Hypothermia & Shivering

Hypothermia: core body temperature < 36 °C.

Core temperature sites: - Nasopharynx — accurate but epistaxis risk - Distal esophagus — accurate; avoid in stricture / varices - Tympanic membrane — accurate; perforation risk - PA catheter thermistor — gold standard - Bladder — lags during thermal swings, esp. low urine output - Rectum — inaccurate with stool, avoid in neutropenia - Skin — always cooler than core

Afferent: A-delta (cold) and C (warm) fibers via spinothalamic tract.

Central control: preoptic-anterior hypothalamus integrates skin and core inputs.

Efferent: - Behavioral (triggered by skin) — seeking clothing, voluntary movement (suppressed under anesthesia) - Autonomic (triggered by core) — only 3 mechanisms: shivering, sweating, vascular tone

Interthreshold range: narrow temp band between cold-induced and warm-induced responses. - Normal: ~0.2 °C - General anesthesia widens it to ~4 °C (20×) - Regional anesthesia widens it to ~0.8 °C (4×)

In decreasing order of importance: 1. Radiation (60%) — IR emission to cooler surroundings 2. Convection (15–30%) — air currents 3. Evaporation (~20%) — surgical prep, exposed viscera, ventilation 4. Conduction (< 5%) — contact with cold table

Phases: 1. Redistribution hypothermia — first hour. Vasodilation from anesthetic shifts heat from core to periphery. Drops core temp ~1 °C in 30 min. 2. Heat loss > heat production 3. Steady state (heat balance)

• Metabolic rate ↓ 8% per 1 °C
• Myocardial protection (↓ O₂ demand)
• CNS protection from ischemic/traumatic injury
• Targeted temperature management (32–36 °C × 24 h) improves outcome after cardiac arrest
• Enables deep hypothermic circulatory arrest for complex aortic surgery
• Possible MH protection

• ↑ Infection rates up to 3-fold (impaired neutrophil function, vasoconstriction → ↓ tissue O₂)
• Delayed wound healing
• Coagulopathy (platelet dysfunction, factor enzyme slowing) — part of trauma's lethal triad
• ↑ Surgical blood loss and transfusion
• Delayed emergence; prolonged drug action — rewarm before extubation
• Left-shifts O₂-Hb curve → impairs tissue O₂ delivery
• ↓ Inotropy & chronotropy, ↑ EKG intervals, arrhythmias, ↑ SVR
• ↑ Systemic stress response, postoperative shivering, PACU LOS

Active warming: - Forced-air (Bair Hugger) — most effective - Heated circulating water pad - Breathing circuit heating / humidification - IV fluid warmer (Ranger for slow, Belmont/Level 1 for fast) - Bladder irrigation with warm fluid - Heating lamp / ↑ ambient room temperature

Passive insulation (less effective): - Cotton blankets, surgical drapes, "space" blanket

Best prophylaxis: preoperative skin warming for 30–60 min to ↑ peripheral compartment temperature before vasodilation occurs — minimizes redistribution.

• Rhythmic muscular activity to generate heat
• ↑ O₂ consumption up to 5×; ↑ CO₂ production; ↑ catecholamines
• Risk for myocardial ischemia, hypoxemia
• Treatment:
• Active warming
• Meperidine 12.5–25 mg IV (κ-opioid effect on shivering)
• Alternatives: dexmedetomidine, clonidine, tramadol, magnesium