Vaughan Williams classification, of antiarrhythmic

Describe the modified Vaughan-Williams classification of antiarrhythmic drugs, and compare and contrast the effects of available antiarrhythmic drugs on ventricular conduction velocity, refractory period, automaticity, and inhibition of specific myocardial ion channels. 

The Vaughan-Williams classification of antiarrhythmic drugs has been criticized for a number of reasons. The classification is based on the effects of drugs on normal, rather than diseased, myocardium. In addition, many of the drugs may be placed into more than one class. For example, the class IA drugs prolong repolarization/refractoriness, either via the parent drug8,9 or an active metabolite,10 and therefore also maybe placed in class III. Sotalol is also a 3-blocker, and therefore fits into class II. Amiodarone inhibits sodium and potassium channels, is a non-competitive inhibitor of 3-receptors, and inhibits calcium... 

Antiarrhythmic Drugs. Table 1 Classification of antiarrhythmic drugs according to Vaughan-Williams... 

Blockers are antiarrhythmics of class II according to the Vaughan-Williams classification, effective in the treatment of both supraventricular and ventricular tachyarrhythmias. These drugs can also reduce ectopic beats, especially if they are a result of sympathetic activity. Sotalol is a racemic mixture of the -blocking L-isomer and the class III antiarrhythmic D-isomer. This racemic mixture as well as D-sotalol are used as class Ill-antiarrhythmic. 

The class IV-antiarrhythmics are the calcium antagonists, but remain limited to verapamil and possibly also diltiazem. The dihydropyridines (nifedipine and related compounds) are unsuitable for antiarrhythmic therapy. The antiarrhythmic activity of verapamil and diltiazem is based upon the impairment of AV conduction and heart rate. A few compounds may be considered to act as antiarrhyth-mics, but they are not included in the Vaughan-Williams classification. 

Vaughan Williams EM. A classification of antiarrhythmic actions reassessed after a decade of new drugs. J Clin Pharmacol 1984 24(4) 129-47. 

The most frequently used classification system is that proposed by Vaughan Williams (Table 6-1). Type la drugs slow conduction velocity, prolong refractoriness, and decrease the automatic properties of sodium-dependent (normal and diseased) conduction tissue. Type la drugs are broad-spectrum antiarrhythmics, being effective for both supraventricular and ventricular arrhythmias. 

Fig. 6. Influences of different types of antiarrhythmic agents (Vaughan-William s classification) on the shape of cardiac action potentials. First row Class I-agents action potentials of ventricular myocardial cells. Second row (from left to right) Action potential of SA-node cells influence of a )0-hlocker (class II). Action potential of ventricular myocardial cells influence of a class Ill-antiarrhythmic. Action potential of AV nodal cells influence of a class IV-antiarrhythmic (verapamil, diltiazem).

Table 2. Examples of antiarrhythmic drugs (Vaughan-William s classification)...

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