LOCAL AND GENERAL ANAESTHESIA

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LOCAL AND GENERAL ANAESTHESIA

• LOCAL
• GENERAL - INHALATION
• - IV

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LOCAL ANAESTHETICS

• First use - coca leaves, cocaine isolated 1860
• 2 groups
• - amides (lignocaine, bupivacaine)
• metabolised slowly in liver (2-3hr t?)
• - esters (cocaine) metabolised by
• plasma esterases (shorter duration)

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LOCAL ANAESTHETICS

• Mechanism of action
• ? action potential by blocking Na channels
  from inside the neuron
• Small nerve fibres are blocked before larger
  myelinated ones (pain fibres before motor axons)
• Uses surface procedures epidurals for labor and
  delivery, surgery

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LOCAL ANAESTHETICS

• Adverse effects (toxicity)
• - due to escape into systemic circulation
• - in CNS ? excitation (agitation,
  tremors,seizure) followed by depression
• - cardiovascular ? myocardial
• depression and vasodilation
• Administration with a vasoconstrictor
  (adrenaline) ??duration, ?toxicity

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GENERAL ANAESTHETIC AGENTS

• Produce unconsciousness and lack of response to
  painful stimuli
• Ether was the first used - 1846
• Ideal anaesthetic ? unconsciousness
• analgesia
• muscle relaxation
• amnesia
• Combination of drugs is needed for balanced
  anaesthesia

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GENERAL ANAESTHETIC AGENTS

• a) Induction of unconsciousness - rapid, by
  action of IV short-acting barbiturate (eg
  thiopentone)
• b) Maintained ( analgesia) by inhalation agents
  (eg nitrous oxide, halothane)
• c) Neuromuscular blocker (tubocurarine)
• Surgical anaesthesia is a stage of deep
  unconsciousness, with varying respiratory
  depression

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PREMEDICATION

• Adjuncts to anaesthesia
• - benzodiazepines (eg diazepam) for sedation,
  ? anxiety
• - opioids (eg morphine) for analgesia
• - anticholinergic drugs (eg atropine) to
• ? vagal heart reflexes (bradycardia)
• - anti-emetic (eg ondansetron) with
• premed or IV during anaesthesia

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INHALATION ANAESTHETICS

• Small lipid soluble molecules
• Mechanism enhance the effects of GABA
  (inhibitory n.t.)
• Minimum alveolar concentration (MAC) minimum
  concentration of drug in the alveolar air able to
  prevent response to pain in 50 of patients
• Low MAC high potency

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INHALATION ANAESTHETICS

• Most inhalation anaesthetics have low MAC (eg
  halothane 0.75)
• MAC of nitrous oxide is very high (105), cannot
  produce surgical anaesthesia alone (but it has
  high analgesic potency)

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PHARMACOKINETICS OF INHALATION AGENTS

• Uptake should be rapid (lipid solubility,
  ventilation rate, lung blood flow)
• Distribution ? rapidly in brain, due to high
  blood flow
• Metabolism little
• Elimination via lungs, leaves brain first (blood
  flow)

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ADVERSE EFFECTS

• Respiratory and cardiac depression
• Sensitising the heart to catecholamines (eg
  adrenaline) ? dysrhythmias
• Malignant hyperthermia (rare), co-administration
  of succinylcholine ? risk
• Aspiration of gastric contents (due to loss of
  reflexes) ? pneumonia
• Toxicity to operating room staff (headache,
  miscarriage)

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INTRAVENOUS ANAESTHETICS

• Used alone or with inhalation agents (to reduce
  dose)
• Short acting barbiturates - induction
• (eg thiopentone)
• Benzodiazepines - induction, and conscious
  sedation (eg midazolam morphine) in minor
  surgery, endoscopy
• Neuroleptic-opioid combination - for minor
  procedures, eg bronchoscopy