2nd Degree AV Block
critical3:1 AV Block
Only every third atrial impulse conducts: high-grade block, a very slow ventricle, and a pacemaker.
3:1 AV block is high-grade block, and the phrase is a category rather than a description of severity. Two or more consecutive non-conducted P waves means conduction is failing most of the time rather than occasionally, which places it between ordinary second-degree block and complete block, and it is managed from the complete-block end. The arithmetic is what makes it urgent: a third of any ordinary sinus rate leaves the ventricles in the twenties. As with any fixed conduction ratio, the strip cannot say whether the lesion sits in the AV node or below it, because the conducted beats never come in pairs to compare. At this rate that question decides the long-term plan rather than the immediate one.
On the trace
Read the strip in this order.
- Count P waves between QRS complexes. Three P waves for every QRS. Two of every three impulses fail before the third gets through.
- Take both rates. The atria are at 72 bpm, unchanged from every other block in this family. The ventricles are at 24 bpm, with an R-R of 2500 ms. Two and a half seconds between beats.
- Count the consecutive failures. Two in a row, every time. That is the definition of high-grade block, and it is what separates this from ordinary second-degree block rather than merely being a worse version of it.
- Measure the PR of the beats that do conduct. 220 ms, constant. As with any fixed ratio, that constancy is forced by the arithmetic and tells you nothing about the level of the block.
- Measure the QRS. 74 ms, narrow here. Narrow does not make this safe. At 24 bpm the rate alone is the emergency.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| P:QRS | 3:1, fixed | 72 P waves to 24 QRS complexes here. |
| Consecutive failures | Two or more | The definition of high-grade block. |
| Ventricular rate | Very slow | 24 bpm here, from an atrial rate of 72. |
| PR of conducted beats | Constant, 220 ms here | Constant by necessity. It cannot classify the block. |
| QRS | Narrow here, 74 ms | A wide QRS would make the outlook worse still. |
| Escape rhythm | Absent on this strip | In a real patient at this rate an escape focus has usually taken over. |
The strip
Mechanism
Conduction fails for two consecutive impulses and then succeeds on the third. Whether the failure is in the node or below it, the consequence is the same at this ratio: the ventricular rate falls to a third of the atrial rate, which at any ordinary sinus rate leaves it in the twenties or low thirties.
That is the point at which the arithmetic stops being an academic question. A rate of 24 bpm does not sustain a conscious adult, and the question is no longer what kind of block this is but how quickly the rate can be supported.
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Why this is not just a worse Mobitz
Second-degree block, of either type, drops isolated beats. High-grade block drops two or more in a row, which means conduction is failing most of the time rather than occasionally. It sits between second-degree and complete block on a spectrum, and it is managed from the complete-block end of it: pacing, urgently, rather than observation and a search for reversible causes.
What the strip does not show
At 24 bpm a real heart almost always produces an escape rhythm, junctional or ventricular, and those beats interrupt the pattern. Their presence does not contradict the diagnosis, and their absence here is a simplification of the simulation rather than a claim about what you will see. What matters clinically is that an escape rhythm at this point is a rescue, not a reassurance, and it must never be suppressed.
Narrow QRS is not reassurance here
In 2:1 block a narrow QRS genuinely shifts the odds towards a nodal, reversible lesion. At 3:1 the rate has already fallen far enough to be dangerous on its own, so the level of block changes the long-term plan rather than the immediate one.
The ratio can move
High-grade block is frequently unstable. A patient may be 3:1 on one strip and 2:1 or complete on the next, and the worst ratio recorded is the one to plan around.
Clinical impact
Two or more consecutive non-conducted P waves is a marker of severe conduction system disease in its own right, and the rate it produces is usually symptomatic before anyone has finished classifying it.
Management
Support the rate first
Transcutaneous pacing for the symptomatic or unstable patient, with a transvenous wire as the bridge. Classification can wait; perfusion cannot.
Arrange permanent pacing
High-grade block is a pacing indication once reversible causes are excluded, regardless of where the lesion turns out to sit.
Look for reversible causes in parallel
AV-nodal blocking drugs, hyperkalaemia, ischaemia and Lyme carditis, none of which should delay pacing while they are being investigated.
Never suppress an escape rhythm
If escape beats are propping the rate up, they are the only thing doing so.
Differential
The next step, and the one to exclude. There the P waves bear no relationship to the QRS complexes at all, whereas here every third P conducts with a fixed PR.
The same mechanism at a gentler ratio. The classification problem is identical; the rate is twice as fast and the urgency correspondingly less.
What usually appears at this rate in a real patient, and it can obscure the underlying pattern. Look for the non-conducted P waves marching through.
Also slow and regular with a narrow QRS. The P waves are the difference: here there are three for every QRS.
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, Fixed Ratio Blocks — Life in the Fast Lane, ECG Library, 2024