Case 15 — Case Discussion

This dog has a supraventricular tachycardia (SVT) with a heart rate of approximately 300 beats/minute. The characteristics present include an extremely steady rate and narrow, normal-appearing QRS complexes. Most SVTs are due to reentry, where the reentrant pathway uses both atrial myocardium and the AV nodal region.

Diagram illustrating a reentrant circuit involving the AV node and atrial myocardium.
Schematic of AV nodal reentry in a patient.

Physical maneuvers for acute termination include ocular pressure (increases vagal tone but commonly fails), alpha agonists IV, edrophonium (Tensilon), and precordial blow. We prefer the precordial blow — the dog is placed on its right side, the left apex beat located, and the area struck with a fist (~5 joule “shock”).

Three classes of antiarrhythmic drugs are used: digitalis glycosides, beta blockers, and calcium channel blockers (verapamil, diltiazem). For acute termination, diltiazem is generally preferred due to less negative inotropic potential.

Mac was treated with diltiazem 0.15 mg/kg IV over 3 minutes.

Two ECG traces after diltiazem. First trace (20 seconds after): mostly atrial bigeminy. Second trace (one minute later): sinus rhythm at 150 bpm.
First trace (20 sec after diltiazem): atrial bigeminy. Second trace (1 min later): sinus rhythm at 150 bpm.

Mac stayed in ICU overnight. SVT did not recur. Blood work was unremarkable. Echocardiogram was normal. He had systemic hypertension secondary to glomerular disease. He was started on diltiazem 30 mg q12h PO and never had a recurrence. He was eventually weaned off diltiazem with no recurrence over 3 years. His renal disease worsened and he ultimately died of mesenteric artery thromboembolism secondary to antithrombin III loss.

↑