When a severely hypothermic patient is warmed, the blood in the peripheral tissues — which has been very cold and has accumulated the metabolic byproducts of reduced circulation, including lactic acid — begins to flow back into the core as the peripheral blood vessels dilate in response to warming.

This cold, acidic blood can cause the core temperature to drop further, paradoxically, even as the warming is proceeding.

It can also contribute to the cardiac arrhythmias that are a major risk in severe hypothermia.

Managing afterdrop requires warming the core preferentially, before the periphery, to the extent possible.

This is why modern hypothermia protocols emphasize active core warming — warming the blood directly, warming the air the patient breathes, warming the fluids introduced into the body — rather than simply applying external heat to the surface.

In 1980, at the Fosston hospital, the tools available for active core warming were limited.