General Anesthesia Overview

General Anesthesia

• General anesthetics are drugs that induce a reversible loss of sensation and consciousness.

• Classification: Anesthetics are classified into inhalational and intravenous types.

• Inhalational:

  • Gases: Nitrous oxide (N₂O), Xenon
  • Liquids: Ether, Halothane, Enflurane, Isoflurane, Desflurane, Sevoflurane

• Intravenous:

  • Inducing agents: Thiopentone sodium, Propofol, Methohexitone, Etomidate
  • Slow-acting: Ketamine
  • Benzodiazepines: Diazepam, Lorazepam, Midazolam
  • Opioid analgesics: Fentanyl

Stages of General Anesthesia

• Stage I (Analgesia): Loss of pain sensation, amnesia, and decreased awareness of pain.

• Stage II (Excitement): Delirium, possible combative behavior; rapid acting agents like propofol used before inhalation.

• Stage III (Surgical Anesthesia): Gradual loss of muscle tone and reflexes, ideal stage for surgery, continuous monitoring is crucial.

• Stage IV (Medullary Paralysis): Severe depression of respiratory and vasomotor centers; death is imminent without intervention.

Mechanism of Action

• Inhalational: Potentiate the action of GABA to open chloride channels. Enhance glycine's inhibitory action in the spinal cord and medulla. Some also inhibit excitatory NMDA glutamate receptors. Certain fluorinated anesthetics and barbiturates also inhibit nicotinic cholinergic receptors.

• Intravenous: Different mechanisms exist but several enhance GABA's action to increase chloride channel opening.

Nitrous Oxide (N₂O)

• Characteristics: Cheap, low potency anesthetic, but good analgesic, fast onset and recovery, 70% N₂O + 25-30% O₂ + 0.2-2% other potent anesthetic (eg halothane).

• Uses: Surgical procedures, obstetrics, and emergency situations.

• Adverse Effects: Interacts and degrades vitamin B12. Contraindicated in pneumothorax, air embolus and obstructive conditions.

• Additional Notes: It has little effect on respiration, heart, or blood pressure. It is non-flammable, non-irritating, and considered the safest inhalational anesthetic. Adverse effects include diffusional hypoxia and megaloblastic anemia.

Halothane

• Characteristics: Volatile liquid with a sweet odor, potent anesthetic but poor analgesic. Dilates bronchi so preferred for asthmatics. Used at 2-4% induction and 0.5-1% maintenance.

• Adverse Effects: Hepatitis, hyperthermia, hangover, shivering, and jaundice. Contraindicated in raised cerebrospinal fluid pressure.

Thiopentone Sodium

• Characteristics: Ultrashort-acting barbiturate given intravenously causes unconsciousness within 20 seconds. Supplemented with analgesics and muscle relaxants.

• Uses: Induction of anesthesia, control of convulsions, protection from cerebral ischemia.

• Adverse Effects: Laryngospasm, shivering, and delirium during recovery.

Ketamine

• Characteristics: Dissociative anesthetic; produces profound analgesia, immobility, and amnesia with light sleep. Heart rate and blood pressure are elevated due to sympathetic stimulation. Respiration is not depressed and reflexes are not abolished.

Propofol

• Characteristics: Most commonly used intravenous anesthetic. Consciousness occurs within 45 seconds and lasts 15minutes. Anti-emetic, non-irritant to airways. Suitable for day care and short procedures.

• Adverse Effects: Pain during injection and drop in blood pressure.

Preanesthetic Medication

• Purpose: To enhance the safety and ease of anesthesia procedures by relieving anxiety, preventing allergic reactions and nausea, and providing analgesia.
• Examples: Benzodiazepines (anxiety), antihistamines (allergy), antiemetics (nausea), opioids (analgesia), proton pump inhibitors (acidity), and atropine (bradycardia and secretion).

This quiz covers the classification and stages of general anesthesia. Dive deep into the types of anesthetic agents and their effects on patients during surgical procedures. Test your knowledge on inhalational and intravenous anesthetics, and understand the key stages of anesthesia.