General Anesthetics: Classification, Mechanism, Therapeutic Uses, and Types of Anesthesia

Classification of General Anesthetics

• Inhalational Anesthetics
  • Volatile Liquids: Halothane, Isoflurane, Sevoflurane, Desflurane
  • Gases: Nitrous oxide, Xenon
• Intravenous Anesthetics
  • Barbiturates: Thiopental sodium
  • Non-Barbiturates: Propofol, Etomidate, Ketamine, Dexmedetomidine
  • Benzodiazepines: Midazolam, Diazepam
  • Opioid Analgesics: Fentanyl, Remifentanil

Mechanism of General Anesthetics

General anesthetics work by depressing the central nervous system, leading to a reversible loss of consciousness, analgesia, muscle relaxation, and amnesia. Their mechanisms include:

• Enhancement of Inhibitory Neurotransmission: Many anesthetics (e.g., propofol, benzodiazepines) enhance the action of GABA on GABA_A receptors, increasing chloride ion flow, hyperpolarizing neurons, and reducing excitability.
• Inhibition of Excitatory Neurotransmission: Agents like ketamine inhibit glutamate neurotransmission through NMDA receptors, reducing excitatory signaling.
• Membrane Stabilization: Some inhalational anesthetics affect ion channels and lipid membranes, altering neuronal excitability and reducing communication between neurons.

Therapeutic Uses of General Anesthetics

• Induction and Maintenance of Anesthesia: Used in surgeries for unconsciousness and loss of sensation.
• Sedation for Procedures: Short-acting agents like propofol are used for minor procedures.
• Emergency Management: Agents like ketamine are used for short, painful procedures, especially when cardiovascular stability is needed.

Adverse Effects of General Anesthetics

• Cardiovascular Effects: Hypotension, arrhythmias, and myocardial depression, especially with agents like halothane and propofol.
• Respiratory Depression: Reduced respiratory rate and volume, with potential for apnea.
• Nausea and Vomiting: Common post-operatively, particularly with volatile agents.
• Neurotoxicity: Cognitive dysfunction, particularly in the elderly.
• Other Effects: Hepatotoxicity (halothane), nephrotoxicity (methoxyflurane), and malignant hyperthermia (triggered by volatile anesthetics).

Balanced Anesthesia

Balanced anesthesia uses a combination of drugs to achieve anesthesia’s desired effects—amnesia, analgesia, muscle relaxation, and unconsciousness—while minimizing doses to reduce side effects. Typically involves:

• Induction Agent (e.g., propofol or thiopental for rapid induction)
• Analgesic (e.g., fentanyl for pain relief)
• Muscle Relaxant (e.g., rocuronium)
• Maintenance Agent (e.g., sevoflurane or isoflurane)

This approach allows precise anesthesia control and reduces adverse effects from high doses of a single agent.

Dissociative Anesthesia

Dissociative anesthesia is mainly induced by ketamine, creating a trance-like state where the patient appears detached but conscious. Characterized by:

• Profound Analgesia and Amnesia: The patient is unresponsive to pain but conscious, with no memory of the procedure.
• Minimal Respiratory Depression: Ketamine preserves airway reflexes and breathing, ideal for emergencies.
• Increased Sympathetic Activity: Unlike other anesthetics, ketamine stimulates the cardiovascular system, increasing heart rate and blood pressure.