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Physiological Changes During Hypothermia
MD Anaesthesiology Exam Notes
Definition & Classification
Hypothermia = core body temperature < 36°C (perioperative definition) or < 35°C (clinical definition).
| Severity | Temperature |
|---|---|
| Mild | 35°C – 32.2°C (95°F – 90°F) |
| Moderate | < 32.2°C – 28°C (90°F – 82.4°F) |
| Severe | < 28°C (< 82.4°F) |
1. CARDIOVASCULAR SYSTEM
Mild Hypothermia (35–32°C)
- Peripheral sympathetic vasoconstriction → ↑ SVR, ↑ BP, ↑ CVP
- Redistribution of blood volume to central compartment
- Shivering + ↑ sympathetic tone → ↑ cardiac output, tachycardia
- Propensity for atrial arrhythmias
- Cardiac cycle prolongation
Moderate Hypothermia (< 32°C)
- Cardiac conduction delays develop
- Osborn (J) waves on ECG — positive deflection at QRS-ST junction = abnormal early ventricular repolarization; harbinger of VF
- Progressive bradycardia from decreased spontaneous depolarization of pacemaker cells
- ↓ cardiac output progressively
- Increased atrial and ventricular arrhythmias
Severe Hypothermia (< 28°C)
- Very high risk of cardiac arrest (especially at 28–24°C)
- Risk of ventricular fibrillation — patient must be handled gently and kept horizontal
- At < 30°C: ventricular irritability and cardiac arrest may occur
"Gentle handling and maintaining horizontal positioning in hypothermic patients can minimize the likelihood of arrhythmia or cardiovascular collapse." — Miller's Anesthesia, 10e
2. RESPIRATORY SYSTEM
| Severity | Changes |
|---|---|
| Mild | Tachypnoea → progressive ↓ in respiratory minute volume; bronchorrhoea; bronchospasm |
| Moderate | Hypoventilation; 50% ↓ in CO₂ production per 8°C drop; ↓ O₂ consumption |
| Severe | Respiratory arrest; pulmonary oedema; apnoea |
Key Mechanisms
- Tidal volume, respiratory rate, and compliance all decrease
- Sensitivity to hypercapnia is diminished → hypoventilation and acidosis
- Impaired ventilation can exacerbate electrolyte abnormalities
- In mechanically ventilated hypothermic patients, significant Pa-PETCO₂ gradient exists — arterial blood gases needed for accurate monitoring
3. CENTRAL NERVOUS SYSTEM
| Temperature | CNS Effect |
|---|---|
| 35–32°C | Linear ↓ in cerebral metabolism; amnesia; apathy; dysarthria; impaired judgment; maladaptive behaviour |
| < 32°C | EEG abnormalities; progressive ↓ consciousness; pupillary dilation; paradoxical undressing; hallucinations |
| < 28°C | Loss of consciousness common; absent reflexes, absent papillary reactivity |
| Severe | Absent EEG activity; cerebral metabolic rate → near zero |
Key Points
- Cerebral blood flow decreases in proportion to reduced metabolic rate
- This forms the basis of neuroprotection during hypothermic cardiac arrest and neurosurgery
- Paradoxical undressing = inappropriate removal of clothing in response to cold stress (maladaptive)
- Decreased CMRO₂ ≈ 6–7% per 1°C drop in temperature
4. RENAL SYSTEM & ENDOCRINE
Early/Mild Hypothermia
- Cold diuresis ("hypothermic diuresis"):
- ↑ Renal blood flow (central redistribution) + ↓ ADH activity → diuresis
- Can lead to significant hypovolaemia
Progressive Hypothermia
- ↓ Renal blood flow + ↓ GFR
- Diuresis may persist despite ↓ RBF due to impaired tubular water reabsorption
- Peripheral oedema in damaged tissues → further depletes circulating volume
Endocrine Changes (Mild Hypothermia)
- ↑ Catecholamines
- ↑ Adrenal steroids
- ↑ Triiodothyronine (T3) and Thyroxine (T4)
- ↑ Metabolic rate (initially, due to shivering)
5. HAEMATOLOGIC SYSTEM
| Change | Mechanism |
|---|---|
| ↑ Blood viscosity | Diuresis → haemoconcentration; cold-induced |
| Sludging & stasis | Vasomotor abnormalities in microcirculation |
| ↓ Platelet count | Sequestration in liver and spleen |
| Platelet dysfunction | Impaired activation; ↓ enzymatic function |
| Coagulopathy | Coagulation factor function impaired (temperature + pH sensitive) |
| ↓ Leukocyte count | With severe hypothermia |
Coagulopathy
- Even small decreases in temperature significantly impair coagulation factors and platelet function
- Results in clinical coagulopathy (though standard PT/APTT tests done at 37°C may not reflect true in vivo coagulopathy)
- Risk of DIC in severe hypothermia
Key clinical note: Standard coagulation tests run at 37°C underestimate coagulopathy in hypothermic patients — TEG/ROTEM better reflect actual status.
