GENERAL (SYSTEMIC) ANESTHETICS

• Stages of general anesthesia

  1. Stage I (cortical stage) → analgesia

  2. Stage II (delirium) – inc BP, inc heart rate (should be bypassed using sedative eg. Thiopental)

  3. Stage III (surgical anesthesia)

  4. Stage IV (medullary depression) – not desirable (leads to death)
• Inhalational anesthetics


  Increase neuronal threshold for firing leading to a decrease in neuronal activity. Hyperpolarization of neurons via activation of K currents.
• Specific inhalational anesthetics (arranged in order of increasing potency)


  • Nitrous Oxide (least potent)
  • Desflurane
  • Sevoflurane
  • Isoflurane
  • Enflurane
  • Halothane
  • Methoxyflurane (most potent) – used for pregnant patients
• Halothane


  • Moderate potency
  • Ease of changing depth of anesthesia
  • May cause uterine relaxation (tocolytic)
  • Reduces myocardial contractility and causes hypotension
  • Tachyarrhythmias exaggerated by adrenergic agonists (e.g., epinephrine)
  • Aggravate conditions of elevated intracranial pressure (ICP)
  • Hepatic necrosis (hepatotoxic)
  • Malignant hyperthermia
• Enflurane


  • Rapid, smooth adjustments of depth of anesthesia
  • Less incidence of arrhythmias, post-operative shivering, nausea, and vomiting
  • Skeletal muscle relaxation
  • Less likely to cause tachyarrhythmias with epinephrine
  • Resp. depression and circulatory collapse with deep anesthesia
  • Seizures
  • Contraindicated in labor
  • Malignant hyperthermia
  • Hepatic necrosis
• Isoflurane


  • Lower blood:gas partition coefficient than enflurane
  • Induction in less than 10 minutes
  • Epinephrine can be used in greater amounts
  • Muscle relaxants
  • Preferred anesthetic in neurosurgery
  • No report of hepatic and renal toxicity
  • More pungent odor than halothane
  • Progressive respiratory depression and hypotension
  • Uterine relaxation in labor
  • May cause "steal" phenomenon
• Sevoflurane


  • Nonflammable, non-irritating
  • More rapid induction of, and recovery from, anesthesia
  • Circulatory and respiratory effects similar to isoflurane
  • 3% metabolized
• Desflurane


  • Smooth and rapid induction of anesthesia
  • No nephrotoxicity and no hepatic failure symptoms observed
  • No "steal" phenomenon
  • May increase ICP
  • No reported case of malignant hyperthermia
• Nitrous Oxide


  • Rapid onset of, and recovery from its effects
  • Weak agent, no muscle relaxant activity, weak anesthetic
  • Causes hypoxia if used alone in large amounts
  • Expansion of air pockets in closed spaces bone marrow depression
• Ultra-short acting barbiturates


  • Use: induction of anesthesia (pre-anesthetics)
  • Toxicity: respiratory and circulatory depression
• Benzodiazepines


  • Use: as pre-medications
  • Mechanism of Action: potentiate action of GABA
  • Advantage: cause antegrade amnesia
  • Toxicity: antagonized by Flumazenil
  • Midazolam is the only water-soluble form
• Etomidate


  • Potent ultra-short-acting non-barbiturate hypnotic without analgesic property
  • Mechanism of Action: GABA-mimetic action
• Propofol


  • 30% reduction in systemic arterial pressure
  • Profound respiratory depression with apnea occurring for 30 seconds
  • Reduction in cerebral blood flow
  • Seizures
• Ketamine


  • Dissociative anesthesia (patient is awake but unconscious)
  • MOA: blockade of the membrane effects of the excitatory neurotransmitter glutamic acid at the NMDA receptor (blockade of NMDA receptor inhibits convulsions)
  • Produces "emergence" phenomena
  • Sensation of dissociation in 15 seconds
  • Unconsciousness in 30 seconds
  • Duration of unconsciousness = 10-15 minutes
  • Duration of analgesia 40 minutes, Duration of amnesia = 1-2 hours
• Opioid analgesics

  • Morphine
  • Meperidine
  • Fentanyl
  • Sufentanil
  • Alfentanil
  • Methadone
• Neuroleptanalgesia

  droperidol + fentanyl
• Neuroleptanesthesia


  droperidol + fentanyl + 65% N2O in O2