Case 15 — Text

Text from “Small Animal Cardiovascular Medicine”

Supraventricular Tachycardia

Definition and Causes

Supraventricular tachycardia (SVT) is defined as repetitive supraventricular premature depolarizations. They can originate from atrial myocardium or AV junctional tissue. They most commonly occur because of reentry, although automatic SVT can occur. Reentrant SVT produces a very constant heart rate, while automatic SVT from an atrial focus can be irregular. Few electrophysiologic studies have been performed in dogs, but response to therapy (most SVTs respond to calcium channel blockers) suggests reentry is also the most common mechanism in dogs. Most cases present with a very regular rhythm and respond to drugs that alter conduction in the AV junctional tissue.

ECG Findings

Most SVT in veterinary medicine occurs in dogs. The ectopic rate varies widely (150 to 350 bpm). Most SVTs are very regular (constant R-R interval). QRS complexes are typically narrow and upright in lead II. In some cases, a coexisting bundle branch block makes differentiation from ventricular tachycardia difficult. P waves may or may not accompany the QRS complexes.

Clinical Features

SVT commonly occurs secondary to organic heart disease or serious systemic disease. Dogs with slow SVT exhibit no clinical signs. Dogs with fast SVT (usually >250 bpm) generally present for weakness or collapse with evidence of poor peripheral perfusion — pale mucous membranes, prolonged capillary refill time, and weak pulses. This occurs because of inadequate diastolic filling.

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