2nd Degree AV Block
caution2:1 AV Block
Every other P wave conducts, which halves the rate and hides whether the block is Mobitz I or II.
2:1 AV block is the one second-degree block that cannot be classified from the strip, and knowing why is more useful than any feature on it. Mobitz I is recognised by a PR interval that lengthens from one conducted beat to the next, and at a 2:1 ratio there are never two conducted beats in a row, so the evidence that would decide the diagnosis cannot exist. The PR of the conducted beats looks reassuringly constant, but it is constant by arithmetic rather than by nature. What is left is indirect: QRS width, the length of the conducted PR, and what happens to the ratio when the atrial rate changes. The distinction is worth the effort, because nodal block is often reversible and infranodal block is a pacemaker.
On the trace
Read the strip in this order.
- Count P waves against QRS complexes. Two to one, on every cycle. 36 QRS complexes in thirty seconds against 72 P waves.
- Take both rates. The atria are at 72 bpm with a P-P of 833 ms. The ventricles are at 36 bpm with an R-R of 1667 ms, exactly double. Both rhythms are individually regular, which is why this reads as a plain bradycardia until you look for the extra P waves.
- Find the P waves you are missing. The blocked one sits on the baseline roughly 600 ms after each QRS, clear of the T wave and plainly visible once you know to look. It is easy to read as a notch in the ST segment and dismiss. At faster atrial rates it moves earlier and does hide inside the T, which is where this rhythm usually gets missed.
- Measure the PR of the conducted beats. 220 ms every time. Now notice what that does not tell you: there are never two conducted beats in a row, so there is no way to know whether the PR would have lengthened. This is the whole problem with 2:1 block.
- Measure the QRS. 74 ms, narrow. That is the one real clue on this strip, and it points towards the node rather than below it.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| P:QRS | 2:1, fixed | 72 P waves to 36 QRS complexes here. |
| Both rhythms | Regular | Atrial 833 ms, ventricular 1667 ms. The regularity is what disguises it. |
| The blocked P wave | Clear of the T here | On the baseline between complexes at this rate. Faster atrial rates push it into the T, where it is missed. |
| PR of conducted beats | Constant, 220 ms here | Constant by necessity, not by nature. It cannot classify the block. |
| QRS | Narrow here, 74 ms | Narrow favours nodal, wide favours infranodal. The most useful clue available. |
| Classification | Neither Mobitz I nor II | The strip cannot decide. Say so rather than guessing. |
The strip
Mechanism
Every second atrial impulse fails to reach the ventricles. Where it fails is the question the strip cannot answer, and the answer is what matters, because nodal block behaves like Wenckebach and infranodal block behaves like Mobitz II.
Both produce an identical picture at a 2:1 ratio. Mobitz I needs two consecutive conducted beats to show its lengthening PR, and 2:1 conduction never provides them. So the pattern that would classify the block is arithmetically unavailable, and the diagnosis has to be made from indirect evidence.
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The evidence you do have
A narrow QRS points to the node, a wide one below it, and this is the most dependable single clue on the strip. A very long PR in the conducted beats, past roughly 300 ms, also favours the node, because that degree of delay is characteristic of nodal tissue. Neither is proof. Taken together with the clinical context they are usually enough to decide how urgently to act.
Provocation, which settles it properly
Change the atrial rate and watch what the conduction does. Exercise or atropine speeds the atrial rate and improves nodal conduction, so nodal block often improves to 3:2 or 1:1 and reveals itself as Wenckebach. The same manoeuvres make infranodal block worse, because faster impulses give diseased His-Purkinje tissue less time to recover, and the ratio may fall to 3:1. Carotid sinus massage does the reverse and is the other half of the same test.
The trap in the rate
At 36 bpm with a narrow QRS this looks like sinus bradycardia, and it will be read as such by anyone who does not go looking for the second P wave. The habit worth forming is to check any bradycardia at close to half a plausible sinus rate for hidden P waves in the T waves, whatever the QRS width.
When it is not second-degree block at all
A 2:1 ratio that persists while the ratio elsewhere on the recording is 3:1 or worse is high-grade block, and that is managed as advanced conduction disease rather than as this.
Clinical impact
The immediate problem is the rate. The longer-term question is the level of block, and it decides whether this needs a pacemaker or a change of medication.
Management
Work out the level of block
QRS width first, then the PR interval of the conducted beats, then a previous ECG for comparison. A gradual change favours the node; an abrupt appearance favours disease below it.
Provoke a change in the ratio
Exercise or atropine improves nodal block and worsens infranodal block. Watching which way the ratio moves is more informative than any static feature.
Treat it as Mobitz II when the QRS is wide
A wide QRS, or symptoms with an unclear level of block, is managed as infranodal disease. Permanent pacing rather than observation.
Correct reversible causes when the QRS is narrow
A narrow QRS with a long PR and an AV-nodal blocking drug or high vagal tone is often reversible, and withdrawing the cause may restore 1:1 conduction.
Differential
The rhythm this is mistaken for, because the QRS is narrow and the ventricles are regular. The difference is a second P wave hiding in every T.
What this may be underneath. Provoke a faster atrial rate and a nodal block will often improve to 3:2 and show its lengthening PR.
The other thing this may be underneath, and the reason a wide QRS here is treated urgently.
A benign mimic that also halves the ventricular rate. Its blocked P waves are early rather than on time, so the P-P interval alternates instead of staying constant.
References
- 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay — Circulation, 2019
- Second-Degree Atrioventricular Block: A Reappraisal — Mayo Clinic Proceedings, 2001
- AV Block: 2nd Degree, Fixed Ratio Blocks — Life in the Fast Lane, ECG Library, 2024