2nd Degree AV Block (Mobitz I / Wenckebach)

Mobitz I is the AV node failing in the way it is built to fail. Each impulse arrives before the node has fully recovered from the last, so each conducts a little more slowly than the one before, until one arrives too early to conduct at all and is dropped. The pause that follows lets the node recover, and the cycle restarts. Four findings follow from that single mechanism and are worth reading as a set: the PR lengthens, the increment by which it lengthens shrinks, the R-R intervals therefore shorten, and the pause comes out shorter than two full cycles. The rhythm is usually nodal, often vagal, and in an asymptomatic person it is frequently a normal variant rather than a disease.

On the trace

Read the strip in this order.

1. Look for the gaps. The beats arrive in groups of three with a pause after each group. Group beating is worth recognising from across the room, because it narrows the diagnosis before you have measured anything.
2. Find the P wave inside the pause. It is there, on time, at the same spacing as every other P. The atria never missed a beat. Only the ventricles did.
3. Measure the PR of each beat in a group. 200 ms, then 300, then 340. It lengthens every beat until one impulse fails to get through.
4. Measure how much it lengthened by each time. 100 ms, then 40. The increment shrinks, and everything below follows from that.
5. Measure the R-R intervals. 933 ms, then 873. They shorten as the PR lengthens, because each beat is delayed less than the one before it was.
6. Measure the pause. 1527 ms, against 1746 for the two cycles it replaces. Shorter than two. A completely blocked beat would give exactly two.
7. Look at the PR after the pause. Back to 200 ms, the shortest on the strip. The node has recovered and the cycle starts again.

How to recognise it

The strip

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Mechanism

AV nodal tissue recovers slowly, and each impulse arrives before recovery from the last is complete. The node conducts anyway, but more slowly each time, so the PR stretches. Eventually one impulse arrives while the node is still refractory and does not get through at all. The pause lets the node recover fully, and the next impulse conducts with the shortest PR of the cycle.

This is decremental conduction: normal AV nodal behaviour pushed one step too far. The node is designed to fail this way, which is why the rhythm is usually benign.

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Why the increment shrinks

The relationship between how early an impulse arrives and how long the node takes to conduct it is a curve, not a straight line, and it is steepest at the start. The first delayed beat therefore gains the most, and each subsequent beat gains less. It is the arithmetic of that curve, and it is what produces the shortening R-R and the pause that is less than two cycles. Both of those are secondary findings, derived from the same fact.

Atypical Wenckebach

Longer cycles, 8:7 and beyond, frequently break the pattern: the increments stop shrinking reliably and the R-R intervals stop shortening. This is common enough that the absence of shortening does not exclude the diagnosis. What stays true in almost every case is progressive PR lengthening and a pause shorter than two cycles.

Why the location matters more than the pattern

Wenckebach conduction is usually nodal, and nodal block is stable, often reversible, and rarely progresses to complete block. That is the basis for treating it as benign. Wenckebach-pattern block does occasionally occur in the His-Purkinje system, and there it carries the prognosis of infranodal disease instead. A wide QRS is the warning sign, so the QRS width is worth checking even when the pattern looks reassuring.

The exercise test in one sentence

Nodal block improves with exercise and with atropine, because both withdraw vagal influence and speed nodal conduction. Infranodal block gets worse with both, because a faster atrial rate gives diseased His-Purkinje tissue less time to recover.

Clinical impact

The AV node recovers incompletely between beats

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Each impulse conducts more slowly than the last until one fails

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Nodal, often vagal, rarely progressiveUsually benign

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A wide QRS moves the block below the node and changes the prognosisWatch

In a healthy person, often during sleep or in a trained athlete, this needs nothing. It becomes a clinical problem when the ventricular rate falls far enough to cause symptoms, or when the QRS is wide.

Management

1

Decide whether the patient has symptoms

Dizziness, presyncope and fatigue at the point where beats are dropped. An asymptomatic Wenckebach in a fit young person, particularly at night, is a normal variant.

2

Review AV-nodal blocking drugs

Beta-blockers, calcium-channel blockers, digoxin and amiodarone all produce it, and stopping the drug usually resolves it.

3

Atropine only if the block is nodal

It speeds nodal conduction and helps here. It does not help infranodal block and can worsen it by increasing the atrial rate, so a wide QRS is a reason to be cautious.

4

Pace only for symptoms that persist

Permanent pacing is reserved for symptomatic block after reversible causes have been addressed, or where the block proves to be infranodal.

Differential

2nd Degree AV Block (Mobitz II)

The dangerous one. Its PR interval does not move at all before the drop, and its pause is exactly two cycles rather than less. Measure two consecutive PR intervals before the pause and compare them.

2:1 AV Block

Every other P conducts, so there are never two conducted beats in a row and there is no way to see whether the PR was lengthening. The pattern cannot be classified from the strip alone.

Blocked (Non-Conducted) PAC

Also produces an unexpected pause. The P in that case is early and deforms the preceding T wave, whereas the blocked P here arrives exactly on schedule.

1st Degree AV Block

A long PR that is not lengthening. If you only have a few beats, watch for longer before concluding the interval is fixed.

References

1. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay — Circulation, 2019
2. Second-Degree Atrioventricular Block: A Reappraisal — Mayo Clinic Proceedings, 2001
3. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy — European Heart Journal, 2021