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Cardiac Arrhythmias Quiz
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According to the Vaughan-Williams classification, which class of antiarrhythmic drugs is defined mainly by blocking potassium channels?
- A Class III antiarrhythmics You picked ✓
- B Drugs outside the classification, such as digoxin and adenosine You picked ✕
- C Class II antiarrhythmics You picked ✕
- D Class I antiarrhythmics You picked ✕
- E Class IV antiarrhythmics You picked ✕
Correct!
Incorrect
The answer is A: Class III antiarrhythmics
Explanation
Why A is correct
See The Vaughan-Williams Classification. Option A is correct because the Vaughan-Williams classification groups antiarrhythmic drugs into four main classes by their dominant effect on the heart cell, and class III is the group of potassium channel blockers. The MSD Manual describes class III drugs as primarily potassium channel blockers that prolong the action potential duration and refractoriness (the period when a cell cannot fire again). Potassium leaving the cell drives repolarisation (phase 3), the return of the cell to its resting voltage, so blocking potassium channels delays it. Amiodarone and sotalol are examples. Class I drugs are sodium channel blockers, class II are beta-blockers and class IV are the non-dihydropyridine calcium channel blockers. Digoxin, adenosine and ivabradine were not part of the original system. The mnemonic Some Block Potassium Channel DAAM gives the order: sodium (I), beta-blockers (II), potassium (III), calcium channel blockers (IV), then digoxin, atropine, adenosine and magnesium, which sit outside the four classes. Several drugs have actions in more than one class, and amiodarone, for example, also has class I, II and IV effects.
- Class III: potassium channel blockers.
- Effect: prolong the action potential and refractoriness.
- Phase: delays repolarisation (phase 3).
Why the other options are incorrect
- B. Drugs outside the classification, such as digoxin and adenosine: Digoxin and adenosine were not part of the original classification, and they do not work mainly by blocking potassium channels. Adenosine actually opens potassium channels in nodal cells. Your answer
- C. Class II antiarrhythmics: Class II drugs are beta-blockers. They reduce sympathetic stimulation, and they act mainly on the pacemaker tissue of the nodes, not on potassium channels. Your answer
- D. Class I antiarrhythmics: Class I drugs block sodium channels, not potassium channels. They slow the upstroke of the action potential and so slow conduction in working heart muscle. Your answer
- E. Class IV antiarrhythmics: Class IV drugs are calcium channel blockers such as verapamil and diltiazem. They act mainly on the nodes, where depolarisation depends on calcium. Your answer
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1
Which of the following best describes the primary electrophysiological effect of class I antiarrhythmic drugs on cardiac muscle cells?
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2
A 19-year-old woman is given an intravenous non-dihydropyridine calcium-channel blocker for symptomatic supraventricular tachycardia. Which phase and ionic current of the sinoatrial node action potential does this drug class mainly inhibit?
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