Hypothermia

Temperature monitoring - An esophageal probe in the lower third of the esophagus is the most accurate core site once an advanced airway is in, and it is the site to use during rewarming. Rectal and bladder temperatures lag true core, and oral and tympanic thermometers are unreliable below 35 C.

Why anesthesia makes it worse - General anesthesia raises the hypothalamic threshold sensitivity 10 to 20 fold and blocks the vasoconstriction response. Neuraxial block drops the shivering and vasoconstriction thresholds about 0.6 C and stops peripheral thermoreceptors reporting heat loss to the CNS.

Flag the high-risk patient preop - Older age, low body mass index, ASA II to V, preoperative hypothermia, a long fast, major or intermediate surgery with a large incision, prolonged anesthetic duration, combined general plus regional, high spinal block level, OR temperature below 21 C, cold IV fluids, cold irrigation, cold blood, and blood loss.

Passive warming is not enough - Blankets cut heat loss only about 33% in an awake person, and a heated cotton blanket buys roughly 10 minutes. Active warming — forced-air or an underbody conductive mat — is what actually holds normothermia. In one 972-patient series, 16.7% of patients became hypothermic when active warming was not used versus 3.5% overall.

Prewarm - Forced-air warming applied preoperatively blunts the redistribution drop at induction, and preoperative plus intraoperative active warming beats intraoperative warming alone.

Cardiac cost - Cooling triggers a sympathetic and adrenomedullary catecholamine surge that raises cardiac output, heart rate, and blood pressure, and with it myocardial demand and ischemia. Hypothermic patients are about three times more likely to have a cardiac event.

Bleeding - Hypothermia cuts thromboxane release and impairs platelet aggregation, and the cold-sensitive coagulation enzymes prolong the aPTT, so blood loss and transfusion requirement both rise. Remember the lab warms the sample to 37 C before running it, so a normal coagulation panel does not exclude cold coagulopathy at the field.

Surgical site infection - Mild perioperative hypothermia triples the risk of surgical site infection. Vasoconstriction cuts subcutaneous oxygen delivery and slows collagen synthesis while neutrophil and macrophage activity, chemotaxis, phagocytosis, and antibody production all slow down.

Delayed emergence and prolonged block - Cold disables the temperature-sensitive enzymes that clear anesthetics, so propofol, fentanyl, and morphine last longer and vecuronium, atracurium, and rocuronium block is prolonged. Confirm recovery with a quantitative train-of-four ratio rather than clinical impression before extubation.

Shivering on emergence - It raises metabolic rate, oxygen consumption, catecholamine release, and cardiac output. That is a real problem in anyone with intrapulmonary shunting, limited lung reserve, or limited cardiac output.

It costs the patient days - Hypothermic patients stay in hospital roughly 20% to 35% longer than normothermic ones. Between 34.9 C and 36.9 C, every 1 C higher intraoperative temperature cuts ICU length of stay by about 31%.

Handle the severely hypothermic patient gently - In moderate to severe accidental hypothermia the myocardium is irritable and jostling can precipitate ventricular fibrillation. Atrial fibrillation is the most common rhythm. VF has a favorable neurologic outcome if ACLS starts immediately, but asystole is generally refractory until the patient is rewarmed toward 35 C.

ECG changes - Slowed conduction through potassium channels prolongs the intervals, and the J point rises into an Osborn wave whose height tracks the degree of hypothermia, most visible in the precordial leads.

Rewarming ladder - Mild: passive external rewarming at 0.5 to 2 C per hour, which needs intact glucose stores to fuel shivering. Moderate to severe: active external forced-air plus warm IV fluid at 40 to 42 C and humidified gas. Extremity immersion at 44 to 45 C risks afterdrop and cardiovascular collapse as peripheral vasodilation returns cold blood to the core. Refractory or arrested: extracorporeal — hemodialysis 2 to 3 C per hour, arteriovenous rewarming 4.5 C per hour, CPB or VA-ECMO 7 to 10 C per hour.

Labs in accidental hypothermia - Fingerstick glucose, CBC, and a metabolic panel, with electrolytes rechecked about every 4 hours while resuscitating a moderate to severely hypothermic patient. Hemoglobin and hematocrit run high from cold diuresis. A potassium above 12 mEq/L in a hypothermic arrest marks irreversible tissue death and cell lysis.

Drugs and diagnoses that blunt the cold response - General anesthetics, beta-blockers, clonidine, meperidine, neuroleptics, ethanol, sedative-hypnotics, and phenothiazines. When the vital signs do not match the measured temperature, go looking for hypothyroidism, adrenal insufficiency, sepsis, hypoglycemia, carbon monoxide poisoning, or overdose.

Targeted temperature management - After ROSC the target is 32 to 34 C reached as fast as possible, held 12 to 24 hours, then rewarmed at a controlled 0.2 to 0.5 C per hour. Expect cardiac output down about 25%, SVR and blood pressure up, and sinus bradycardia; serious arrhythmias are uncommon in the 32 to 34 C window but the risk climbs below 30 C, which is easy to overshoot when the measured site lags true core.

Shivering defeats cooling - Shivering starts below 36.5 C and raises oxygen consumption, energy expenditure, and induction time, canceling much of the benefit. It has to be suppressed pharmacologically or non-pharmacologically for TTM to work.

TTM electrolytes and glucose - Cold diuresis plus intracellular ion shift drops magnesium, potassium, phosphate, and calcium, often needing high-dose replacement. Rewarming reverses the shift and hyperkalemia appears, so rewarm slowly. Cooling also blunts insulin secretion and sensitivity, and hyperglycemia tracks with worse neurologic outcome.

Ventilation during cooling - CO2 production falls with metabolic rate. Leave the ventilator alone and you get hypocapnia and respiratory alkalosis, cerebral vasoconstriction, and reduced cerebral blood flow. The oxyhemoglobin dissociation curve also shifts left, cutting tissue oxygen availability and feeding metabolic acidosis.

Drug dosing under TTM - Propofol, midazolam, vecuronium, rocuronium, and phenytoin all show higher serum concentrations, slower clearance, and longer duration during cooling, returning to baseline on rewarming. Lower the doses.

Hypothermia is a core temperature below 35 C, graded mild (32 to 35 C), moderate (28 to 32 C), and severe (under 28 C). The hypothalamus normally defends temperature with cold-induced peripheral vasoconstriction, shivering that raises heat production 2 to 5 fold, and a catecholamine, thyroid, and adrenal response. Anesthesia dismantles all of that. General anesthesia widens the hypothalamic threshold 10 to 20 fold and inhibits vasoconstriction, while spinal and epidural block lower the shivering and vasoconstriction thresholds about 0.6 C and cut off peripheral thermal input entirely, so the patient never registers the heat loss. That is why inadvertent perioperative hypothermia — core under 36 C from an hour before induction through 24 hours after — is so common. Cold slows metabolic rate 5% to 7% per degree, prolongs drug effect, impairs coagulation and immune function, and makes the myocardium irritable.