General Anesthesia Overview

General Anesthesia

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

Classification of General Anesthetics

• 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. Sensory transmission interference. Progresses from conscious state to analgesia. Amnesia and reduced awareness occurs as Stage II approaches.
• Stage II (Excitement): Delirium and combative behavior may occur. Rapid-acting agent (e.g., Propofol) often administered intravenously to proceed to Stage III.
• Stage III (Surgical Anesthesia): Progressive loss of muscle tone and reflexes. Skeletal muscle relaxation. The ideal stage for surgery. Continuous careful monitoring is required.
• Stage IV (Medullary Paralysis): Severe respiratory and vasomotor depression. Death can quickly ensue if circulation and respiration are not maintained.

Mechanism of Action of General Anesthetics

• Inhalational Anesthetics: Enhance GABA's (inhibitory neurotransmitter) action, enabling chloride channels to open. They also augment the actions of glycine (another inhibitory neurotransmitter) in the spinal cord and medulla.
• Barbiturates, Benzodiazepines, and Propofol: Potentiate GABA's action, opening chloride channels.
• Ketamine: Inhibits the excitatory NMDA type glutamate receptor.
• Certain Fluorinated Anesthetics and Barbiturates: Inhibit neuronal cation channel gated by nicotinic cholinergic receptors

Nitrous Oxide (N₂O)

• Cheap and commonly used anesthetic.
• Low potency, but effective analgesic.
• Fast onset and recovery.
• Typically used in combination with other anesthetics (70% N₂O + 25-30% O₂ + 0.2-2% other potent anesthetic).
• Minimal impact on respiration, heart rate, and blood pressure.
• Uses: Surgical procedures, obstetric procedures, emergency medical situations.
• Adverse effects: Interacts and degrades vitamin B12, potential for pneumothorax, air embolus, obstructive issues.

Halothane

• Volatile liquid with a sweet odor and is non-irritating and non-explosive.
• Potent anesthetic, but poor analgesic.
• Dilates bronchial tubes, often preferred for asthmatic patients.
• Uses: Induction (2-4%) and maintenance (0.5-1%) of anesthesia.
• Adverse effects: Hepatitis, hyperthermia, hangover, shivering, jaundice, raised intracranial pressure.
• Contraindications include pre-existing conditions like raised cerebrospinal fluid pressure, or jaundice.

Thiopentone Sodium

• Ultrashort-acting barbiturate administered intravenously (IV).
• Rapidly induces hypnosis and anesthesia.
• Suppresses CNS activity, reducing muscle tone and reflexes, within minutes.
• Uses: Induction of anesthesia, control of convulsions, protection against cerebral ischemia (lack of blood flow to brain).
• Adverse effects include laryngospasm, shivering, and delirium during recovery.

Ketamine

• Dissociative anesthetic.
• Produces profound analgesia, immobility, and amnesia with light sleep.
• Heart rate and blood pressure increase due to sympathetic stimulation.
• Respiration not significantly suppressed; reflexes remain active.
• Emergence delirium, hallucinations, and involuntary movements can occur.
• Dangerous for patients with hypertension or ischemic heart disease (IHD).

Propofol

• Most commonly used IV anesthetic.
• Rapid onset (unconsciousness within 45 seconds).
• Duration of action approximately 15 minutes.
• Anti-emetic properties.
• Non-irritating to airways, suits day care surgery well.
• Adverse effects: Pain during injection, blood pressure fluctuations.

Preanesthetic Medication

• Drugs administered prior to general anesthesia to enhance safety and patient comfort.
• Relieves anxiety (benzodiazepines)
• Prevents allergies and nausea (antihistamines, antiemetics).
• Provides analgesia (opioids).
• Prevents acidity (proton pump inhibitor)
• Prevents bradycardia (atropine).