Sick Sinus Syndrome

Sick Sinus Syndrome refers to a group of disorders involving the sino-atrial node usually with further involvement of the atrio-ventricular node, His bundle and/or bundle branches. Frequently, multiple areas of the conduction system are involved.

ECG findings (may include any of the following):

1. A slow and irregular atrial rate (impaired sino-atrial activity), severe sinus bradycardia, or long pauses of asystole with no escape beats
2. Evidence of impaired atrio-ventricular conduction such as 2nd degree heart block
3. The ventricular rate is slow and irregular.
   • Ventricular escape beats are common, though they are often slow to manifest (follow a longer pause than usual)
4. Paroxysms of supraventricular tachycardia alternating with severe sinus bradycardia

Etiology:

1. May have SA nodal artery disease – ischemia
2. Fibrosis of SA node with partial involvement of the rest of conduction system
3. Possibly inherited in some breeds as Miniature Schnauzer, West Highland White terrier
4. Usually idiopathic

Consequences:

1. Periods of asystole are associated with syncope, pre-syncope, weakness
2. The paroxysms of supraventricular tachycardia may also cause clinical signs

Treatment:

1. Atropine is usually not effective (or at least there is a suboptimal atropine response), though some anticholinergics or sympathomimetics may accelerate the ventricular rate and provide a temporary response
2. Permanent pacemaker implantation is the only definitive therapy