Case Studies In Small Animal

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Case 15

Supraventricular Tachycardia Section from "Small Animal Cardiovascular Medicine" On-Line

Text from "Small Animal Cardiovascular Medicine"

Supraventricular Tachycardia

Definition and Causes

Supraventricular tachycardia (SVT) is defined as repetitive supraventricular premature depolarizations (SPDs). As with SPDs, SVT can originate from atrial myocardium or AV junctional tissue. They most commonly occur in humans because of reentry although automatic SVT can occur. Reentrant supraventricular tachycardia produces a very constant heart rate (the interval between QRS complexes is extremely regular). Supraventricular tachycardia due to an automatic focus in atrial myocardium can be irregular. Few electrophysiologic studies have been performed in dogs but response to therapy (most SVTs respond to calcium channel blocker administration) suggests that reentry is also the most common mechanism underlying SVT in the dog. In most humans, the reentrant pathway is within the AV node or uses the AV junctional tissue as part of the reentrant circuit. Again, this has not been studied in dogs. However, most cases of SVT that are seen in dogs present with a very regular rhythm and most are responsive to drugs that specifically alter conduction and refractoriness in the AV junctional tissue. Consequently, most cases of SVT in dogs are most likely also due to AV junctional reentry. One case report in the veterinary literature has described an electrophysiologic study in a dog with tricuspid valve dysplasia and an incessant atrial tachycardia which was due to reentry in the right atrium.17

ECG Findings

Most SVT in veterinary medicine occurs in dogs. The rate of the ectopic focus varies widely in dogs. We have seen rates as low as 150 beats/minute and as high as 350 beats/minute. On an ECG, most SVTs are very regular (the R-R interval is constant) and many occur for prolonged periods. Some dogs, however, can have very short runs of SVT. Rarely, the rhythm will be irregular. In most cases, the QRS complexes are typical of supraventricular complexes being narrow and upright in lead II. In some cases there is a coexisting bundle branch block that makes it difficult to impossible to differentiate an SVT from a ventricular tachycardia by examining the ECG. As with SPDs, there may or may not be P waves accompanying the QRS complexes and, if present, the P waves can be positive or negative in lead II.

Clinical Features

Supraventricular tachycardia commonly occur secondary to organic heart disease or to serious systemic disease. Dogs with slow SVT exhibit no clinical signs. Dogs with fast SVT (heart rate usually > 250 beats/minute) generally present for weakness or collapse. On physical examination they have evidence of poor peripheral perfusion with pale mucous membranes, a prolonged capillary refill time, and weak pulses. This occurs because of inadequate diastolic filling. When viewed on an echocardiogram during bursts of SVT, the left ventricle has a normal end-systolic diameter and a very small end-diastolic diameter resulting in a decreased shortening fraction because of inadequate filling.

Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.