2nd Degree AV Block
caution2nd Degree AV Block (Mobitz II)
A QRS dropped without warning, PR interval unchanged before and after: infranodal, and a pacing indication.
Mobitz II is the dangerous second-degree block, and everything that makes it dangerous follows from where it sits. The AV node is working; the damage is below it, in the His bundle or the bundle branches. That tissue does not slow down under stress the way the node does, so there is no progressive PR lengthening to warn you. Conduction is all-or-none: beats pass at a fixed PR until one simply does not. Because the lesion is infranodal the conducted QRS is usually wide, and because it is infranodal the escape rhythm available if conduction fails completely is a slow and unreliable ventricular one. That combination is why permanent pacing is indicated whether or not the patient has any symptoms.
On the trace
Read the strip in this order.
- Measure the QRS before anything else. 132 ms, wide, with a broad terminal S and a T wave pointing the opposite way to the R. Every conducted beat looks like this. A wide QRS in a second-degree block places the lesion below the AV node, and that is the finding that decides what happens to the patient.
- Measure two consecutive PR intervals. 220 ms and 220 ms. Then the next, and the next. It does not move. Nothing creeps, nothing warns.
- Find the beat that is missing. A P wave arrives exactly on time and no QRS follows it. The beat before it looked identical to every other beat on the strip.
- Measure the pause. 1667 ms, against an R-R of 833. Exactly two cycles. The atria kept perfect time through the gap, and the ventricles simply missed one.
- Check when the next drop comes. It does not follow a pattern here. Runs of normal conduction are interrupted without warning, which is what makes a short strip so unreliable in this rhythm.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| PR interval | Fixed | 220 ms here, on every conducted beat, including the one before the drop. |
| The drop | Sudden, unannounced | No lengthening beforehand. This is the definition. |
| The pause | Exactly two R-R intervals | 1667 ms against 833. Wenckebach's pause is shorter than two. |
| QRS | Wide, 132 ms here | Usually wide, because the block is infranodal. Block inside the His bundle itself gives a narrow QRS and is the minority. |
| Pattern of drops | Unpredictable here | Can also settle into a fixed ratio, in which case it becomes unclassifiable from the strip. |
| Risk | Progression to complete block | The reason it is paced. |
The strip
Mechanism
The AV node is conducting normally. The damage is below it, in the His bundle or the bundle branches, and that tissue does not have the node's ability to slow down gracefully. It conducts or it does not.
So there is no warning. Every conducted impulse takes the same time to arrive, because the diseased tissue either passes an impulse at its usual speed or blocks it outright. When enough of the His-Purkinje system fails at once, conduction stops altogether and the patient is in complete heart block with whatever escape rhythm is available underneath.
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All-or-none, and why that is worse
Decremental conduction is a safety feature. Tissue that slows down under stress gives warning, fails gradually, and recovers. His-Purkinje tissue does not behave that way: it has a long refractory period and a steep response, so it works until it does not. The loss of the warning is exactly what makes Mobitz II dangerous, because the first sign of decompensation can be syncope rather than a change on the ECG.
Why the escape rhythm is the real problem
When infranodal conduction fails completely, the escape focus has to come from below the lesion, which means ventricular muscle at 20 to 40 beats a minute. Those foci are slow, unreliable and can stop without warning. A patient in complete block from nodal disease usually has a junctional escape to fall back on; a patient with infranodal disease usually does not.
The strip may not show it
Long runs of 1:1 conduction are common, so a normal-looking twelve-lead does not exclude the diagnosis in someone with syncope and a wide QRS. Ambulatory monitoring is what settles it.
Pacing, and why the threshold is so low
Permanent pacing is a Class I indication in Mobitz II irrespective of symptoms. That is unusual, and the reason is the unpredictability: the rhythm gives no reliable warning of progression, so waiting for symptoms means waiting for syncope or worse. It is the clearest example in conduction disease of treating a mechanism rather than a complaint.
Clinical impact
Unlike Wenckebach, this is a structural problem and it does not resolve. The absence of symptoms is not reassurance, because the mechanism gives no warning before it decompensates.
Management
Admit and monitor
Continuous monitoring from the moment the rhythm is recognised. Progression to complete block cannot be predicted from the strip.
Arrange permanent pacing
A Class I indication whether or not the patient has symptoms, once reversible causes are excluded.
Do not rely on atropine
The block is below the node, where atropine has no useful effect. By speeding the atrial rate it can increase the number of impulses that fail and make the block worse.
Have transcutaneous pacing ready
For the symptomatic or unstable patient, as a bridge to a transvenous wire and then to a permanent system.
Differential
The benign one, and the distinction that matters most. Its PR lengthens before the drop and its pause is shorter than two cycles. Measure two consecutive conducted PR intervals.
When the drops settle into every other beat there are no consecutive conducted beats to compare, and the strip can no longer tell you which type this is.
Where this rhythm is heading. There the P waves and QRS complexes lose all relationship; here most beats still conduct with a fixed PR.
A benign mimic. Its P is early and buried in the preceding T wave, and the rest of the conduction system is normal.
References
- 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
- AV Block: 2nd Degree, Mobitz II (Hay Block) — Life in the Fast Lane, ECG Library, 2024