Ideal inhalational anaesthetic agent
- The ideal inhalational anaesthetic agent should have several
properties
- In its preparation is should be
- Easily administered
- Boiling point above ambient temperature
- Low latent heat of vaporization
- Chemically stable with long shelf-life
- Compatible with soda-lime, metals and plastics
- Non-flammable
- Cheap
- Pharmacokinetic
- Low solubility
- Rapid onset, rapid offset, adjustable depth
- Minimal metabolism
- Predictable in all age groups
- Pharmacodynamic
- High potency - allows high FiO2
- High therapeutic index
- Analgesic
- Adverse actions
- Minimal toxicity
- Minimal unwanted effects - nausea, vomiting, cardiac arrhythmias
- No toxicity with chronic low-level exposure of staff
Maintenance of anaesthesia
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. |