Drugs Used in Anaesthesia

Anaesthesia is defined as the absence of feelings, sensation, or pain. Anaesthetics are drugs that reduce or abolish sensation, affecting either the whole body (general anaesthetics) or a particular area or region (local anaesthetics).

Local Anaesthetics

Local anaesthetics provide brief periods of anaesthesia in a small localized area of the skin and adjacent tissues. They may be administered in two ways: topically for surface anaesthesia and by injection for infiltration anaesthesia.

Topical anaesthetics are usually applied to the skin or the mucous membrane to relieve itching, insect bites, hemorrhoids, pruritus, and minor surgical procedures.

Infiltration anaesthesia may be achieved by injecting a local anaesthetic into the immediate area of surgery. It is commonly used during dental extraction and biopsies.

Examples:

• Lidocaine (lignocaine)
• Bupivacaine
• Mepivacaine

Note: Lignocaine is sometimes combined with epinephrine (adrenaline), a powerful vasoconstrictor, that decreases blood flow to the tissue where it is injected. Adrenaline controls bleeding and also prolongs the anaesthetic action of lignocaine.

General Anaesthetics

General anaesthetic drugs are normally given IV or by inhalation to produce rapid, reversible loss of consciousness and insensibility to surgical stimuli.

Examples: 

Inhaled Anaesthetics:

• Halothane
• Nitrous oxide
• Ether

Intravenous Anaesthetics:

• Ketamine
• Midazolam
• Propofol

Ketamine

Available Preparations:

• Injection: 50 mg/ml

Available Brands: Ketajex®, Ketalar®

Pharmacokinetics: Ketamine is rapidly and well absorbed after IM injection, rapidly enters the CNS, metabolized by the liver, and excreted in urine.

Indications:

• Induction and maintenance of anaesthesia
• Pain relief
• Diagnostic maneuvers and procedures not involving intense pain

Contraindications:

• Thyrotoxicosis
• Hypertension (including pre-eclampsia)
• History of cerebrovascular accident
• Raised intracranial pressure
• Psychiatric disorders, particularly hallucinations
• Severe cardiac disease
• Recent myocardial infarction
• Stroke
• Known hypersensitivity to ketamine
• Cerebral trauma
• Eye injury

Dosage:

Induction:

• Intravenous Injection: 1-4.5 mg/kg (2 mg/kg usually produces anaesthesia lasting 5-10 minutes)
• Intramuscular Injection: 6.5-13 mg/kg (duration of anaesthesia up to 25 minutes)
• IV Infusion: 0.5-2 mg/kg initially, then infuse at 10-45 mcg/kg/minute, adjust according to response

Maintenance:

• Intravenously: Increments of half or full dose repeated as required
• Analgesic for painful procedures: IV 1-1.5 mg/kg slowly over 2-5 minutes. Give half dose every 10 minutes if required for prolonged procedures

Administration Instructions:

• Dilute dose with an equal volume of water for injection, sodium chloride 0.9%, or glucose 5% before IV injection
• Give IV slowly; rapid administration may result in respiratory depression and enhanced hypertensive response

Side Effects:

• Raised blood pressure and pulse rate
• Increased muscle tone
• Lacrimation
• Hypersalivation
• Raised intracranial pressure
• Redness of the skin
• Postoperative nausea and vomiting
• Pain on injection
• Irrational behavior during recovery

Drug Interactions:

• Inhalation anaesthetics such as halothane may prolong the effect of ketamine and delay recovery
• Prolonged recovery occurs when barbiturates or opioids are given concurrently with ketamine
• Ketamine should not be used with ergometrine
• Concomitant use with thyroid hormones may cause hypertension and tachycardia

Key Issues to Note:

• Warn the patient to avoid tasks requiring motor coordination and/or mental alertness for 24 hours after anaesthesia
• Keep verbal, tactile, and visual stimulation to a minimum during induction and recovery