Classifications, Vaughan-Williams

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. 

Vaughan-Williams Classification of Anti-arrhythmic Drugs... 

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... 

Adenosine and digoxin are agents used for the management of arrhythmias that do not fit into the Vaughan Williams classification. aSlows conduction, prolongs refractory period, and reduces automaticity in SA node and AV node tissue, but not in the ventricles. 

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. 

The Vaughan-Williams classification of anti-arrhythmic drugs is the most commonly used classification. Despite its many peculiarities the... 

There are many dmgs available to treat arrhythmias and the choice of drug often depends on the type of arrhythmia. Anti-arrhythmic drugs have been classified according to their mode of action into the so-called Vaughan-Williams classification. (See Table 4.2.)... 

Table 4.2 Vaughan-Williams classification of anti-arrhythmic drugs...

The Vaughan—Williams classification is of academic interest. Clinically, the type of arrhythmia against which the drugs are effective is of more importance than their mode of action. 

Class 1, II, III, and IV drugs A method for classifying antiarrhythmic drugs, sometimes called the Vaughan-Williams classification based loosely on the channel or receptor affected... 

The Vaughan Williams classification (shown as types in Table 8.1) and therapeutic ranges for cardiovascular drugs are summarized in Table 8.1. ... 

Vaughan Williams Classification and Therapeutic Ranges of Cardiovascular Drugs... 

This section is mainly concerned with the class I antiarrhythmics, which also possess some local anaesthetic properties, and with class IH antiarrhythmics. Antiarrhythmics that fall into other classes are dealt with under beta blockers , (p.833), digitalis glycosides , (p.903), and calcium-channel blockers , (p.860). Some antiarrhythmics that do not fit into the Vaughan Williams classification (see Table 9.1 , (below)) are also included in this section (e.g. adenosine). Interactions in which the an-tiarrhythmic drug is the affecting substance, rather than the drug whose activity is altered, are dealt with elsewhere. 

Table 9.1 Antiarrhythmics (modified Vaughan Williams classification) Class I Membrane stabilising drugs...

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