Pharmacology for Dentistry

192 Section 4/ Drugs Acting on Cardiovascular & Urinary System

digitalised patients, arrhythmias associated
with Wolff-Parkinson-White (WPW) syn-
drome.

LIGNOCAINE

It is an amide local anaesthetic and has
rapid onset of action. It depresses diastolic
depolarization and automaticity in ectopic
foci in ventricular tissue. Phase 4 depolar-
ization in partially depressed Purkinje fibres
and after depolarizations are antagonised.
It does not depress AV conduction and de-
creases action potential duration, effective
refractory period. It has no effect on BP.

Adverse effects include ventricular fi-
brillation, hypotension or massive cardiac
arrest due to overdose, dizziness, paraesthe-
sia, drowsiness, seizures, disorientation, res-
piratory arrest, nausea, vomiting, circulatory
collapse and blurred vision.

It is indicated in prophylaxis or treat-
ment of ventricular arrhythmias associated
with Ml, digitalis intoxication, ventricular
tachyarrhythmia, in patients predisposed to
ventricular arrhythmias during general ana-
esthesia.

PHENYTOIN SODIUM

It is a anticonvulsant drug and de-
presses the ventricular automaticity and
accelerates the AV conduction. It also re-
duces the duration of action potential like
quinidine. It also shortens the QT interval.
It mainly blocks inactivated Na+ channels.
It is used for the suppression of ectopic
beats and for prophylaxis of recurrent par-
oxysmal tachycardia and also for the treat-
ment of rapid supraventricular or ven-
tricular tachycardia.

It is also used in digitalis induced ven-

tricular arrhythmia as it reverse the conduc-

tion block while accentuating the depression

of automaticity (The detailed pharmacology

is discussed in chapter ‘Antiepileptic agents’).

BETA-ADRENEGIC BLOCKERS

The detailed pharmacology of beta blockers

is discussed in chapter ‘Adrenergic block-

ing agents’ and ‘Antihypertensive agents’.

The antiarrhythmic action is due to

cardiac adrenergic blockade. It decreases the

slope of phase 4 depolarization and

automaticity in SA node, Purkinje fibres and

other ectopic foci. It also prolongs the

effective refractory period of AV node and

impedes AV conduction. ECG shows

prolonged PR interval. It is useful in sinus

tachycardia, atrial and nodal extrasystoles.

It is also useful in sympathetically mediated

arrhythmias in pheochromocytoma and

halothane anaesthesia.

DRUGS PROLONGING ACTION POTENTIAL

By prolonging repolarization, AP is wid-

ened and ERP is increased, so the

tissues remain refractory even after full

repolarization.

AMIODARONE

It is a long acting antiarrhythmic drug.

It contains iodine and may cause disorders

of thyroid function.

It blocks inactivated sodium channels.

It also decreases calcium current and tran-

sient outward, delayed rectifier and inward

rectifier potassium currents. It is a potent

inhibitor of abnormal automaticity. It pro-

longs duration of action potential, refracto-