Cardiac arrhythmias reentry

O Cardiac arrhythmias may be caused by abnormal impulse formation (automaticity), abnormal impulse conduction (reentry), or both. 

It is estimated that 80 to 90% of clinical arrhythmias have a reentry mechanism. One explanation of how an antiarrhythmic agent may abolish reentry is by converting unidirectional block to bidirectional block. A second mechanism to explain the action of antiarrhythmic drugs is that they can prevent reentry by increasing the ERP of the cardiac hbers within or surrounding the region of the reentrant circuit. 

Verapamil and diltiazem are prototypic calcium channel blockers. As indicated previously, these drugs influence cardiac function by blocking inward calcium movement through L channels. In so doing they block conduction velocity in SA and AV node cells. They are used therapeutically to treat reentry arrhythmias through the AV node as well as paroxysmal supraventricular tachycardias. In fact, verapamil has been reported to terminate 60-80 percent of paroxysmal supraventricular tachycardias within several minutes. However, because of their potent effect on AV conduction, these drugs are contraindicated in patients with preexisting conduction problems since they may produce complete AV block. 

Correct answer = A. A bidirectional block can decrease arrhythmias caused by reentry. All antiarrhythmic drugs exert some negative inotropic effect and decrease cardiac output. The i side effects of this group of drugs are serious and include arrhythmias that can lead to sudden death. Some antiarrhythmic drugs affect K+ or Ca++ channels, or p adrenoreceptors. 

C. Clinical Use and Toxicities Calcium channel blockers are effective for converting atrioventricular nodal reentry (also known as nodal tachycardia) to normal sinus rhythm. Their major use is in the prevention of these nodal arrhythmias in patients prone to recurrence. These drugs are orally active verapamil is also available for parenteral use (Table 14—2). The most important toxicity of verapamil is excessive pharmacologic effect, since cardiac contractility, AV conduction, and blood pressure can be significantly depressed. See Chapter 12 for additional discussion of toxicity. Amiodarone has moderate calcium channel-blocking activity. 

The procedure causes immediate depolarization, interrupting reentry circuits (abnormal impulse conduction that occurs when cardiac tissue is activated two or more times, causing reentry arrhythmias) and allowing the SA node to resume control. 

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