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Hypothermia

Created: 1/6/2006
Updated: 8/1/2007
 

Hypothermia

  • Definition
    • Body temperature below normal
      • Normal 36.7-37.0°C
    • Mild 34-36.5°C, moderate 27-34°C, deep 17-27°C, profound <17°C
  • Mild hypothermia
    • Benefits
      • Cerebral and myocardial protection, decreased risk of MH, decreased MAC requirement
    • Risks
        • Increased O2 demand during cooling, myocardial ischaemia, arrhythmia, impaired immunity, pharmacokinetic changes, discomfort
  • Effects in detail
    • Neurological
      • Decreased CBF, CMRO2 (7% per °C), EEG activity, MAC requirement, IOP
      • Neuroprotection via decreased neurotransmitter release, membrane stabilization, effects on response to ischaemia
      • Impaired conscious state at 33°C, coma at 30°C (dilated pupils)
      • Increased neuromuscular blockade
        • via decreased axonal depolarization, increased ACh release, decreased excitation-contraction coupling
      • Decreased TOFC (causing overestimate of paralysis)
    • CVS
      • Increased myocardial O2 demand if shivering, decreased CO if anaesthetized
      • Severe: arrhythmias, decreased HR, O2 consumption
        • Contractility and SV relatively preserved
      • Vasoconstriction, poor peripheral perfusion, increased SVR, CVP
      • ECG
        • SB, increased PR , wide QRS, prolonged QT
        • <32°C: J wave in II, V6, then anterior leads
        • Nodal rhythms, PVCs, AV block, fibrillation
    • Respiratory
      • Increased VO2 with shivering (up to 300%)
      • Increased PVR, V/Q mismatch, impaired hypoxic vasoconstriction
      • Decreased ventilatory drive, decreased bronchial tone, increased dead space
      • Increased gas solubility
    • Renal
      • Decreased RBF, GFR directly due to increased sympathetic tone, decreased CO
      • Cold diuresis due to decreased Na+ reabsorption
    • Hepatic, GIT
      • Decreased blood flow, metabolic and excretory functions
      • Decreased gut motility
    • Haematological
      • Decreased platelet function, increased visceral sequestration
      • Decreased coagulation, increased fibrinolysis
      • Increased haematocrit, rouleaux formation, left shift of Hb-O2 dissociation curve
    • Endocrine and metabolic
      • Decreased BMR 5-7% per °C if not shivering (decreased PCO2, increased glucose)
      • Metabolic acidosis, pH and PCO2 changes (a-stat)
      • Acute rise in K+ with rewarming
      • Decreased insulin, insulin resistance, hyperglycaemia with increased sympathetic tone
      • Increased thyroid hormone secretion (long term)
    • Pharmacokinetic
      • Decreased metabolism and excretion (hepatic and renal)
      • Increased protein binding, decreased Vd of circulating compartment

Obesity

  • Definition
    • BMI (height in m ÷ weight in kg squared)
      • >25 overweight, >30 obese
    • Waist-hip ratio
      • 0.8 ideal, >1.0 overweight
  • Risks
    • Increased perioperative morbidity and mortality, average hospital stay
    • Associated diseases
      • Respiratory
        • Decreased FRC, increased closing capacity
        • Hypoventilation, OSA
      • Cardiovascular
        • Increased CO 0.1 L/min/kg fat
        • HT, LVH
        • Cor pulmonale
      • Other
        • Diabetes, cholelithiasis, cirrhosis
      • 30% overweight --> 40% increase in IHD mortality, 50% increase in CVA mortality
    • Anaesthetic risks
      • Reduced physiological reserve
      • Increase in difficult intubation
      • Decreased FRC, increased ventilatory pressures
      • Increased fasting gastric volume
        • 75% have >25 ml, pH <2.5 at 140 kg
      • Pressure areas
      • Monitoring difficulties, vascular access
      • Larger fat compartment in pharmacokinetics
    • Postoperative risks
      • Postoperative hypoxia
      • Wound infection
      • DVT, PE

Kindly provided by Dr James Mitchell from his pharmacodynamics series


ArticleDate:20060601
SiteSection: Article
 
   
    
                                            
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