Sinus Arrest
Sinus Arrest
The sinus node stops firing: a flat pause with no P wave, whose length is not a multiple of the sinus cycle.
On the trace
Read the strip in this order.
- Find the pause.
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The rhythm runs at 833 ms between beats and then simply stops for 2311 ms. Over two seconds of nothing. - Look inside the pause, not at its edges.
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It is flat. No P wave, no QRS, no atrial activity of any kind. That matters: in the blocks there is always a P sitting in the gap, because the atria kept going. Here nothing fired at all. - Check the rhythm on either side.
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Regular, 833 ms, with an upright P before every QRS and a fixed PR of 175 ms. The sinus node is normal right up to the moment it is not. - Divide the pause by the sinus cycle.
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2311 over 833 is 2.77. Now do it for the other pauses on the strip: 2.28, 2.51, 2.54. None of them is a whole number, and none is the same as another. - Understand why that is the diagnosis.
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A node that fires on time, and whose impulse then fails to escape, produces a pause that is an exact multiple of the cycle: that is sinoatrial exit block. A node that fails to fire at all resumes whenever it recovers, which is governed by nothing. Non-integer, and different each time, is what says arrest.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| The pause | 1.9 to 2.3 s here | Over 3 s is the threshold usually quoted as significant. |
| Inside the pause | Nothing at all | No P wave. This is what separates it from every AV block. |
| Pause as a multiple | 2.28 to 2.77 cycles | Not a whole number, and not repeatable. The whole diagnosis. |
| The underlying rhythm | Normal sinus | Regular, upright P, PR 175 ms, narrow QRS. |
| Escape beats | May terminate a long pause | A junctional or ventricular escape is a rescue, never to be suppressed. |
| Mean rate | Misleadingly low | 57 bpm here, but no single beat is at 57. |
The strip
Mechanism
The sinus node's pacemaker cells drift up to threshold and fire. In sinus arrest that drift fails, or the cells depolarise and produce an impulse too feeble to leave the node. Nothing reaches the atria, so there is no P wave, and nothing reaches the ventricles, so there is no QRS. The trace goes flat until the node recovers and resumes.
Because the failure is in the pacemaker rather than in the timing of its output, the length of the pause has no relationship to the rhythm around it. That is the arithmetic the diagnosis rests on.
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Arrest against exit block, in one calculation
Divide the pause by the underlying P-P interval. An exact multiple means the node kept its own time and one impulse failed to get out, which is type II sinoatrial exit block. A number that is not a multiple means the node stopped keeping time, which is arrest. Third-degree exit block is indistinguishable from arrest on a surface ECG, and separating those two needs an electrode in the node, so the ECG answer is often as far as it goes.
What fills the gap, and why it must not be suppressed
A pause long enough gives a subsidiary pacemaker time to reach threshold. A junctional escape appears at 40 to 60 with a narrow QRS; a ventricular one is slower and wide. Either is the only thing standing between the patient and asystole, so the escape beat is never the thing to treat.
The causes worth separating
Reversible: high vagal tone, which produces pauses during sleep, vomiting, straining or carotid pressure; and drugs, which means beta-blockers, calcium-channel blockers, digoxin and amiodarone. Structural: sick sinus syndrome, in which the node has become unreliable, often alternating with atrial tachyarrhythmias as the tachycardia-bradycardia syndrome. Also ischaemia of the right coronary territory, hyperkalaemia, hypothyroidism and obstructive sleep apnoea.
Deciding what a pause is worth
Length alone is a weak guide. A three-second pause in a sleeping athlete is vagal and needs nothing; a two-second pause that reproduces the patient's syncope is a pacing indication. The question is whether the pause and the symptom happen at the same moment, which is what ambulatory monitoring exists to answer.
Clinical impact
The pause is well tolerated while the patient is lying down and asleep, and dangerous while they are standing on a staircase. What decides the significance is the correlation between the pause and the symptom, not the duration on its own.
Management
Correlate the pause with the symptom
Ambulatory monitoring with a symptom diary. A pause that coincides with syncope is a diagnosis; an incidental one during sleep usually is not.
Review the drug list and the vagal triggers
Rate-slowing drugs are the commonest reversible cause. Pauses confined to sleep, or to straining and vomiting, are vagal and benign.
Never suppress an escape rhythm
If escape beats are ending the pauses, they are holding the patient up.
Pace for symptomatic pauses that are not reversible
Symptomatic sinus node dysfunction after reversible causes have been excluded is a pacing indication.
Differential
The commonest cause of an unexpected pause, and the benign one. There is a premature P hiding in the T before the pause. Here the gap is genuinely empty.
Also drops beats without warning, but the P waves keep marching through the pause on time. Look for atrial activity in the gap before calling this arrest.
Also uneven sinus rhythm, but it varies smoothly with the breath rather than stopping, and the variation never approaches a pause of this length.
What the mean rate suggests if you do not look at individual intervals. This rhythm is normal-rate sinus interrupted, not uniformly slow.
References
- Sinoatrial Exit Block — Life in the Fast Lane, ECG Library, 2024
- 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay — Circulation, 2019
- 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy — European Heart Journal, 2021