore, one has to be very careful in assessing brain death while a patient remains hypothermic. Low temperatures cause the myocardium to become irritable and cardiovascular abnormalities are common.15 These may include initial tachycardia followed by progressive bradycardia with an increase in systemic vascular resistance. Arrhythmias are common at core temperatures below 32°C (90°F), and ventricular fibrillation may occur spontaneously when the temperature is below 28°C (82°F).14 Systemic blood pressure is often decreased in patients with severe hypothermia. Other clinical manifestations of hypothermia are listed in Table 110-2.