Sinus pause & arrest
A transient failure of the sinoatrial node to generate an impulse. A brief interruption is called a sinus pause and a longer one sinus arrest, but the distinction is one of duration rather than a hard cutoff.
Key ECG features
- ✓A sudden gap with no P wave and no QRS
- ✓Underlying rhythm otherwise sinus
- •Pause not an exact multiple of the P-P interval
- ±Ended by a sinus beat or an escape beat
✓ required · • typical · ± may be present
ECG characteristics
A sudden gap with no P wave and no QRS interrupting an otherwise sinus rhythm. The pause is not an exact multiple of the underlying P-P interval, which helps separate it from a blocked atrial premature complex. A junctional or ventricular escape beat may end it.
This illustration is schematic. Dashed line is a normal rhythm. The grid is not drawn to scale.
Common causes
Short pauses are frequently vagally mediated and benign, especially in dogs with high resting vagal tone. Long or frequent ones point toward sinus node dysfunction, and they also occur with raised intracranial pressure and drugs that raise vagal tone.
Why it matters
What matters is whether the pauses correlate with clinical signs and whether they abolish with atropine or exercise. What ends the pause matters too: a sinus beat is reassuring, an escape beat says the node failed for long enough that a backup pacemaker had to take over.
Differentiate from
- Atrial premature complexesA blocked atrial premature complex also produces a pause, but a premature P wave is hidden in the preceding T wave. A sinus pause has no P at all.
- Second-degree AV blockA dropped beat in AV block leaves a P wave with no QRS after it; here neither appears.
- Sinus node dysfunctionLong, frequent, or symptomatic pauses point to sinus node dysfunction as the underlying disorder.
- Sinus arrhythmiaMarked respiratory sinus arrhythmia can leave gaps that resemble a pause, but its slowing is gradual and cyclic rather than abrupt.