31. Anaesthesia. Kinds of anaesthesia

Topic 31: Anaesthesia – Kinds of Anaesthesia

Definition

Anaesthesia is a reversible state of loss of sensation, with or without consciousness, induced for surgical or diagnostic purposes.

It can also involve loss of pain, reflexes, muscle tone, and memory.

Classification

1. General Anaesthesia

• Induces complete unconsciousness.
• Patient is unresponsive and unaware.
• Requires airway protection (endotracheal tube or mask).
• Common agents: Propofol (IV), Sevoflurane (inhaled), opioids, muscle relaxants.

2. Local Anaesthesia

• Blocks nerve conduction at the site of application.
• No effect on consciousness.
• Used for minor procedures (e.g. suturing, dental). Drugs:
• Lidocaine – Fast-acting, short duration, Surface Anaesthesia – Applied on skin or mucosa
• Bupivacaine – Long-acting, used in nerve blocks/spinal
• Articaine – Excellent for dental use
• Prilocaine – Used in EMLA cream (topical mix with lidocaine)

3. Regional Anaesthesia

• Blocks nerve supply to a larger body region (e.g. limb).
• Consciousness is preserved.
• Includes spinal and epidural techniques.

Peripheral Nerve Block

Blocks specific nerves or plexuses (outside spinal canal)

Examples:

• Brachial plexus block – for upper limb surgery
• Femoral nerve block – for knee/hip procedures

Spinal Anaesthesia

• Injection into subarachnoid space (usually L3–L4).
• Fast onset, used for lower body surgeries.
• Complications: hypotension, post-spinal headache.

Epidural Anaesthesia

• Drug injected into the epidural space.
• Slower onset, adjustable dose, common in labor analgesia.
• Unlike spinal, no CSF leakage occurs.

Inhalational Anaesthetics - General

These agents are inhaled via mask or endotracheal tube and absorbed through the alveoli into blood.

From pulmonary circulation, they reach the brain and induce anaesthesia.

Their effect is based on achieving a sufficient concentration in the CNS, though exact mechanisms remain unclear.

Minimum Alveolar Concentration (MAC)

MAC is the alveolar concentration that prevents movement in 50% of patients during surgery exposed to surgical stimulus.

It reflects anaesthetic potency: lower MAC = higher potency.

MAC is additive and influenced by factors like age, medications, and temperature.

Factors decreasing MAC: old age, hypothermia, sedatives

Factors increasing MAC: fever, chronic alcohol use

Nitrous Oxide (N₂O)

• Colorless, odorless gas. Stored as a liquid under pressure.
• Combined with volatile agents to reduce their dose
• Weak anaesthetic but strong analgesic (at >20%).
• Should not exceed 70% to avoid hypoxia.
• Not metabolized; excreted unchanged by lungs.

Special effects:

• Concentration effect: N₂O is more soluble than nitrogen → more of it diffuses into blood
• This shrinks the alveolar volume, increasing concentration of remaining gases
• → Leads to faster onset of action
• Second gas effect: When combined with another gas (e.g. isoflurane), speeds up its onset.

Volatile Anaesthetic Agents