General Anaesthetics

General Anaesthetics

1. Definition & components of the anaesthetic state

• General anaesthetics (GAs) are drugs that produce a reversible loss of all sensation and consciousness, with a low therapeutic index, used to permit surgery and unpleasant procedures (KDT 8e Ch.27, p.399; G&G 14e Ch.24, p.471).
• The cardinal features ("components") of general anaesthesia are five distinct end points — no single available agent achieves all five well (Katzung 16e Ch.25, p.464; G&G 14e Ch.24, p.472):
  • Unconsciousness (hypnosis)
  • Amnesia (loss of recall of perioperative events)
  • Analgesia (loss of pain sensation)
  • Immobility / inhibition of motor response to noxious (surgical) stimulation
  • Attenuation of autonomic reflexes (and skeletal-muscle relaxation) (Katzung 16e Ch.25, p.464).
• Because no one drug delivers all components safely, modern practice uses balanced anaesthesia — a combination of inhaled and intravenous drugs, each chosen for a specific component (induction agent, maintenance agent, opioid analgesic, neuromuscular blocker), exploiting favourable properties while minimising the side effects of each (KDT 8e Ch.27, p.399; Katzung 16e Ch.25, p.464).
• The three overarching objectives of general-anaesthesia delivery are (G&G 14e Ch.24, p.471):
  • Minimise the direct/indirect deleterious effects of the agents and techniques.
  • Sustain physiological homeostasis during surgery (blood loss, ischaemia–reperfusion, fluid shifts, cold exposure, impaired coagulation).
  • Improve post-operative outcome by blunting the surgical stress response.