Hypothermia and Peripheral Cold Injuries¶
Chapter 477 | Part 15: Disorders Associated with Environmental Exposures
KEY CLINICAL POINTS¶
- Hypothermia is defined as an unintentional drop in core body temperature below 35°C (95°F), with severe cases (<28°C) associated with high mortality.
- Primary accidental hypothermia is geographically pervasive, with risk factors including age extremes (elderly/neonates), environmental exposure, malnutrition, and endocrine disorders.
- Frostbite occurs when tissue temperature drops below 0°C (32°F), causing ice crystal formation and microvascular thrombosis; classification includes superficial and deep frostbite.
- Rewarming strategies depend on core temperature: passive rewarming for mild cases (<32°C), active rewarming (forced-air blankets, intravenous fluids) for severe hypothermia.
- Extracorporeal life support (ECLS) is critical for nonperfusing patients, with specific rewarming rates and circuit configurations (e.g., continuous venovenous rewarming).
1. DEFINITION & OVERVIEW¶
Accidental hypothermia results from direct cold exposure in previously healthy individuals. Secondary hypothermia occurs as a complication of systemic disorders. Frostbite is a freezing injury caused by tissue temperatures below 0°C (32°F), while nonfreezing cold injuries (e.g., chilblain, immersion foot) result from wet cold exposure.
Table 477-1 Risk Factors for Hypothermia¶
| Category | Risk Factors |
|---|---|
| Age extremes | Elderly, Neonates |
| Environmental exposure | Immersion, Inadequate clothing |
| Toxicologic/pharmacologic | Ethanol, Anesthetics, Antipsychotics |
| Endocrine-related | Diabetes mellitus, Hypothyroidism, Adrenal insufficiency |
| Neurologic | Cerebrovascular accident, Spinal cord injury |
| Other | Malnutrition, Burns, Immobility, Sepsis |
1.1 Thermoregulation¶
The preoptic anterior hypothalamus orchestrates thermoregulation via autonomic responses (norepinephrine release, shivering) and endocrine mechanisms. Cold-induced vasoconstriction conserves heat, while prolonged exposure stimulates the thyroid axis.
1.2 Cold Injury Classification¶
Cold injuries are categorized as freezing (frostbite) or nonfreezing (chilblain, immersion foot). Frostbite severity is graded by cyanosis location (grade 1–4), while nonfreezing injuries involve vasospasm and tissue damage without ice crystal formation.
2. EPIDEMIOLOGY¶
Primary accidental hypothermia is seasonal but common in warmer regions. Mortality is higher in secondary hypothermia (e.g., from sepsis, trauma). Frostbite incidence peaks in cold climates, with military and outdoor workers at highest risk.
2.1 Demographics¶
Elderly and neonates are most vulnerable due to diminished thermoregulation. Military personnel, hunters, and climbers face elevated risks from environmental exposure.
2.2 Risk Factors¶
Malnutrition, hypoglycemia, endocrine disorders (hypothyroidism, adrenal insufficiency), and medications (antipsychotics, ethanol) increase susceptibility. Immobility and socioeconomic factors compound risks.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Hypothermia results from heat loss exceeding generation. Frostbite involves ice crystal formation and microvascular thrombosis, while nonfreezing injuries cause vasospasm and tissue ischemia. Endocrine dysfunction (e.g., hypothyroidism) impairs thermogenesis.
Table 477-2 Physiologic Changes Associated with Accidental Hypothermia¶
| Severity | Body Temperature | Central Nervous System | Cardiovascul ar | Respiratory | Renal & Endocrine | Neuromuscu lar |
|---|---|---|---|---|---|---|
| Mild | 35°C–32.2°C | Linear depression of cerebral metabolism; amnesia | Tachycardia fi bradycardia | Tachypnea fi decreased respiratory volume | Diuresis; increased cat echolamines | Increased muscle tone fi fatiguing |
| Moderate | <32.2°C–28° C | EEG abnormalities; decreased co nsciousness | Progressive hypotension; arrhythmias | Hypoventilatio n; absent airway reflexes | 50% increase in renal blood flow | Hyporeflexia; impaired insulin action |
| Severe | <28°C | Loss of autor egulation; coma | Cardiac arrest; ventricular fibrillation | Pulmonic congestion; apnea | Decreased renal blood flow; oliguria | No motion; peripheral areflexia |
3.1 Mechanisms of Heat Loss¶
Radiation (55–65%), conduction (10–15%), convection (wind), respiration, and evaporation. Cold water increases conduction, while humidity affects evaporation.
3.2 Frostbite Pathophysiology¶
Tissue freezing causes cellular dehydration, microvascular thrombosis, and ischemia. Thawing leads to progressive dermal ischemia, edema, and necrosis.
4. CLINICAL FEATURES¶
Hypothermia presents with progressive organ system depression, paradoxical undressing, and altered mental status. Frostbite manifests as sensory loss, mottled skin, and vesiculation. Nonfreezing injuries cause cyanosis, edema, and erythema.
