Clinical meaning
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.
Diagnosis & workup
Diagnostics & workup: - Core temperature measurement: esophageal probe (most accurate in intubated patients), rectal (may lag behind true core temp by 15-30 min), bladder (unreliable during lavage). Standard oral thermometers do not read below 34°C - ECG: progressive changes with cooling - sinus bradycardia → Osborn waves (J-waves: positive deflection at J-point, pathognomonic but not specific) → atrial fibrillation → prolonged QT → ventricular fibrillation → asystole - ABG: interpret at patient temperature (temperature-corrected) vs uncorrected (alpha-stat); mixed metabolic/respiratory acidosis is common - Labs: CBC (hemoconcentration), CMP (hyperglycemia from catecholamine surge and insulin resistance; hypo/hyperkalemia), coagulation (prolonged PT/PTT from impaired enzyme function at cold temperatures), lipase (pancreatitis can precipitate or result from hypothermia), lactate, thyroid function (rule out myxedema), cortisol - Blood glucose: hypoglycemia may be cause or consequence of hypothermia - CXR: aspiration pneumonia (common in hypothermic patients who are obtunded)