6. ELECTROLYTE & ACID-BASE
| Finding | Severity | Mechanism |
|---|---|---|
| Hypokalemia | Mild | Intracellular shift |
| Hyperglycaemia | Mild | ↓ Insulin secretion & sensitivity |
| Hyperkalemia | Progressive | K⁺ leaks out of cells; marker of cell lysis |
| Metabolic acidosis | Moderate–Severe | ↓ Lactate clearance (hepatic impairment) + lactic acid from shivering |
| Respiratory alkalosis | — | ↑ Gas solubility → ↓ PaCO₂ (at same dissolved CO₂) |
Blood Gas (pH-stat vs Alpha-stat)
- With ↓ temperature: ↑ gas solubility → ↓ PaO₂ & PaCO₂, ↑ pH (temperature-corrected values)
- Alpha-stat: do NOT temperature-correct → accept the alkalosis; used in most adult cardiac surgery
- pH-stat: add CO₂ to correct PaCO₂; ↑ cerebral blood flow; used in paediatric cardiac surgery
Hyperkalemia Warning
- K⁺ > 12 mmol/L in hypothermic arrest = grave prognosis, may preclude resuscitation
- Cardiac toxicity threshold for K⁺ diminishes with progressive hypothermia
- Succinylcholine (which raises K⁺) should be used with caution
7. METABOLIC & HEPATIC
- BMR decreases approximately 6% per 1°C fall in temperature
- Hepatic impairment: decreased clearance of lactate and other metabolic byproducts
- Delayed drug metabolism: prolonged effect of anaesthetic agents, muscle relaxants, opioids
- Increased postoperative protein catabolism and stress response
- Impaired wound healing
- Increased risk of infection (impaired immune function)
8. NEUROMUSCULAR
| Severity | Neuromuscular Effect |
|---|---|
| Mild | Increased pre-shivering muscle tone |
| Moderate | Shivering fatigues and decreases |
| Severe | Muscular rigidity; absent neuromuscular function |
Shivering — Anaesthetic Significance
- Increases O₂ consumption up to 5-fold
- ↑ CO₂ production, ↑ catecholamine release
- ↑ Cardiac output, HR, BP
- ↑ Intracranial and intraocular pressure
- May precipitate myocardial ischaemia in at-risk patients
9. PERIOPERATIVE PATTERN OF TEMPERATURE LOSS
Three phases during general anaesthesia:
| Phase | Time | Mechanism | ΔT |
|---|---|---|---|
| Phase 1 | 0–1 h | Heat redistribution from core to periphery (anaesthetic vasodilation) | ↓ 1–2°C rapid drop |
| Phase 2 | 1–4 h | Continuous heat loss to environment > metabolic heat production | Slower gradual decline |
| Phase 3 | > 4 h | Steady state — heat loss = metabolic production | Plateau |
Most of phase 1 drop is due to redistribution (not environmental loss). Can be greatly reduced by preoperative warming.
SUMMARY TABLE — Deleterious Effects of Hypothermia (Morgan & Mikhail)
| System | Effect |
|---|---|
| Cardiovascular | Arrhythmias, ischaemia, ↑ peripheral vascular resistance |
| Haematologic | Reversible coagulopathy (platelet dysfunction) |
| Respiratory | ↓ Minute ventilation, hypoventilation |
| CNS | Altered mental status, ↓ CMRO₂ |
| Renal | Impaired renal function |
| Metabolic | Delayed drug metabolism |
| Wound | Impaired healing, ↑ infection risk |
| Other | ↑ Postoperative protein catabolism, stress response |
OSBORN (J) WAVE — High-Yield ECG Finding
- Positive deflection at the QRS-ST junction (J point)
- Seen at temperatures < 32°C
- Indicates abnormal early ventricular repolarization
- Harbinger of ventricular fibrillation
- Disappears with rewarming
ANAESTHETIC RELEVANCE — High-Yield Points
- Perioperative hypothermia is associated with increased mortality
- Hypothermia left-shifts the Hb-O₂ dissociation curve → ↓ O₂ delivery to tissues
- Hypothermia causes coagulopathy not reflected in standard lab tests at 37°C
- Drug effects prolonged — especially NMBDs, volatile agents, opioids
- Shivering on awakening → 5× ↑ O₂ consumption → dangerous in cardiac patients
- Hypothermia reduces MAC of volatile anaesthetics (approximately 5% per 1°C)
- K⁺ > 12 mmol/L in arrest = poor prognosis; do NOT use succinylcholine when hyperkalemia suspected
- Neuroprotective: CMRO₂ ↓ ~6-7% per °C — basis of therapeutic hypothermia and deep hypothermic circulatory arrest (DHCA)
Sources: Miller's Anesthesia 10e; Morgan & Mikhail's Clinical Anaesthesiology 7e; Harrison's Principles of Internal Medicine 22e
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