The cardiac arrhythmias that are the primary cause of death in hypothermia management — specifically ventricular fibrillation, in which the heart’s electrical system loses its organized rhythm and the heart stops pumping effectively — are most likely to occur when the temperature drops below approximately 82 degrees Fahrenheit or when warming is done too rapidly.

Jean Hilliard’s heart, apparently, did not fibrillate.

It slowed to near-imperceptible rates, but it maintained an organized rhythm that could be gradually restored to normal.

This is not something that can be guaranteed in any given hypothermia patient.

Some patients do fibrillate during cooling or rewarming, and ventricular fibrillation in a severely hypothermic patient is notoriously difficult to convert to a normal rhythm until the heart has been warmed.

The cold heart simply does not respond to defibrillation in the same way that a normothermic heart does.