chronic suppurative otitis media

ANAESTHESIA—LOCAL
P.Charters
Mode of Action
Local anaesthetic (LA) agents produce a reversible block in the transmission of impulses along nerve fibres.
They vary in terms of potency, toxicity, water solubility, ability to penetrate mucous membranes and
duration of action. When applied to different sites their efficacy is influenced by these properties and the
local blood supply can have implications for both duration of action and liability to toxic effects. Removal
from the site of action tends to be by the circulation, diffusion, metabolism or dilution which reduce local
tissue concentration of the LA to restore nerve function. Local anaesthetics are weak bases and the degree
of ionization depends on the individual drug’s pKa and the surrounding pH. At a low pH, more of the drug
remains in the ionized form, preventing it from diffusing across the axonal membrane and therefore
reducing efficacy.
There are two broad classes of LA:
Classification
• esters (e.g. benzocaine, cocaine and procaine);
• amides (e.g. lignocaine, prilocaine, bupivacaine, ropivacaine and levobupivacaine).
Complications
Complications associated with the use of local anaesthesia may not be common but can be life threatening
and for this reason their use should be restricted to areas where resuscitation facilities are to hand.
1. Toxicity. This depends on the dose of the agent and speed of uptake into the circulation. Where the
circulation is poor or when vasoconstrictors (e.g. adrenaline) are administered with the LA, larger doses can
be tolerated. For ENT surgery, toxicity most commonly results from inadvertent direct intravascular
injection or rapid absorption via mucous membranes and toxicity from any concomitant vasoconstrictor is
likely. The blood supply may be increased in infected tissue where the choice of local anaesthesia must be
questioned. Manifestations of toxicity usually start during drug administration or very shortly afterwards.
They include:
• CNS: lightheadedness, perioral paraesthesia, slurred speech, tinnitus, facial twitching, convulsions and
coma.