Issue Description Sick sinus syndrome (SSS) is a primary
conduction abnormality resulting from sinoatrial node disease. Many
dogs and people with SSS also appear to have coexisting dysfunction of
the atrioventricular (AV) nodal and intraventricular subsidiary
pacemakers, resulting in a failure to generate appropriate escape
rhythms. The syndrome so far has been reported in humans and dogs.
Canine cases are mostly described in older female miniature
schnauzers, dachshunds, cocker spaniels, west highland white terriers
and pugs. The inability of the animal to maintain an adequate heart
rate, when there is pathological change affecting all pacemakers,
contributes to the generation of clinical signs, such as weakness,
stumbling, confusion, and syncope. Other Names Lazy Sinus Syndrome, Bradycardia-tachycardia
Syndrome, Sluggish Sinus Syndrome, Sinoatrial Syncope, SSS
Causes and Risk Factors Idiopathic
Genetic inheritance, especially in female miniature schnauzers
Cardiomyopathy characterized by fibrous tissue replacing sinoatrial
Ischemic heart disease
Treatment Treatment is unnecessary in asymptomatic
Symptomatic dogs can be subdivided into those that show primarily
bradycardia or sinus arrest and those that have supraventricular
tachycardia followed by sinus arrest.
Clinical response to medical treatment often inconsistent
Permanent artificial pacemaker insertion used to treat patients that
fail to respond to medical treatment or who cannot tolerate side
effects of anticholinergics
Pacemaker also indicated in animal in which treatment of
tachyarrhythmia t may aggravate bradyarrhythmia, or vice versa.
Drugs and Fluids Symptomatic dogs with bradycardia or sinus
arrest that are atropine responsive are treated with anticholinergic
drugs (propantheline [Probanthine]--small dogs, 3.75-7.5 mg PO
q8h-q12h; medium dogs, 15 mg PO q8h; large dogs, 30 mg PO q8h). Dogs
with bradycardia-tachycardia can be given medication to abolish the
tachycardia component which may eliminate overdrive suppression
resulting in sinus arrest. Digoxin (0.22 mg/m2 or 0.0055-0.01 mg/kg PO
q12h) or propranolol (Inderal, 0.2-1 mg/kg PO q8h) can be
administered. Monitor closely for bradycardia.
Long-acting forms of theophylline (Theo-Dur, 20 mg/kg PO q12h) in dogs
with bradycardia and sinus arrest.
Contraindications/Possible Interactions Caution should be taken when treating
bradycardia-tachycardia syndrome. The drug treatment for
supraventricular tachycardia may worsen the bradycardia/sinus arrest
and vice versa. These patients generally require an artificial
Side effects with anticholinergic drugs are common and include
constipation, dry mucous membranes, emesis, and keratoconjunctivitis
Follow-Up If patient is asymptomatic, monitor with serial
ECG for progression
Owner should watch for development of clinical signs including
lethargy, weakness, and syncope.
If patient is treated medically or by insertion of pacemaker, monitor
routinely by ECG.
In animals that have pacemaker inserted and no signs of CHF, prognosis
Prognosis varies in animals treated medically; clinical response is
often inconsistent and the disease may progress.