190 Section 4/ Drugs Acting on Cardiovascular & Urinary Systemb. Automaticity: Quinidine decreases the
slope of slow diastolic depolarisation
(phase 4 of action potential) and thus
decreases the spontaneous rate of fir-
ing of pacemakers. By depressing the
entry of sodium into the cell during
depolarization, quinidine depresses
diastolic depolarization and ultimately
automaticity.
c. Conductivity: Quinidine depresses in-
terventricular and atrioventricular con-
ductivity. PR and QRS intervals are
also prolonged. Also decreases the rate
of rise of action potential.
d. Effective refractory period: Quini-
dine depresses the potassium efflux
during repolarization and prolongs
repolarization. The refractory period
increases due to its antivagal action.Its antivagal action prolongs the re-
fractory period of atrium and short-
ens that of AV node. The antivagal
action on the AV node causes para-
doxical tachycardia in a patient of
atrium fibrillation.
e. Contractility: Quinidine produces a
negative inotropic action on the heart
and contractility is depressed with
toxic doses.
f. AV conduction: Quinidine depresses
the conduction in atrium and Purkinje
system.
g. Electrophysiological effect (effect on
ECG):- It reduces the rate of rise of action
potential i.e. phase zero of action
potential which is due to
depolarisation.
Table 4.4.1: Classification of antiarrhythmic agents.
I. Class I: Sodium channel blockers
Quinidine (NATCARDINE) 200-400 mg TDS-QID
Procainamide (PRONESTYL) 50 mg/kg/day oral, 50 mg/min slow IV
Disopyramide (NORPACE) 100-200 mg TDS-QID, 2 mg/kg slow IV
Lignocaine (XYLOCARD) 1 mg/kg slow IV (bolus) then 1-3 mg/min IV infusion
Phenytoin sodium (DILANTIN) 100-400 mg/day oral, 100 mg IV (max 600 mg/day)
II.Class II: Beta adrenergic blockers
Propranolol, etc. (Detailed pharmacology is discussed in chapter ‘Antihypertensive agents’ and Adrenergic
blocking agents’).
III.Class III: Drugs that prolong effective refractory
period by prolonging action potential
Amiodarone (ALDARONE) 200 mg TDS
Bretylium 5-10 mg/kg bolus IV then 0.5-2 mg/min IV infusion
IV.Class IV: Calcium channel blockers
Verapamil, diltiazem etc. (Detailed pharmacology is discussed in chapter ‘Antihypertensive agents’).
V.Miscellaneous
Adenosine (ADENOCOR) 6-12 mg IV bolus.
Atropine 0.6-2 mg IM (Used for AV block)
Sympathomimetics (e.g. adrenaline, isoprenaline and orciprenaline are also used for AV block).
Digitalis (DIGOXIN) 0.25-0.5 mg IV for paroxysmal supraventricular
tachycardia (PSVT), atrial flutter and atrial fibrillation