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Basics ⬇

Basics

Definition

A disorder of impulse formation within, and conduction out of, the sinus node; dysfunction of subsidiary pacemakers and other segments of the cardiac conduction system frequently coexist with the sinus node dysfunction.

ECG Features

  • Arrhythmias noted with SSS include any or all of the following: inappropriate sinus bradycardia, sinus pauses (representing sinus arrest or sinoatrial exit block), slow ectopic atrial rhythm, or alternating periods of sinus bradyarrhythmias and SVT (Figure 1).
  • Paroxysms of SVT may alternate with prolonged periods of sinus node inertia and often AV nodal inertia as well, producing tachycardia-bradycardia syndrome, a variant of SSS.
  • P waves and QRS complexes are usually normal.
  • P waves may be abnormal or absent with slow atrial ectopic rhythm or junctional escape rhythm.

Pathophysiology

  • ECG manifestations may precede development of clinical signs.
  • Clinical signs usually result from the failure of subsidiary pacemakers to generate escape rhythms when sinus node dysfunction occurs.
  • The common clinical manifestations reflect transient decreases in organ perfusion, particularly reduced cerebral and skeletal muscle perfusion.
  • Rarely, congestive heart failure develops.

Systems Affected

  • Cardiovascular.
  • Nervous, musculoskeletal, and renal systems may be secondarily affected because of hypoperfusion.

Genetics

  • May be heritable in miniature schnauzers and West Highland white terriers.
  • Doberman pinschers and boxers can have syncope associated with long sinus pauses, suggestive of sick sinus syndrome.

Signalment

Species

Dog

Breed Predilections

  • Miniature schnauzer (may be heritable).
  • Noted commonly in cocker spaniel, dachshund, and West Highland white terrier.

Mean Age and Range

Most dogs >6 years old

Predominant Sex

Female

Signs

Historical Findings

  • Clinical signs vary from asymptomatic to weakness, syncope, collapse, and/or seizures.
  • Sudden death is infrequent.

Physical Examination Findings

  • Heart rate may be abnormally rapid or abnormally slow.
  • Pauses may be noted.
  • Some patients appear normal.

Causes

  • Idiopathic
  • Familial in miniature schnauzers
  • Metastatic disease
  • Ischemic disease

Diagnosis ⬆ ⬇

Diagnosis

Differential Diagnosis

  • Healthy dogs may exhibit sinus bradycardia (rate as low as 30 beats/minute) and sinus pauses (as long as 3.5 seconds) normally during sleep.
  • Bradycardia and sinus arrest due to normal or enhanced vagal tone.
  • Drug-induced (digitalis, -adrenergic antagonists, 2-adrenergic agonists, calcium channel antagonists, cimetidine, opioids).
  • Seizures or syncope due to non-cardiac disease.
  • Atrial standstill secondary to hyperkalemia or atrial disease.
  • Weakness due to neurologic, musculoskeletal, or metabolic diseases.

CBC/Biochemistry/Urinalysis

Normal

Imaging

Breeds predisposed to SSS are also predisposed to degenerative valvular disease; echocardiography is used to confirm presence of significant valvular disease when heart murmur is present.

Diagnostic Procedures

  • Atropine response testing-indicated in dogs with sinus bradycardia, sinus arrest, and sinoatrial exit block. Administer atropine (0.04 mg/kg IM), and evaluate the ECG 20–30 minutes later. A normal (positive) response is >50% increase in heart rate with abolishment of pauses; dogs with SSS generally have no response or an incomplete response to atropine.
  • Electrophysiologic testing of sinus node recovery time and sinoatrial conduction time.
  • 24-hour ambulatory ECG (Holter) or event recording to correlate clinical signs with arrhythmia.

Pathologic Findings

Vary with cause

Treatment ⬆ ⬇

Treatment

Appropriate Health Care

  • Hospitalization rarely necessary except for electrophysiologic testing or pacemaker implantation.
  • Do not treat asymptomatic animals.

Activity

Avoid vigorous exercise and stressful situations

Diet

Modifications unnecessary

Client Education

Owner should be aware that medical management is often ineffective.

Surgical Considerations

  • Permanent artificial pacemaker necessary for dogs failing to respond to medical treatment and those exhibiting unacceptable medication side effects.
  • Permanent artificial pacemaker usually required for dogs with bradycardia-tachycardia syndrome.
  • Transvenous placement of a pacing lead in the right atrium or auricle may successfully abolish the sinus pauses.

Follow-Up ⬆ ⬇

Follow-Up

Patient Monitoring

  • ECG in asymptomatic patients-to detect progression of disease.
  • ECG in patients treated medically or with pacemaker implantation.

Possible Complications

  • Rarely, reduced cerebral or renal perfusion results in chronic renal dysfunction or CNS damage.
  • Presence of significant valvular disease has implications for type of permanent pacing mode selected.

Expected Course and Prognosis

  • Good, following pacemaker implantation in animals without congestive heart failure.
  • Medical management-often ineffective; initial beneficial effects often not sustained.

Miscellaneous ⬆

Miscellaneous

Synonyms

  • Bradycardia-tachycardia syndrome
  • Sinus node dysfunction
  • Tachycardia-bradycardia syndrome

Abbreviations

  • AV = atrioventricular
  • CNS = central nervous system
  • ECG = electrocardiogram
  • SSS = sick sinus syndrome
  • SVT = supraventricular tachycardia

Authors Larry P. Tilley and Francis W.K. Smith, Jr.

Consulting Editors Larry P. Tilley and Francis W.K. Smith, Jr.

Acknowledgment The editors acknowledge the prior contribution of Janice McIntosh Bright.

Client Education Handout Available Online

Suggested Reading

Kraus MS, Gelzer ARM, Moise S. Treatment of cardiac arrhythmias and conduction disturbances. In: Smith FWK, Tilley LP, Oyama MA, Sleeper MM, eds., Manual of Canine and Feline Cardiology, 5th ed. St. Louis: Saunders Elsevier, 2015 (in press)

Tilley LP, Smith FW. Essentials of Electrocardiography. Interpretation and Treatment, 4th ed. Ames, IA: Wiley Blackwell Publishing, 2016 (in preparation).