Page 68 - Basic Monitoring in Canine and Feline Emergency Patients

atrium. A single supraventricular ectopic beat that couplet and triplet can be used to describe two or
occurs before the next expected sinus beat is called a three SVPCs in a row, respectively; more than three
VetBooks.ir single (or isolated) supraventricular premature com- VPCs in succession is termed supraventricular tachy-
cardia (SVT) (see Fig. 3.12); and periods of SVT
plex (SVPC). As with ventricular ectopy, the terms
alternating with sinus rhythm are often called ‘runs’
or ‘paroxysms’ of SVT. SVT is perfectly regular
AV node Bundle
SA node of His (identical R–R intervals between beats), and gener-
ally causes the most dramatically high heart rates of
any tachycardia (often ~280–300 bpm in dogs). The
RA abnormal P′ morphology will be consistent through-
LA
out every beat of the tachycardia. Unlike sinus
tachycardia, the onset and termination of SVT from
Accessory normal sinus rhythm are typically abrupt.
pathway
Treatment of SVT is directed at prolonging AV
nodal conduction time such that the supraventricu-
LV
Bundle lar circuit is extinguished, allowing normal SA
branches RV depolarization to resume control of heart rate and
rhythm. Vagal maneuvers (such as manual pressure
on the globes, nasal philtrum, or carotid sinus) can
Purkinje be attempted to slow AV nodal conduction. Vagal
fibers maneuvers may cause transient conversion to nor-
mal sinus rhythm, confirming the diagnosis of SVT.
Diltiazem is the first-line medical treatment for SVT;
Fig. 3.11. Schematic representation of cardiac it is generally given as small IV boluses until conver-
conduction system with a bypass tract between the sion to sinus rhythm occurs, and continued as a
right ventricular free wall and the right atrium. An continuous rate infuser (CRI) if needed (see Table
impulse conducted antegrade through the AV node 3.6). Long-term management typically involves oral
may return to re-activate the atria via the retrograde diltiazem. Other drugs that may be considered in
accessory pathway; that impulse can then be some cases include sotalol or atenolol (see Table 3.6).
conducted again through the AV node in a re-entrant
loop of SVT. LA, left atrium; LV, left ventricle; RA, right Radiofrequency ablation is also available in select
atrium; RV, right ventricle. veterinary centers for dogs with SVT secondary

Fig. 3.12. Lead II ECG (25 mm/s, 10 mm/mV) showing normal sinus rhythm with onset of supraventricular
tachycardia. The heart rate in SVT is approximately 300 bpm. The onset of the SVT from normal sinus rhythm is
abrupt. The SVT is regular (R–R intervals are identical). The QRS complexes in SVT are identical to this patient’s
normal sinus complexes (narrow and positively deflected in lead II). During the tachycardia, no obvious P waves are
noted. However, there is an irregularity in the S–T segment of every beat during SVT that likely represents a P′ wave.

60 J.L. Ward