Heat loss: Mechanisms

Hypothermia can be defined as core body temperature less than 360C. Maintaining normal core temperature is important for the function of enzyme systems.

Hypothermia can lead to:

1. Coagulopathy, as the clotting cascade is temperature dependent

2. Prolongation of anesthesia recovery, largely as a result of altered drug metabolism

3. Increased oxygen consumption through postoperative shivering

4. Arrhythmias and myocardial ischemia

5. Delayed wound healing and increased rate of wound infection

Phases of heat loss under anesthesia:

1. Redistribution: Typically in the first hour after induction of anesthesia, where vasodilatation causes redistribution of heat from core to periphery. Typically patients’ loss 0.5-1.5 °C. Redistribution results because anesthetics inhibit the tonic vasoconstriction that normally maintains a large core-to-peripheral temperature gradient. Redistribution during neuraxial anesthesia is typically restricted to the legs.

2. Linear phase: Core temperature then decreases linearly at a rate determined by the difference between heat loss and production. Typically, this occurs over the next 2-3 hours.

3. Plateau phase: When surgical patients become sufficiently hypothermic, they eventually trigger thermoregulatory vasoconstriction, which restricts core-to-peripheral flow of heat. This mechanism might be impaired in diabetics with autonomic neuropathy and during combined general and regional anesthesia.

Mechanisms of heat loss (during the linear second phase), in order of importance:

1. Radiation: Contributes to most of heat loss – up to 60%. It is proportional to the difference of the fourth powers of room wall (i.e., ambient) and skin temperature.

2. Convection: Due to loss of heat to air immediately surrounding the body. It contributes up to 30% of heat loss. It is proportional to the difference between skin and air temperatures multiplied by the square root of air speed.

3. Evaporation: Occurs from cleaning fluids, skin, respiratory, bowel and wound surfaces.

4. Conduction: Due to heating surfaces in contact with the body such as OR table or cold fluids. Minimal contribution.