Operations are termed “open heart” when the functions of the heart are assumed by an extracorporeal pump and gas exchange unit (cardiopulmonary bypass, CPB). This can result in significant thermal changes through the active cooling of blood, which lowers the core temperature rapidly. Peripheral vasoconstriction due to core hypothermia reduces peripheral blood flow so that core temperature on bypass falls quicker than peripheral temperature. This is the only situation in which the normal core-peripheral gradient is reversed. Conversely on rewarming core temperature rises rapidly but rewarming of the peripheries is slower.
On separation from bypass, core temperature falls rapidly by 1-2°C. This is because the periphery fails to rewarm adequately on bypass and is relatively cool on separation from bypass. Post bypass body heat is transferred from the warmer core to the cold peripheries, resulting in a fall in core temperature. “Afterdrop”.
Strategies to minimise afterdrop
- Minimise the amount of cooling on bypass.
- Prolonged rewarming on bypass until the peripheral temperature is above 35°C.
- Pharmacological vasodilatation with sodium nitroprusside (SNP) during rewarming improves peripheral blood flow and increases the rate of peripheral rewarming. Patients who are chronically vasodilated with ACE inhibitors also rewarm faster than controls.