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Supraventricular Arrhythmias

1
Romain Pariaut, DVM, DACVIM (Cardiology), DECVIM-CA (Cardiology) and
Amara H. Estrada, DVM, DACVIM (Cardiology) 2
1 Department of Clinical Sciences, Cornell University, Ithaca, NY, USA
2 Department of Small Animal Clinical Sciences, University of Florida, Gainesville, FL, USA

Arrhythmias are classified as supraventricular or ven­ hypertension and an abnormal respiratory pattern,
tricular based upon the location of the cardiac tissues characterized by periods of apnea.
responsible for their occurrence. Supraventricular ana­
tomic structures include the sinus and atrioventricular Signalment
(AV) nodes, the His bundle, the atrial myocardium, the There is no breed or age predisposition. These animals
ostia of the coronary sinus, pulmonary veins and venae are usually free of any underlying cardiac disease.
cavae (Figure 21.1).
Arrhythmias can be further divided into bradyarrhyth­ History and Clinical Signs
mias (heart rate <60 bpm in dogs; <100 bpm in cats) or
tachyarrhythmias (heart rate typically >180–200 bpm in Most dogs and cats do not display clinical signs that can
dogs; >220–240 bpm in cats). Major bradyarrhythmias solely be attributed to sinus bradycardia. However,
include sinus bradycardia, sick sinus syndrome (sinus extreme cases of sinus bradycardia may cause lethargy
node dysfunction), atrial standstill and atrioventricular and collapse.
(AV) blocks. Among tachyarrhythmias, atrial fibrillation
(AF) is by far the most common. Other forms of Diagnosis
supraventricular tachycardia (SVT) are less commonly On the surface ECG, P‐waves are accompanied by QRS
encountered in clinical practice (Table 21.1). complexes and the QRS complexes are in direct associa­
tion with the preceding P‐wave, such that the PR interval
is relatively constant. The P‐waves are typically positive in
Sinus Bradycardia leads II, III, and aVF. When markedly elevated vagal tone
is the cause for the bradycardia in dogs, a wandering pace­
Etiology/Pathophysiology maker, which corresponds to a variation in the amplitude
of the P‐wave usually in relation to the respiratory cycle,
Sinus bradycardia (or exaggerated sinus arrhythmia) is may be present. The QRS complexes are usually narrow
rarely a primary disorder and is usually benign in small (duration <60 ms in dogs; 40 ms in cats) (Figure 21.2).
animal patients. It results from an underlying disease
responsible for excessive vagal tone, particularly gastro­ Therapy
intestinal, respiratory, neurologic, and ocular diseases.
Drugs (digoxin, opioids, calcium channel blockers, beta‐ Treatment is usually not necessary. However, vagally
blockers), hypothermia, and hypothyroidism are other induced bradyarrhythmias respond to the administra­
extrinsic causes of sinus bradycardia. Though rare, sinus tion of parasympatholytic medications. An increase in
bradycardia, when diagnosed in an animal with impaired heart rate following the intravenous administration of
consciousness, should raise the suspicion of elevated atropine or glycopyrrolate confirms the contribution of
intracranial pressure leading to brainstem compression. excessive vagal tone to the bradycardia. Transient AV
The clinical picture of this physiologic response, known block frequently occurs following parenteral administra­
as the Cushing’s reflex, combines bradycardia, systemic tion of atropine, most likely because of an initial blockade

Clinical Small Animal Internal Medicine Volume I, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical