Page 1830 - Cote clinical veterinary advisor dogs and cats 4th

916 Sick Sinus Syndrome

Synonyms account for the development of SSS in bradyarrhythmias, especially when compli-
humans.
Sinus node dysfunction (SSS), bradycardia- • Occlusion or sclerodegenerative changes of cated by degenerative valvular disease in older
VetBooks.ir notropic incompetence, sinoatrial syncope, the sinus node artery rarely play a role in • Echocardiography (p. 1094) can assess the
small-breed dogs.
tachycardia (brady-tachy) syndrome, chro-
severity of valvular, myocardial, or structural
sinoatrial node disease
the development of this disease.
Epidemiology (including neoplastic) cardiac disease, if
present.
SPECIES, AGE, SEX DIAGNOSIS
Small-breed, older dogs Diagnostic Overview Advanced or Confirmatory Testing
The two most common clinical contexts that • Video recording of episodes by the owner
GENETICS, BREED PREDISPOSITION would raise the suspicion of this disorder are may help differentiate syncope from seizures
Middle-aged to older, female miniature a history of syncope in a dog of a susceptible if the episodes occur infrequently and cannot
schnauzers are most commonly afflicted. West breed and incidental discovery of contextually be classified based on historic description
Highland white terriers and cocker spaniels and inappropriate bradycardia. The diagnosis is alone.
their crosses are also overrepresented. confirmed electrocardiographically. In some • A 24-hour Holter monitor or cardiac event
patients, an in-hospital resting electrocardio- monitor may be indicated in cases when the
ASSOCIATED DISORDERS gram (ECG [p. 1096]) is sufficient; in others, history, clinical signs, ECG, and atropine
Because of their similar breed and age pre- the intermittent nature of the arrhythmia response test are inconclusive. A Reveal/Linq
dispositions, SSS and chronic degenerative mandates the use of telemetry or ambulatory device (ECG loop recorder) can be implanted
valvular disease often occur at the same time. ECG (p. 1120). when episodes are very infrequent.
However, there is no known causative link
between the two. Differential Diagnosis TREATMENT
Other bradyarrhythmias:
Clinical Presentation • High-grade second-degree AV block Treatment Overview
HISTORY, CHIEF COMPLAINT • Third-degree AV block • Dogs showing no overt clinical signs (inci-
Sinus node dysfunction may be recognized • Persistent atrial standstill dentally discovered arrhythmia): periodic
because of sinus bradycardia on routine exam • Vagally mediated bradyarrhythmias monitoring and client education to detect
or during general anesthesia. Clinical signs asso- • Metabolic disorders that slow SA nodal progression of disease
ciated with SSS include syncopal/seizure-like activity • Dogs showing overt clinical signs such as
episodes or, less commonly, episodic weakness. ○ Hyperkalemia (p. 495) syncope
Some animals with severe, long-standing sinus ○ Hypothermia (p. 523) ○ Bradycardia: increase the heart rate to alle-
bradycardia may develop congestive heart failure ○ Hypothyroidism (p. 525) viate the syncopal episodes using medical
(CHF) and are presented with respiratory ○ Dysautonomia therapy or pacemaker implantation.
distress. • Seizures of any cause (p. 903) ○ Brady-tachy syndrome: bradyarrhythmia
should be addressed before starting therapy
PHYSICAL EXAM FINDINGS Initial Database aimed at reducing the supraventricular
• Bradyarrhythmia with prolonged episodes of • ECG: essential to characterize the bradyar- tachycardia (p. 96).
asystole (due to sinus arrest without activa- rhythmia and diagnose SSS
tion of subsidiary AV or intraventricular ○ Although exact criteria have not been Acute General Treatment
pacemakers) is classic. established for the diagnosis of SSS, sinus • In the absence of syncope, significant
• Paroxysms of tachycardia followed by arrest with or without various degrees of cardiomegaly, or CHF, periodic ECGs and
pauses (brady-tachy syndrome) may be AV block or supraventricular tachycardia client education are indicated. General anes-
detected. raises suspicion. thesia should be avoided, or if unavoidable,
• Varied-intensity, left apical systolic murmurs • Atropine response test can determine whether temporary cardiac pacing should be available.
of mitral insufficiency are common but the bradyarrhythmia is vagally (entirely or • Dogs showing overt clinical signs
unrelated (small breed, older dogs). partially) mediated. If vagally mediated (i.e., ○ Vagolytic drugs display variable and often
• Exam may be normal if high sympathetic physiologic), the test reveals increased heart temporary efficacy in animals with SSS,
tone associated with veterinary visit over- rate. If pathologic, there is no change in the although a positive atropine response
comes bradycardia; this may obscure the ECG. helps predict the likelihood of success for
diagnosis. ○ Obtain initial ECG. medical therapy. One option is to begin
○ Administer atropine 0.04 mg/kg IV, IM, with a sympathomimetic agent (e.g.,
Etiology and Pathophysiology or SQ. Obtain follow-up ECG approxi- theophylline, terbutaline), and if clinical
• The etiologic basis for SSS in dogs has not mately 10 minutes later (if administered signs persist, add a parasympatholytic
been determined. IV) or 20-30 minutes later (if IM or SQ). (anticholinergic such as propantheline).
• Fibrosis and/or fatty infiltration with atrophy ○ An increase of at least 50% in the heart ■ Theophylline 10 mg/kg PO q 8h
of the SA node have been identified in rate is expected when the bradycardia is ■ Terbutaline (instead of theophylline)
humans and dogs with SSS. vagally mediated (i.e., physiologic). A 1.25-5 mg/kg PO q 8-12h
• The absence of morphologic abnormalities in positive atropine response test predicts ■ Propantheline bromide 0.25-0.5 mg/
some humans with sinus node dysfunction the success of medical treatment. kg PO q 8h
suggests that alterations of neural innervation ○ The absence of any response to atropine ○ Pacemaker implantation: dogs with overt
or neural regulation may contribute to the in a dog with appropriate signalment and clinical signs caused by SSS that fail to
bradyarrhythmias. clinical signs is strongly suggestive of SSS. respond or become refractory to positive
• Autoantibodies to proteins isolated from • CBC, serum chemistry profile, and urinalysis chronotropic drugs require permanent
sinus node cells have been identified in to detect underlying electrolyte imbalances cardiac pacing.
humans, suggesting an immune-mediated or metabolic disorders that may contribute
process in some cases. to bradyarrhythmias or seizures. Chronic Treatment
• Changes in ion channels, often sodium, or • Thoracic radiographs: cardiomegaly or CHF • Positive chronotropic drugs: if animals are
gap junctions within the sinus node may may develop in the face of long-standing responsive, therapy is maintained lifelong;

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