Sinus node dysfunction (SND)
A disorder of the sinoatrial node that fails to pace the heart reliably.
Key ECG features
- ✓Sinus rate or pauses inappropriate for the patient's state
- •Inappropriate sinus bradycardia
- •Prolonged sinus pauses or arrest
- ±Junctional or ventricular escape beats rescuing the pauses
- ±Alternating bradycardia and supraventricular tachycardia
✓ required · • typical · ± may be present
ECG characteristics
Inappropriate sinus bradycardia, prolonged sinus pauses or arrest, and often alternating bradycardia and supraventricular tachycardia (sick sinus syndrome), with junctional or ventricular escape beats rescuing the long pauses.
This illustration is schematic. Dashed line is a normal rhythm. The grid is not drawn to scale.
Common causes
Degenerative disease of the sinus node, classically in older small-breed female dogs - Miniature Schnauzers, West Highland White Terriers, Cocker Spaniels and Dachshunds are over-represented. It is essentially a canine diagnosis and the equivalent is rare in cats.
Why it matters
Pauses that persist despite atropine and correlate with weakness or syncope support the diagnosis. It is a leading indication for pacemaker implantation.
Differentiate from
- Sinus bradycardiaSinus bradycardia is a finding. This is the diagnosis behind a bradycardia or pauses that persist despite atropine and correlate with weakness or syncope.
- Sinus pause & arrestPauses are the finding; sinus node dysfunction is the underlying disorder when they are long, frequent, or symptomatic.