Evaluation and Treatment of Hypothermia, Colin K. Grissom, M.D.
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Physiology of Hypothermia
Initially hypothermia causes shivering thermogenesis and an increase in metabolic
rate (oxygen consumption or VO
2
), ventilation, heart rate, cardiac output, peripheral
vascular resistance and mean arterial pressure. These responses are predominately
triggered by skin cooling.
Activity
and
metabolic
rate largely determine the heat produced by the body.
Shivering is an important mechanism that increases muscle activity and heat production
by several times. Increased heat production from shivering can overcome heat loss and
warm the body's core temperature in a mildly hypothermic patient as long as warm, dry,
insulation is provided. In critically ill patients, however, even mild hypothermia may
require active rewarming because shivering thermogenesis is impaired due to illness or
drugs.
At lower core body temperatures, less than about 30 to 32°C (86 to 90°F),
shivering thermogenesis ceases and VO
2
, heart rate, cardiac output, mean arterial
pressure and ventilation progressively decrease. This is associated with a progressive
deterioration in the level of consciousness initially causing confusion and stupor as core
temperature approaches 30
°C. Obtundation and then coma occur as core temperature
decreases below 30
°C. Poikilothermia, the point at which body temperature will
equilibrate with the surrounding environment, occurs at less than 30°C. Patients who
have reached this state require active rewarming methods.
The
term
core temperature afterdrop refers to a further decline in an individual's
core temperature after removal from the cold exposure. The two processes that contribute
to afterdrop are simple temperature equilibration across a gradient from the core to the
body surface, and increased circulation of the blood from the core to the cooler periphery,
returning cooler blood back to the core. Increased perfusion of cold tissues is the reason
why hypothermic patients who ambulate or exercise after removal from a cold exposure
have a greater core temperature afterdrop than if they remain supine and are rewarmed
(Giesbrecht 1987).
Clinical Presentation: Mild Hypothermia
In early exposure to cold the body attempts to preserve a normal temperature by
increasing heat production and by vasoconstricting blood vessels in the skin to reduce
radiant and convective heat loss. In mild hypothermia the patient is awake and responsive
but may have an altered mental status. During mild hypothermia heat production is
increased by shivering. The increased sympathetic tone occurring during mild
hypothermia results in tachycardia, increased cardiac output, and increased respiratory
rate. At the same time the heat conserving mechanism of skin vasoconstriction results in
slight elevation of the blood pressure, delayed capillary refill, and mottled skin.
Clinical Presentation: Moderate Hypothermia
Moderate hypothermia is characterized by cessation of shivering and progressive
slowing of metabolism. The heart rate falls due to declining atrial and ventricular
pacemaker activity. At temperatures below 30°C stroke volume decreases. This in
combination with bradycardia decreases cardiac output. Vasoconstriction relaxes and
hypotension may occur. Atrial flutter and fibrillation are common. Although respiratory
