(Mode of Action of Drugs)

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Chapter

1.4

Chapter

4.4 Antiarrhythmic Agents

CARDIAC ARRHYTHMIAS

Cardiac arrhythmias is a group of dis-
order characterized by an abnormal cardiac
rhythm and arise as a result of disorders of
impulse formation or conduction or both.

Tachyarrhythmias (sinus rate more
than 100 per minute) are produced by a
disturbances of impulse generation or of
impulse conduction in the heart.
Tachyarrhythmias due to disturbed im-
pulse formation are associated with ir-
regular and rhythmic discharge from ec-
topic pacemaker activity in areas of the
heart other than the SA node. The charac-
teristic of myocardial cells, which enables
them to generate spontaneous depolariza-
tion, is called automaticity.

Bradycardia can be due to depressed
sinus automaticity and AV block.
Bradyarrhythmias manifest as slow heart
rate (less than 50 to 60 beats per minute in
sleep). Depressed SA nodal automaticity
lead to missing beats and bradycardia. AV
block can be due to high vagal activity and
side effect of certain drugs e.g. digitalis and
β-blockers.

Antiarrhythmic drugs can be classified

as in table 4.4.1.

SODIUM CHANNEL BLOCKERS

By limiting the conductance of Na+ (and K+)

across cell membrane, they interfere with

depolarization and decrease responsiveness

to excitation thereby reducing rate of phase

of phase 4 depolarisation in automatic cells.

QUINIDINE

It is an alkaloid obtained from the

bark of cinchona and is a dextro isomer

of anti-malarial drug ‘quinine’. Its so-

dium channel blocking property results

in an increased threshold for excitability

and decreased automaticity. As a conse-

quence of its potassium channel blocking

properties, it prolongs action potential in

most cardiac cells.

Pharmacological Actions

Cardiac actions:

a. Excitability: Quinidine depresses the

excitability of cardiac tissues.

Antiarrhythmic

Agents