Pediatric Temperature Variation in the MRI Scanner Under General Anesthesia

It is known that following the induction of anesthesia, core hypothermia occurs in three stages;

1. Redistribution
2. Linear phase
3. Plateau phase Redistribution This accounts for the largest drop in core temperature of the three stages. Vasodilatation causes redistribution of heat from the core to the periphery.

Linear Phase This begins at the start of surgery as the patient is exposed to cold cleaning fluids and cool air flow in the theatre. Heat loss exceeds heat production, and most surgery does not extend past the linear phase.

• Radiation contributes the most (40%) and is proportional to environment/core, temperature difference to the power of four.
• Convection contributes up to 30% and is proportional to air velocity.
• Conduction contributes up to 5% and is proportional to the difference in surface temperatures.
• Evaporation contributes up to 15% and occurs from cleaning fluids, skin, respiratory and wound. A laparotomy can contribute up to 50% of the total.
• Respiratory contributes 10% (8% evaporation of water; 2% heating of air) and is enhanced by the cooling effect of cold anesthetic gases.
• Cold intravenous fluids. Plateau Phase Once core temperature falls below the thermoregulatory threshold, peripheral vasoconstriction increases and acts to limit the heat loss from the core department. When core heat production = heat loss to the peripheral compartment, core temperature reaches a plateau. Patients with an autonomic neuropathy (diabetics) have impaired sympathetic vasoconstriction and are unable to establish a core plateau in phase 3. Combined general and regional anesthesia will have a similar effect as the regional anesthetic (spinal/epidural) will prevent vasoconstriction in the legs; i.e. failure to establish a core plateau.

The ability to maintain body temperature is also compromised because anesthesia impairs intrinsic thermoregulatory response.