Active rewarming of a patient with moderate or severe hypothermia should be avoided in the field due to what primary concern?

The primary concern with actively rewarming a patient with moderate or severe hypothermia in the field is that rewarming too quickly can lead to a fatal cardiac dysrhythmia. This occurs because the body's normal thermoregulatory responses are disrupted in hypothermia. Rapid warming can cause a sudden release of cold blood from the extremities into the core circulation, which can affect cardiac function and lead to arrhythmias.

In moderate to severe hypothermia, the heart is already vulnerable, and sudden temperature changes can increase the likelihood of dysrhythmias, including ventricular fibrillation, which can be life-threatening. Therefore, a careful and gradual rewarming process is recommended to avoid these potentially fatal complications.