3rd Degree (Complete) AV Block
critical3rd Degree (Complete) AV Block
Nothing crosses the block: the atria and a slower escape pacemaker beat entirely independently.
In complete heart block no atrial impulse reaches the ventricles at all. The sinus node carries on unaware, the atria contract at their own rate, and the ventricles are kept going by whichever subsidiary pacemaker sits closest below the lesion. Both rhythms are regular and neither has anything to do with the other, so any apparent PR interval is coincidence and drifts from beat to beat. The atria are always the faster of the two, and that is worth holding onto: it is what separates block from the dissociation produced by a junctional or ventricular focus outrunning a healthy sinus node. The escape rhythm carries the prognosis. A narrow junctional escape near 50 is reasonably stable; a wide ventricular escape in the twenties or thirties can stop without warning.
On the trace
Read the strip in this order.
- Count both sets of complexes. There are far more P waves than QRS complexes: 72 P waves in thirty seconds against 19 QRS. That imbalance is the first sign, and it is easier to see than any interval.
- Time the P waves against each other. 833 ms apart, every time. 72 bpm, perfectly regular. The sinus node is healthy and has no idea anything is wrong.
- Time the QRS complexes against each other. 1579 ms apart, every time. 38 bpm, also perfectly regular. Two metronomes, each keeping good time.
- Try to measure a PR interval. You cannot. The gap between a P wave and the QRS after it reads 91 ms on one beat and 482 ms three beats later, drifting steadily. P waves appear before a QRS, inside one, and on top of a T wave, in turn. There is no PR interval here, only coincidence.
- Ask which chamber is faster. The atria, 72 against 38. That is the arithmetic of block: impulses cannot get through, so something below has taken over more slowly.
- Measure the QRS. 173 ms, wide, with an inverted T. The escape focus is in ventricular muscle, below the His-Purkinje system, and that is the worse of the two possibilities.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| Relationship | None | Complete AV dissociation. The defining finding. |
| Atrial rate | Regular, and the faster | 72 bpm here. Faster than the ventricles, which is what says block. |
| Ventricular rate | Regular, and slow | 38 bpm here. Set by whichever escape focus took over. |
| PR interval | Meaningless | Any apparent value drifts from beat to beat. |
| Escape QRS | Wide, 173 ms here | Wide and slow means ventricular. Narrow at 40 to 60 means junctional, and is more stable. |
| Pacing | Class I indication | Once reversible causes are excluded. |
The strip
Mechanism
No atrial impulse reaches the ventricles. The sinus node carries on at its own rate and the atria contract normally, but every impulse dies at the level of the block, so nothing crosses to the ventricles at all.
Left with no input, the ventricles rely on whichever subsidiary pacemaker is closest below the lesion. Block in the node leaves the junction available, which paces at 40 to 60 with a narrow QRS. Block below the His bundle leaves only ventricular muscle, which paces slower, with a wide QRS, and much less reliably. Which escape appears is therefore a statement about where the block is, and it is the most useful prognostic information on the strip.
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Why the escape rhythm decides the prognosis
A junctional escape is an established pacemaker doing a job it is designed for. It is reasonably fast, reasonably stable, and it responds a little to exercise and catecholamines. A ventricular escape is none of those things: it is slow, it does not respond, and it can stop firing without warning, which is the mechanism of a Stokes-Adams attack. The two carry quite different risks, and the QRS width is what separates them at a glance.
Dissociation is not the diagnosis
Atrioventricular dissociation means the chambers are beating independently, and complete block is only one cause. The discriminator is which chamber is faster. Here the atria lead and the ventricles are being paced by default, which means impulses cannot get through. In a junctional or ventricular tachycardia the ventricles lead, because a focus has outrun a sinus node that is working perfectly well and nothing is blocked. Compare the two rates before reaching for the word block.
Inferior against anterior infarction
Block complicating an inferior myocardial infarction is usually nodal, often vagally mediated, generally produces a narrow escape and frequently resolves. Block complicating an anterior infarction means enough of the septum has infarcted to destroy the bundle branches, produces a wide escape, and carries a much worse prognosis. The same rhythm label covers two very different situations.
Reversible causes worth excluding first
Hyperkalaemia, AV-nodal blocking drugs, Lyme carditis, endocarditis with an aortic root abscess, and the early phase of an inferior infarction. All can produce complete block that resolves, and all are worth identifying before a permanent device is implanted, though none of them should delay temporary pacing in a compromised patient.
Clinical impact
A narrow junctional escape in the fifties may be tolerated for a long time. A wide ventricular escape in the twenties or thirties is a different situation, and the danger is not the rate but the possibility of it stopping altogether.
Management
Pace temporarily, straight away, if the patient is compromised
Transcutaneous pacing immediately, then a transvenous wire as a bridge. Bradycardia of this severity is never a wait-and-reassess finding.
Arrange permanent pacing for almost everyone
A Class I indication once reversible causes are excluded. An escape rhythm cannot be relied on to keep going.
Do not let drug trials delay it
Atropine works on the node and this block is usually at or below it. By speeding the atrial rate it can occasionally make matters worse.
Treat reversible causes alongside, not instead
Correct potassium, withdraw AV-nodal blocking drugs, and treat ischaemia or infection while pacing is being arranged.
Differential
High-grade block, and the closest mimic. There every third P still conducts with a fixed PR, so a relationship exists. Here no PR value repeats.
The mirror image, and the reason to compare rates first. Also dissociated, but the ventricles are the faster chamber because a focus has usurped the sinus node rather than anything being blocked.
What the escape rhythm looks like on its own when the sinus node is the problem rather than the conduction system. There the atria are slow or silent; here they are fast and simply unheard.
The same wide, slow escape without the atrial activity above it. If P waves are marching through at 72, this is complete block rather than an idioventricular rhythm.
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
- Atrioventricular Dissociation — StatPearls, NCBI Bookshelf NBK563205, 2023