Page 114 - Small Animal Internal Medicine, 6th Edition

86 PART I Cardiovascular System Disorders

occasionally occurs transiently, usually in association with Symptomatic Bradyarrhythmia
trauma or surgery. Acute AF without signs of heart disease
VetBooks.ir or failure may convert to sinus rhythm spontaneously or in Atropine challenge; treat underlyingdisease,
response to drug therapy or electrical cardioversion. Drugs
and/or discontinue predisposing drugs
that might be effective include amiodarone, diltiazem (e.g.,
PO for ≈3 days), or possibly sotalol or other class III or IC Effective Ineffective
agents. Acute-onset AF associated with high vagal tone may
convert with IV lidocaine. Quinidine PO or IM has been
used for acute AF conversion in large dogs without signs of Try PO anticholinergic, Dopamine or Pacemaker
heart disease, but adverse effects can include increased ven- ± methylxanthine, or isoproterenol implantation
infusion, or
tricular response rate from the drug’s vagolytic effects, ataxia, terbutaline terbutaline
and, most seriously, seizures or polymorphic ventricular
tachycardia. If effective, the drug is discontinued after sinus Ineffective
rhythm is achieved. Dogs that do not convert to sinus rhythm
are given either digoxin or diltiazem for rate control. Alter- FIG 4.5
natively, if the ventricular rate is consistently low at rest, dogs A therapeutic approach to managing symptomatic
bradyarrhythmias. See Box 3.1 and Table 4.2 for drug
can be monitored periodically without therapy, but rapid doses and text for more information.
heart rates are still likely with exercise or excitement. Lone
AF precedes the development of DCM in some cases.

BRADYARRHYTHMIAS convulsive syncope (Stokes-Adams seizures) triggered by
Sinus Bradycardia cerebral hypoxia. Older female Miniature Schnauzers and
Slow sinus rhythm (or arrhythmia) can be a normal finding, West Highland White Terriers most commonly are affected,
especially in athletic dogs. Sinus bradycardia also has been but the syndrome also is seen in Dachshunds, Cocker Span-
associated with various drugs (e.g., xylazine, thorazine tran- iels, Pugs, and mixed-breed dogs. Affected dogs have epi-
quilizers, some anesthetic agents, medetomidine, digoxin, sodes of marked sinus bradycardia with sinus arrest (or SA
calcium entry blockers, β-blockers, parasympathomimetic block). Sick sinus syndrome is extremely rare in cats.
drugs), trauma or diseases of the CNS, organic disease of the Abnormalities of the AV conduction system can coexist,
sinus node, hypothermia, hyperkalemia, and hypothyroid- causing the activity of subsidiary pacemakers to be depressed
ism, among other disorders. Conditions that increase vagal and leading to prolonged periods of asystole. Some affected
tone (for example, respiratory or gastrointestinal [GI] disease dogs also have paroxysmal SVTs, prompting the name
or a mass involving the vagosympathetic trunk) can induce bradycardia-tachycardia syndrome (Fig. 4.6). Premature
sinus bradycardia. Chronic pulmonary disease often is asso- complexes can be followed by long pauses before sinus node
ciated with pronounced respiratory sinus arrhythmia. activity resumes, indicating a prolonged sinus node recovery
In most cases with sinus bradycardia, the heart rate time. Intermittent periods of accelerated junctional rhythms
increases in response to exercise or atropine administration, and variable junctional or ventricular escape rhythms also
and no clinical signs are associated with the slow heart rate. occur.
Symptomatic dogs usually have a heart rate slower than 50 Clinical signs result from bradycardia and sinus arrest,
beats/min and/or pronounced underlying disease. Because paroxysmal tachycardia, or both. Signs can mimic seizures
sinus bradycardia and sinus bradyarrhythmia are extremely stemming from neurologic or metabolic disorders. Concur-
rare in cats, a search for underlying cardiac or systemic rent degenerative mitral valve disease also is often present.
disease (such as hyperkalemia) is warranted in any cat with Some dogs have evidence of CHF, usually secondary to
a slow heart rate. mitral valve regurgitation, although the arrhythmias may be
When sinus bradycardia is associated with signs of weak- a complicating factor.
ness, exercise intolerance, syncope, or worsening underlying ECG abnormalities frequently are pronounced in dogs
disease, an anticholinergic (or adrenergic) agent is given with long-standing sick sinus syndrome. Nevertheless, some
(Fig. 4.5). If sinus bradycardia is the result of a drug effect, dogs have one or more normal resting ECGs. Prolonged
discontinuation, dosage reduction, or other therapy should visual ECG monitoring or 24-hour ambulatory ECG can
be used, as appropriate (e.g., reversal of anesthesia, calcium help establish a definitive diagnosis. An atropine challenge
salts or atropine for calcium entry blocker overdose, dopa- test is done in dogs with persistent bradycardia (see p. 97).
mine or atropine for β-blocker toxicity). If there is inade- The normal response is an increase in the heart rate of 150%
quate increase in heart rate with medical therapy, temporary or to more than 130 to 150 beats/min. Dogs with sick sinus
or permanent pacing is indicated (see Suggested Readings). syndrome generally have a subnormal response.
Therapy with an anticholinergic agent, methylxanthine
Sick Sinus Syndrome bronchodilator, or terbutaline given orally might temporar-
Sick sinus syndrome is a condition of erratic SA function ily help some animals that respond positively to atropine
characterized by episodic weakness, syncope, and, sometimes, challenge. Yet anticholinergic or sympathomimetic drugs