Introduction
General anesthetics are medicines that produce a reversible loss of consciousness and sensation, allowing major surgeries to be performed without pain or awareness. Unlike local anesthetics, which work only on a specific body part, general anesthetics act on the brain. They bring about unconsciousness, muscle relaxation, loss of reflexes and pain relief. Their action depends on dose and patient condition.
Mechanism of Action
General anesthetics work by depressing activity in the central nervous system. Although each drug has unique actions, most enhance the effect of GABA (an inhibitory neurotransmitter) or block excitatory pathways like NMDA receptors. This reduces neuronal firing, leading to unconsciousness, amnesia and immobility.
Stages of General Anesthesia
According to Guedel’s classification, anesthesia develops in four stages:
- Stage I – Analgesia: Patient is conscious but experiences reduced pain. Memory and awareness may start to fade.
- Stage II – Excitement: Patient becomes unconscious but may show irregular breathing, movements, coughing or vomiting. Modern IV agents reduce this stage.
- Stage III – Surgical Anesthesia: Desired stage for surgery. Breathing becomes regular, reflexes disappear and muscles relax.
- Stage IV – Medullary Paralysis: Overdose stage where vital centers fail. Immediate support is required.
Classification of General Anesthetics
- Inhalational anesthetics: Nitrous oxide, Halothane, Isoflurane, Sevoflurane, Desflurane, Enflurane
- Intravenous (IV) anesthetics: Thiopentone, Propofol, Etomidate, Ketamine, Benzodiazepines (Midazolam), Opioids (Fentanyl)
Inhalational Anesthetics
These are gases or vapors inhaled through the lungs. They maintain anesthesia during long surgeries. Their onset depends on solubility, blood flow and ventilation rate.
Nitrous Oxide (N2O)
- Fast onset and recovery
- Poor muscle relaxation
- Used with other agents for balanced anesthesia
- Can cause diffusion hypoxia after discontinuation
Halothane
- Potent anesthetic with smooth induction
- Good bronchodilation
- Risk of hepatotoxicity (halothane hepatitis)
- Causes arrhythmias and hypotension
Isoflurane
- Commonly used inhalational agent
- Minimal effect on heart rhythm
- Causes vasodilation and hypotension
Sevoflurane
- Rapid and smooth induction
- Preferred in children
- Minimal airway irritation
Desflurane
- Very fast induction and recovery
- Airway irritation common
- Useful for outpatient and short procedures
Intravenous Anesthetics
IV anesthetics induce rapid unconsciousness and are often used for induction before shifting to inhalational agents.
Thiopentone Sodium
- Ultra-short acting barbiturate
- Strongly induces sleep within seconds
- Causes respiratory depression
- Not analgesic
Propofol
- Most commonly used IV anesthetic
- Rapid recovery and anti-emetic effect
- Causes hypotension and respiratory depression
Etomidate
- Minimal cardiovascular depression
- Used in cardiac patients
- May cause adrenal suppression with repeated use
Ketamine
- Produces “dissociative anesthesia”
- Strong analgesic effect
- Increases heart rate and blood pressure
- Causes hallucinations during recovery
- Useful in asthmatic patients due to bronchodilation
Benzodiazepines (e.g., Midazolam)
- Used for premedication and procedural sedation
- Provide amnesia and mild sedation
- Can cause respiratory depression if combined with opioids
Pre-Anesthetic Medication
These medicines are given before anesthesia to reduce anxiety, secretions and side effects.
- Anxiolytics: Diazepam, Midazolam
- Analgesics: Opioids
- Anticholinergics: Atropine to reduce salivation
- Anti-emetics: Ondansetron
- H2 blockers: Ranitidine
Adverse Effects of General Anesthetics
- Respiratory depression
- Low blood pressure
- Nausea and vomiting
- Heart rhythm disturbances
- Malignant hyperthermia (rare but serious)
Detailed Notes:
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