• Hypothermia refers to a subnormal core temperature (<37°C) and can be divided into three subcategories:
  • Mild hypothermia: 32–35°C
  • Medium hypothermia: 28–32°C
  • Severe hypothermia: <28°C
• Perioperative hypothermia occurs in three phases:
  • An initial decrease in core temperature of 1–1.5°C in the first hour of anesthesia
  • Slower linear decrease in heat due to heat loss outpacing metabolic heat production
  • After 3–5 hours of anesthesia, a thermal plateau of core temperature

• Body morphology. Insulating properties of fat reduce core-to-peripheral temperature gradient. Risk of hypothermia is inversely proportional to patient body fat percentage.
• Age extremes
  • Infants and young children have a high surface area to mass ratio, allowing intraoperative heat loss to easily exceed metabolic heat production.
  • Geriatric patients have a lower metabolic rate, thus produce less metabolic heat.
• Size of incision
• Severe trauma (including burns)
• Chronic illness decreases metabolic heat production (e.g., cardiovascular disease).

• All general anesthetics cause a loss in core body temperature, mostly from reducing thermoregulatory controls and promoting vasodilation.
• Heat loss can result from:
  • Radiation. Describes the transfer of heat from one surface to another by emission of photons. Heat from the patient radiates into the cool operating room air and accounts for ~90% of heat loss.
  • Convection. Describes the carrying of heat from the core to the periphery.
    • Second largest contributor to heat loss (after radiation)
    • Dependent on peripheral blood flow and core-to-peripheral temperature difference
  • Conduction. Adjacent tissue-to-tissue transfer of heat.
    • Slow
    • Dependent on tissue characteristics (insulation properties of tissue, diffusion coefficient) and differences in surface temperatures.
  • Evaporation. Derived from heat of vaporization of water (0.58 kcal/g)
    • Respiratory evaporative losses are small, usually <10% of basal metabolic rate.
    • Substantial heat loss occurs from evaporation through the surgical incision.
    • Skin-prep (especially alcohol-based) solutions contribute to evaporation heat loss (–0.2 to –0.7°C/m² depending on solution).
  • Cold intravenous fluids. The body warms cold fluids via conduction from blood and tissues. IV fluid at room temperature decreases mean body temperature ~0.25°C per liter.
  • Irrigation. Irrigation fluid can reduce core body temperature by carrying heat way from core organs (via conduction).
• Neurologic effects
  • Cerebral metabolic rate of oxygen (CMRO₂) decreases by 6–7% for every 1° decrease in temperature.
  • Delayed awakening from anesthesia
• Cardiac effects of hypothermia
  • Heart rate and cardiac output gradually decrease
  • Hypotension (particularly in severe hypothermia)
  • Atrial and ventricular arrhythmias increase. Atrial fibrillation can occur secondary to atrial distention. Ventricular fibrillation is an immediate hazard at core temperatures <28°C.
  • Prolonged asystole (most commonly occurs at core temperatures below 32°C)
  • J-waves (Osborn waves), positive deflections at the end of the QRS complex, appear on ECG (typically below 30°C). Clinically insignificant and disappear on rewarming.
  • During cardiopulmonary bypass, hypothermia is utilized to decrease the myocardial metabolic rate (decreases oxygen consumption during periods of decreased oxygen supply). The perfusion pump is utilized to rapidly cool (and rewarm) the patient; it dwarfs all other sources of heat transfer (e.g., radiation, convection). Afterdrop is a term used to describe a rapid decrease in core temperature that can occur after discontinuation of bypass. It is caused by a large core-to-peripheral temperature gradient and from insufficient rewarming of peripheral tissues.
• Pulmonary effects of hypothermia
  • Pulmonary vascular resistance (PVR) increases.
  • Significant respiratory depression can occur with severe hypothermia.
  • Reduced cough reflex and cold-induced bronchorrhea can lead to aspiration and difficulty clearing secretions.
• Hepatic effects of hypothermia
  • Enzyme activity (e.g., conjugation, detoxification) can decrease with a resultant decrease in drug metabolism.
  • Decreased synthesis of coagulation factors (in addition factors are also sensitive to decreases in temperature).
• Hematologic effects of hypothermia
  • Leukocyte and platelet counts commonly decrease secondary to splenic, hepatic, and intravascular sequestration; this effect reverses with rewarming
  • Hematocrit levels can increase secondary to volume contraction that results from osmotic diuresis. Consequently, viscosity increases when temperatures drop below 27°C
• Metabolic effects
  • Basal metabolic rate decreases 5–7% per 1°C; at 28°C it is decreased by 50%.
  • Renal function decreases; a "cold diuresis" occurs secondary to depressed oxidative tubular activity resulting in reduced sodium and water reabsorption.
  • Insulin release as well as glucose uptake into cells are impaired, resulting in hyperglycemia. This effect is reversed with rewarming.
• Therapeutic hypothermia
  • Indicated for neuroprotection in procedures likely to cause cerebral ischemia (e.g., cardiopulmonary bypass, neurosurgery). Outcome studies have demonstrated increased cerebral performance scores and greater survival after discharge, with no significant difference in adverse events between hypo- and normothermic groups.
  • Evidence indicates beneficial effect to inducing mild hypothermia within hours of restoring circulation following cardiac arrest (3)[A].
  • No significant benefit has been shown from the use of mild hypothermia in the treatment of traumatic head injury (4)[A], stroke (5)[A], or in coronary artery bypass surgery (6)[A].

• Preoperative (on ward, before premedication)
  • Keep patient comfortably warm. Actively warm patient if hypothermic.
• Prior to induction
  • Measure oral temperature, ask about thermal comfort level.
  • Prewarm all patients for 20 minutes (forced air) to reduce the core-to-periphery temperature gradient.
  • Limit skin exposure.
  • When possible, start anesthesia only when patient is normothermic.

• 54/3,000 total anesthesia claims secondary to burns.
• 64% of burns were due to IV fluid bags or bottles warmed in an oven prior to applying to patient's skin.
• 0 claims attributed to injuries due to forced-air warming (7).

• Circulatory Arrest
• Cardiopulmonary Bypass

991.6 Hypothermia

• T88.51XA Hypothermia following anesthesia, initial encounter
• T88.51XD Hypothermia following anesthesia, subsequent encounter
• T88.51XS Hypothermia following anesthesia, sequela

N. Matthew Decker , BS