4.1 Hypothermia Symptoms¶
Initial: shivering, tachycardia. Moderate: confusion, paradoxical undressing. Severe: coma, arrhythmias, respiratory arrest. Cold-induced ileus and rectus spasm may mimic acute abdomen.
4.2 Frostbite Presentation¶
Superficial: erythema, anesthesia. Deep: waxy, mottled skin; vesiculation with edema. Grades 1–4 correlate with cyanosis location (distal phalanx to metacarpophalangeal joint).
5. DIFFERENTIAL DIAGNOSIS¶
Secondary hypothermia must be differentiated from sepsis, trauma, and drug overdose. Frostbite must be distinguished from nonfreezing injuries (e.g., chilblain) and other dermatologic conditions.
5.1 Secondary Hypothermia¶
Sepsis, myocardial infarction, burns, and drug overdose (e.g., ethanol, antipsychotics) may mimic hypothermia. Neurological injury (e.g., spinal cord trauma) increases susceptibility.
6. INVESTIGATIONS & DIAGNOSIS¶
Core temperature measurement is essential. Arterial blood gases should not be temperature-corrected. ECG abnormalities (J waves) and hypothermic arrhythmias (e.g., bradycardia, asystole) aid diagnosis.
6.1 Diagnostic Criteria¶
Core temperature <35°C confirms hypothermia. Frostbite diagnosis relies on clinical presentation (sensory loss, skin appearance) and exclusion of other causes.
6.2 Monitoring¶
Esophageal temperature probes are preferred. ETCO2 monitoring and echocardiography assess perfusion. Continuous renal and cardiovascular monitoring is critical during rewarming.
7. MANAGEMENT & TREATMENT¶
Passive rewarming (insulation) for mild hypothermia. Active rewarming (forced-air blankets, intravenous fluids) for severe cases. ECLS is indicated for nonperfusing patients. Frostbite requires gradual rewarming and surgical debridement.
Table 477-3 Options for Extracorporeal Life Support¶
| Rewarming Technique | Considerations |
|---|---|
| Continuous venovenous (CVV) rewarming | Circuit: CV catheter to CV, dual-lumen CV, or peripheral catheter; Flow rates 150–400 mL/min; ROR 2–3°C/h |
| Continuous arteriovenous (CAVR) rewarming | Requires systolic BP ‡60 mmHg; Flow rates 225–375 mL/min; ROR 3–4°C/h |
| Rewarming Technique | Considerations |
|---|---|
| Cardiopulmonary bypass (CPB) | Full circulatory support with pump and oxygenator; Perfusate-temperature gradient 5–10°C; Flow rates 2–7 L/min |
7.1 Rewarming Strategies¶
Passive rewarming: 0.5–2°C/h. Active rewarming: 2–9.5°C/h with ECLS. Avoid direct heat application to frozen extremities to prevent afterdrop.
7.2 Pharmacologic Management¶
Low-dose vasopressors (dopamine, nitroglycerin) for hypotension. Avoid high-dose antiarrhythmics; monitor for drug toxicity during rewarming. Levothyroxine is contraindicated in euthyroid patients.
8. PROGNOSIS & COMPLICATIONS¶
Mortality is high in severe hypothermia (core <28°C) with cardiac arrest. Frostbite may lead to amputation, infection, or chronic neuropathy. Complications include electrolyte disturbances, coagulopathy, and post-rewarming arrhythmias.
8.1 Survival Factors¶
Survival with cardiac arrest >7 h is possible. Early rewarming, absence of lethal injuries, and core temperature >10°C improve prognosis. Severe metabolic acidosis (pH <6.5) portends poor outcomes.
8.2 Long-Term Effects¶
Chronic hypothermia may cause anemia, leukopenia, and impaired wound healing. Frostbite survivors may develop sensory loss, joint stiffness, or recurrent infections.
9. SPECIAL CONSIDERATIONS¶
Elderly and neonates require cautious rewarming due to impaired thermoregulation. Pregnant patients may have altered core temperature thresholds. Immunocompromised individuals need antibiotic prophylaxis for infection risk.
9.1 Pediatric Considerations¶
Neonates lose heat rapidly due to high surface-to-mass ratio. Rewarming should be gradual to avoid afterdrop. Hypoglycemia is a common complication.
9.2 Geriatric Patients¶
Elderly patients have diminished thermal perception and are prone to immobility. Hypothyroidism and adrenal insufficiency increase hypothermia risk.
10. KEY POINTS & CLINICAL PEARLS¶
- Core temperature <35°C defines hypothermia; severe cases (<28°C) require immediate ECLS. 2. Frostbite grades correlate with cyanosis location; deep frostbite may require surgical debridement. 3. Rewarming rates depend on severity: 0.5–2°C/h for mild, 2–9.5°C/h for severe. 4. Avoid direct heat application to frozen extremities to prevent afterdrop. 5. Monitor for electrolyte disturbances and coagulopathy during rewarming.