2021B12 Describe the effects of perioperative hypothermia. DO NOT discuss mechanisms of heat loss.

 

Hypothermia: core temp <36°C

 

Direct effects:

Metabolic rate

·     ↓temp 1°C = ↓metabolic rate 7%

o  ↓Rate of enzymatic reactions

o  ↓ATP hydrolysis

o  ↓Oxidative phosphorylation

o  ↓Krebs cycle activity

o  ↓Glycolysis (-> hyperglycaemia)

o  ↓Drug metabolism (e.g. Hofmann degradation of cisatracurium)

Acid-base

·     ↑pH 0.015 per 1°C (Rosenthal equation). Multiple factors:

o  ↓Water dissociation into H+ and OH-

o  ↓Metabolic rate -> ↓VCO2

o  ↑CO2 solubility -> ↓partial pressure

·     N.B. ↑pH ∝ ↑pH of ICF ∝ pH of neutrality

o  Favours alpha stat management: i.e. allow pH to increase

§  Optimal acid-base buffering in ICF

§  Trapping of metabolic intermediates in ICF

§  Preserved function of enzymes and ion channels

§  Preserved CBF autoregulation

o  Alternative: pH stat management

§  Luxury cerebral perfusion

§  Rapid homogenous cooling

Cardiovascular

·     Electrical:

o  Slowed conduction: bradycardia, prolonged QT, (J wave)

o  Arrhythmias: AF, VF (arrest at 28°C)

o  Resistance to defibrillation

·     Haemodynamic:

o  Vasoconstriction -> ↑SVR and ↑PVR

o  If shivering: ↑metabolic rate -> ↑venous return -> ↑cardiac output

o  If paralysed: the opposite

Respiratory

·     Respiratory depression:

o  Mainly premature neonates

o  ↓RR

o  ↓Brainstem sensitivity to CO2

·     Reduced oxygenation (and ↑SvO2)

o  ↓Metabolic rate -> ↓VO2

o  Left shift OHDC -> ↓tissue uptake

Neurological

·     Peripheral:

o  Shivering (if not paralysed)

o  Hyporeflexia

·     Central:

o  ↓Metabolic rate = cerebral protection

o  ↓Seizures

o  ↓MAC, ↓propofol Cp50

o  ↓GCS (confusion) if severe

Renal

·     ↓ADH synthesis -> cold diuresis

Haematological

·     ↑Viscosity -> ↑afterload

·     Coagulopathy:

o  <35°C platelet dysfunction

o  <33°C factor dysfunction

Immunological

·     ↓WBC activity -> ↑susceptibility to infection (if <36°C)

 

Compensation:

Behavioural

Mediated by hypothalamus and cortex, absent under GA:

·     Shelter

·     Insulation

·     Motor activity -> ↑metabolic rate

Autonomic

Mediated by hypothalamus, impaired under GA:

·     Peripheral vasoconstriction -> ↓skin blood flow 3x

·     Shivering -> ↑metabolic rate 2-3x

·     Brown fat metabolism (neonates only)

 

 

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