Hypothermia During Anesthesia

The administering of general anesthesia on a daily basis on postoperative patients commonly presents in hypothermia. Hypothermia remains the most frequent complication of surgery under anesthesia.


Normothermia (normal body temperature) is defined as a core body temperature between 36.5C – 37.5C. In homoeothermic terms – a core body temperature below 1C is considered hypothermia. Therefore Hypothermia is defined as a core temperature less than 36C.

  • Mild hypothermia: 1C to 2C below core body temperature
  • Moderate Hypothermia: a core temperature of 35C
  • Severe hypothermia: is a body core temperature below 35C
  • Deep Hypothermia: below 28C. Consciousness is lost, sinoatrial pacing becomes erratic, ventricular irritability increases, and below 26C rigidity and myoclonus is imminent.

How does heat loss occur?

Heat loss in a patient occurs from 4 primary sources: radiation, evaporation, conduction, and convection.


The heat generated from within the body is given-off to the surrounding atmosphere. This accounts for 60% of heat loss in humans and a predominant source of heat loss in most surgical patients.


Evaporation occurs when heat is lost through the conversion of water to gas, i.e. when the body perspires. Evaporation accounts for 20% of heat loss during anaesthesia. However –patients undergoing major open wound surgery can account for significant heat loss.


Conduction is the process of heat loss through physical contact with another object and accounts for 5% of heat loss. For instance – warm body heat transferred to a cold surface, such as contact with a cold mattress.


Body heat can be lost through convection, for instance air ventilation from sat in front of a fan. Air flow from laminar flow ventilation systems in operating theatres, (primarily used to decrease in air contamination), can account for 10% to 15% of body heat loss.

Heat loss under general anaesthetic?

The first 30 minutes under anesthesia is the most crucial!.. where body temperature declines the most dramatically. Without precautionary measures patients will inevitably develop hypothermia to some degree. After this rapid decline within the first half an hour the body temperature can slowly drop further up to 2 hours after the anaesthetic was administered. The primary factor for hypothermia results from internal redistribution of the body core heat to the extremities and peripheral tissues.-, and considering that the body core temperature is isolated in 50% of the body’s mass, a decline in core body temperature can be understood.

Detrimental affects of hypothermia The three most prevalent complications associated with mild hypothermia are a 300% increase in:

  • Morbid myocardial events. i.e. death occurs when blood flow to the heart muscle is decreased, ‘myocardial ischemia’.
  • Surgical wounds infections. i.e. most likely caused by impairment of the immune system
  • Prolonged hospitalization. As a result of a multitude of negative responses to hypothermia.

The cost of patient warming

It seems that it would prove far more cost-effective in preventing hypothermia by the use of patient warming systems as studies concluded that a core body temperature around 1.5C below the norm caused adverse affects. This ironically added £1600 to £5000 per surgical patient to hospitalization costs!

The operating theatre room temperature is the predominant factor in decreasing core body heat loss. The room temperature should be above 24C pre op to prevent the occurrence of hypothermia. Once patient warming systems have been applied then the operating theatre room temperature can be adjusted to a desirable temperature for theatre staff.


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