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Case 15
 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.
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