CHAPTER 22. LOCAL ANAESTHETICS
Indications180
Dosage180
Complications – lidocaine toxicity180
Contraindications181
Practical procedure181
Sigmund Freud was aware of the analgesic properties of cocaine, but its anaesthetic properties were recognized by his colleague, the Austrian ophthalmologist Karl Koller (1857–1944). He successfully performed eye surgery utilizing the local anaesthetic properties of cocaine in 1884, a significant breakthrough in the developing field of anaesthesia.
INTRODUCTION
Local anaesthetic agents have varying onset times, duration of action and ability to penetrate tissue. Chemically they are separated into esters (cocaine, procaine, amethocaine) and amides (lidocaine, prilocaine, bupivacaine), which is relevant to their metabolism and tendency to toxicity. Esters are rapidly metabolized by serum and liver cholinesterase and therefore metabolism is retarded by low enzyme levels such as pregnancy or liver disease. Amides are metabolized by liver microsomal enzymes, which may be less effective when hepatic function or blood flow are reduced.
Local anaesthetics work by reversibly blocking conduction along nerve fibres through binding to voltage-gated sodium channels in the neuronal membrane. This prevents sodium ion entry during depolarization, which in turn prevents the propagation of the action potential. The membrane in effect becomes stabilized. Consequently, the response to painful stimuli is not effected and pain is not sensed centrally.
Local anaesthetics are weak bases and are most efficacious in alkaline conditions, whereby the un-ionized form crosses the neuronal membrane. Conversely, infected or inflamed tissues have an acidic environment, which reduces the action of local anaesthetics. Furthermore, the increased vascularity of these environments gives rise to an increased risk of systemic absorption and subsequent side-effects. Local anaesthetics affect the smaller nerves such as the autonomic before the medium-sized sensory and large motor nerves. They are more effective at higher concentrations, having a shorter onset and longer duration. The duration of local anaesthetics is prolonged when combined with vasoconstrictors such as weak epinephrine solutions (e.g. 1 in 200, 000).
INDICATIONS
These anaesthetics provide local anaesthesia for painful procedures. The most commonly used local anaesthetic for ward-based procedures is lidocaine (also known as lignocaine). Various lidocaine preparations are available:
• Topical cream – anaesthesia before minor skin procedures, e.g. cannulation.
• Topical gel – e.g. catheterization.
• Injection – e.g. central line insertion, chest and ascitic drain insertion, lumbar puncture, suturing.
• Injection in combination with epinephrine prolongs the local anaesthetic effect by reducing blood flow and thus the rate of systemic absorption. However, this is usually required in specialist circumstances such as facial or scalp anaesthesia, and is not recommended for use in the procedures described in this book.
DOSAGE
The concentration of lidocaine (mg/mL) equals the percentage concentration multiplied by 10. For example:
• 1% lidocaine = 10 mg/mL.
• 2% lidocaine = 20 mg/mL.
The maximum dose of lidocaine (without vasoconstrictor agent) can be calculated as follows:
• For example, for an individual weighing:
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