Maintenance of anaesthesia

• Balanced anaesthesia has three aspects to it

  • Hypnosis = suppression of consciousness
  • Analgesia = suppression of physiological responses to stimuli
  • Relaxation = suppression of muscle tone and relaxation
• Anaesthesia is normally maintained with inhaled volatile gases
• They are lipid soluble hydrocarbons
• They have high saturated vapour pressures
• Modern agents are potent, non-inflammable and non-explosive
• MAC = Minimum alveolar concentration
• It is the alveolar concentration required to keep 50% of population unresponsive

Adverse effects of inhalational anaesthetic agents

Cardiovascular

• Decreased myocardial contractility
• Reduced cardiac output
• Hypotension
• Arrhythmias
• Increased myocardial sensitivity to catecholamines

Respiratory

• Depress ventilation
• Laryngospasm and airway obstruction
• Decreased ventilatory response to hypoxia and hypercapnia
• Bronchodilatation

Central nervous system

• Increased cerebral blood flow
• Reduced cerebral metabolic rate
• Increased risk of epilepsy
• Increased intracranial pressure

Others

• Decreased renal blood flow
• Stimulate nausea and vomiting
• Precipitate hepatitis

Specific anaesthetic agents

Halothane

• Potent anaesthetic but poor analgesic agent (MAC = 0.75)
• Can be used for gaseous induction in children
• 20% is metabolised in the liver and can cause hepatic dysfunction
• Occasionally causes severe hepatitis that can progress to liver necrosis
• Depressed myocardial contractility and can induce arrhythmias

Isoflurane

• Potent anaesthetic but poor analgesic agent (MAC = 1.05)
• Less cardiotoxic but causes greater respiratory depression
• Reduces peripheral resistance and cause a 'coronary steal'
• Few adverse effects have been reported

Nitrous oxide

• Weak anaesthetic agent (MAC = 103)
• Can not be used as an anaesthetic agent alone without causing hypoxia
• Very potent analgesic agent
• Used as 50% N₂O / 50% O₂ mixture = 'Entonox'
• Used in anaesthesia mainly for its analgesic properties

Muscle relaxants

Muscle relaxants are either depolarising or non-depolarising agents

Depolarising agents

• For example - suxamethonium
• Act rapidly within seconds and last for approximately 5 minutes
• Used during induction of anaesthesia
• Side effects:
  • Histamine release producing a 'scoline rash'
  • Bradycardia
  • Somatic pain resulting from fasciculation
  • Hyperkalaemia
  • Persistent neuromuscular blockade = 'scoline apnoea'
    • Affects 1:7000 of population
    • Due to pseudocholinesterase deficiency
  • Malignant hyperpyrexia
    • Affects 1:100,000 of population
    • Due to increased calcium influx and uncontrolled metabolism
    • Rapid increase in body temperature with increased PaCO₂
  • Increased intra-ocular pressure
  • Increased gastric pressure

Non-depolarising agents

• For example - vecuronium
• Act over 2-3 minutes and effects last for 30 minutes to one hour
• Competitive antagonism of acetylcholine receptor
• Used for muscle relaxation

Bibliography

Campagna J A,  Miller K W,  Forman S A.  Mechanism of action of inhaled anesthetics.  N Eng J Med 2003;  348:  2110-2124.

Fox A J,  Rowbotham D J.  Anaesthesia.  Br Med J 1999;  319:  557-560.

Heggie J E.   Malignant hyperthermia:  considerations for the general surgeon.  Can J Surg 2002;  45:  369-372.

Hopkins P M.  Malignant hyperthermia:  advances in clinical management and diagnosis.  Br J Anaesth 2000;  85:  118-128.

Wapper F.  Malignant hyperthermia.  Eur J Anaesthesiol 2001;  18:  632-652.

Last updated: 05 January 2008

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