McKenna's Pharmacology for Nursing, 2e

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P A R T 4  Drugs acting on the central and peripheral nervous systems

Pharmacokinetics The ester local anaesthetics are broken down imme- diately in the plasma by enzymes known as plasma esterases . The amide local anaesthetics are metabolised more slowly in the liver, and serum levels of these drugs (i.e. active metabolites) can increase and lead to toxicity. Contraindications and cautions The local anaesthetics are contraindicated with any of the following conditions: history of allergy to any one of these agents or to parabens to avoid hypersensitivity reactions ; heart block, which could be greatly exacer- bated with systemic absorption ; shock, which could alter the local delivery and absorption of these drugs ; and decreased plasma esterases, which could result in toxic levels of the ester-type local anaesthetics. They should be used during pregnancy and breast- feeding only if the benefit outweighs any potential risk to the fetus or neonate that could occur if the drug is inadvertently absorbed systemically because of the sup- pressive effects on nerves. Adverse effects The adverse effects of these drugs may be related to their local blocking of sensation (e.g. skin breakdown, self- injury, biting oneself). Loss of skin integrity is always a problem if the person is unable to move, and care must

Local anaesthetics are classified as esters or amides. The agent of choice depends on the method of admini­ stration, the length of time for which the area is to be anaesthetised and consideration of potential adverse effects. Esters include benzocaine ( Applicaine, Cepacol ) and amethocaine (generic). Amides include bupiv- acaine ( Marcain ), cinchocaine ( Proctosedyl, Rectinol, Scheriproct ), levobupivacaine ( Chirocaine ), lignocaine ( Xylocaine ), mepivacaine ( Scandonest ), prilocaine ( Citanest ) and ropivacaine ( Naropin ). Therapeutic actions and indications Local anaesthetics work by causing a temporary inter- ruption in the production and conduction of nerve impulses. They affect the permeability of nerve mem- branes to sodium ions, which normally infuse into the cell in response to stimulation. By preventing the sodium ions from entering the nerve, they stop the nerve from depolarising. A particular section of the nerve cannot be stimulated, and nerve impulses directed towards that section are lost when they reach that area. The way in which a local anaesthetic is adminis- tered helps to increase its effectiveness by delivering it directly to the area that is causing or will cause the pain, thereby decreasing systemic absorption and related toxic effects (Figure 27.2). Local anaesthetics are indicated for infiltration anaesthesia, peripheral nerve block, spinal anaesthesia and the relief of local pain.

Without local anaesthetic

To brain and conscious perception of pain

Wound

Pain impulses in sensory nerve

Spinal cord

With local anaesthetic

Peripheral nerve block

Infiltration anaesthesia

No pain perceived

No pain perceived

Wound

Wound

No pain impulses

Pain impulses

No pain impulses

Spinal cord

A

B

FIGURE 27.2  Mechanism of action of local anaesthetics. Top. An injury produces pain impulses (action potentials) that are conducted and transmitted in an area of the brain in which pain is perceived. A. Conduction of the pain impulse has been blocked by infiltration anaesthetics at the site of the injury. B. A nerve block at some distance from the injury. Local anaesthetics block the movement of sodium into the nerve and prevent nerve depolarisation, stopping the transmission of the pain impulse.