General and Local Anaesthesia

• Anaesthesia is defined by the loss of all sensation
• It creates a reversible condition of comfort and quiescence for a patient within the physiological limit
• There are 2 primary types of anaesthetics: general and local

General Anaesthesia (GA)

• Achieves amnesia, analgesia, and possibly muscle paralysis through medication
• Drugs used are called general anaesthetics

Local Anaesthesia (LA)

• Induces the absence of sensation in a specific body part for procedures like surgery or dental work
• Drugs used are called local anaesthetics

Differences Between GA & LA

• General anaesthesia acts on the CNS, while local anaesthesia acts on peripheral nerves
• General anaesthesia affects the whole body, local anaesthesia only a restricted area
• Consciousness is lost with general anaesthesia but remains unaltered with local
• Vital functions require essential care with general anaesthesia but are usually not needed with local
• General anaesthesia is risky for poor health patients, local anaesthesia is safer
• General anaesthesia is possible for use in non-cooperative patients, local not possible
• General anaesthesia is preferred for major surgery, local anaesthesia not preferred
• General anaesthesia is not preferred for minor surgery, local anaesthesia preferred

Local Anaesthetics

• These drugs are used to block nerve conduction in peripheral nervous tissue, abolishing sensation without loss of consciousness
• Examples include lidocaine, cocaine, benzocaine, and procaine

Mechanism of Action for LA

• Local anaesthetics reversibly inhibit action potential conduction by binding to sodium channels
• This binding decreases the nerve membrane's permeability to sodium

Features of LA

• Should have a quick onset of action
• Should not irritate skin and mucous membranes
• The duration of action must be long enough to allow the completion of the desired procedure
• Should be effective on both injection and local application
• Should have low systemic toxicity
• Should not cause any permanent tissue damage
• Should be relatively free from producing allergic reactions
• Should be stable in solution and undergo biotransformation readily

Techniques of LA

• Surface anaesthesia involves applying a local anaesthetic to areas like the nose, mouth, throat, or tracheobronchial tree
• Infiltration anaesthesia involves injecting a local anaesthetic directly into tissue under the skin to sensory nerve endings
• Conduction block involves injecting a local anaesthetic around nerve trunks to anaesthetize and paralyse a distant area
• Field block is achieved by injecting a local anaesthetic subcutaneously around an area to block all nerves coming to a field
• A nerve block involves injecting a local anaesthetic around a nerve leading to the operative site
• Epidural anaesthesia involves analgesia or anaesthesia using the spinal dural space filled with fat for nerve root travel, often for surgical obstetrics
• Spinal anaesthesia involves injecting into the subarachnoid space between L2-3 or L3-4 of the spinal cord to anaesthetize the lower abdomen and hind limbs

General Anaesthesia

• Is produced by gradually depressing the CNS and is divided into four stages

Stages

• During Stage I, the cerebral cortex is gradually inhibited, resulting in decreased response to pain (analgesia), euphoria, and loss of consciousness (sleep)
• Stage II, the 'excitement phase', occurs when the cerebral cortex is fully depressed, leading to increased sympathetic tone, including increased blood pressure, heart rate, respiration, and muscle tone; cardiac arrhythmias may occur
• Stage III, or surgical anaesthesia, is when surgery is most efficiently performed, cardiovascular and respiratory functions return to normal, spinal reflexes are inhibited, and skeletal muscles are relaxed
• In Stage IV, or medullary paralysis, an overdose of general anaesthetic inhibits cardiovascular and respiratory centres in the medulla, leading to death

Mechanism of Action for GA

• Binds with the GABAα (inhibitory) receptor
• Causes chloride ions (CI) to move across the membrane into the cells
• This then causes the membrane to hyperpolarize
• Decreases neuronal membrane excitability where GABAα receptors are continually active

Examples of General Anaesthetics

• Inhalation anaesthetics such as volatile liquids like chloroform and halothane
• Inhalation anaesthetics such as gases like nitrous oxide
• Injectable anaesthetics like ketamine, midazolam, and propofol

This material covers the key distinctions between general and local anaesthesia. General anaesthesia affects the entire body and induces unconsciousness, while local anaesthesia targets specific areas and preserves consciousness. Both play crucial roles in medical procedures.