Pharmacology for Anaesthesia and Intensive Care

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14 Anti-arrhythmics

Mechanism of action

Flecainide prevents the fast Na+flux into cardiac tissue and prolongs phase 0 of the

action potential. It has no effect on the duration of the action potential or the refrac-

tory period. Its effects are particularly pronounced on the conducting pathways.

Side effects


Cardiac – flecainide may precipitate pre-existing conduction disorders and spe-

cial care is required when used in patients with SA or AV disease or with bun-

dle branch block. A paradoxical increase in ventricular rate may be seen in atrial

fibrillation or flutter. When used to suppress ventricular ectopic beats following

myocardial infarction it was associated with an increased mortality. Cardiac fail-

ure may complicate its use due to its negative inotropic effects. It raises the pacing

threshold.


Non-cardiac – dizziness, parasthesia and headaches may complicate its use.

Kinetics

Flecainide is well absorbed from the gut and has an oral bioavailability of 90%. It is

about 50% plasma protein bound and has a volume of distribution of 6–10 l.kg−^1.

Hepatic metabolism produces active metabolites, which along with unchanged drug

are excreted in the urine.

Procainamide

Procainamide has similar effects to quinidine but is less vagolytic.

Uses

Procainamide has been used to treat both SVT and ventricular tachyarrhythmias. It

is as effective as lidocaine in terminating VT. It may be given orally or intravenously.

The oral dose is up to 50 mg.kg−^1 .day−^1 in divided doses and the intravenous dose

is 100 mg slowly up to a maximum of 1 g. This may be followed by an infusion of

2–6 mg.min−^1 ,which should subsequently be converted to oral therapy.

Mechanism of action

Procainamide is a class Ia anti-arrhythmic and as such reduces the rate of rise of

phase 0 of the action potential by blocking Na+channels. In addition, it raises the

threshold potential and prolongs the refractory period without altering the duration

of the action potential. It also antagonizes vagal tone but to a lesser extent than

quinidine.

Side effects

These have limited its use.