Hypothermia (core temperature <35°C/95°F) slows all enzymatic reactions and cellular metabolic processes. Classification: Mild (32-35°C) - shivering thermogenesis active, tachycardia, vasoconstriction, impaired judgment. Moderate (28-32°C) - shivering ceases, consciousness decreases, bradycardia, atrial fibrillation common, J-waves (Osborn waves) appear on ECG at the junction of QRS and ST segment. Severe (<28°C) - loss of consciousness, VF threshold markedly reduced, risk of VF with minimal stimulation (rough handling, central line placement, pacing). At core temperature <30°C, the myocardium becomes extremely irritable: any mechanical stimulation can trigger VF, and the cold myocardium is refractory to defibrillation and ACLS medications (epinephrine, amiodarone have prolonged metabolism and may accumulate to toxic levels). The classic teaching: 'No one is dead until they are warm and dead' - cases of full neurological recovery after prolonged cardiac arrest in hypothermia (especially cold water submersion) are well-documented.