1st Degree AV Block
normal1st Degree AV Block
Every atrial impulse still reaches the ventricles, only late: a PR interval over 200 ms, fixed on every beat.
First-degree AV block is the mildest of the conduction disturbances and the only one in which nothing is actually blocked. Every P wave conducts. The impulse is simply held up on its way through the AV node, so the PR interval stretches past its normal ceiling of 200 ms and then stays there, the same on every beat. In a structurally normal heart it is an incidental finding that changes nothing about rate, output or symptoms. It starts to matter in three situations: when the interval is very long, when the QRS is wide and the delay may not be in the node at all, and when it appears alongside other conduction disease and becomes one line of an argument about the whole system.
On the trace
Read the strip in this order.
- Check that nothing is missing. Every P wave has a QRS after it. Count them if you like: 36 of each in thirty seconds. Whatever is wrong here, it is not a dropped beat.
- Take the rate and the rhythm. 72 bpm, R-R 833 ms, identical on every beat. The block adds a delay, not an irregularity, so the rhythm is whatever is driving the atria underneath.
- Measure from the start of the P to the start of the QRS. 280 ms. The upper limit of normal is 200 ms, which is one large square, so this is a large square and a half.
- Measure it again on the next beat, and the next. 280 ms every time, varying by a millisecond either way. Fixed is the finding. A PR that creeps longer beat by beat is a different diagnosis.
- Measure the QRS. 74 ms, narrow. The impulse is slow getting through the node and perfectly normal once it is through.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| PR interval | Over 200 ms | 280 ms here. Fixed, not creeping. |
| Conduction | 1:1 | Every P conducts. This is the difference between delay and block. |
| Rate and rhythm | Whatever the atria are doing | 72 bpm regular here. First-degree block does not change the rate. |
| QRS | Narrow, 74 ms here | A wide QRS raises the possibility that the delay is below the node. |
| Where it sits | Usually the AV node | The commonest cause is high vagal tone or an AV-nodal blocking drug. |
The strip
Mechanism
The impulse leaves the sinus node normally and crosses the atria normally. At the AV node it is held up for longer than usual, and then it is released and travels on through the His-Purkinje system normally. The PR interval measures that whole journey, so a delay anywhere along it lengthens the PR, and the AV node is where the delay almost always is.
Nothing fails. Every impulse arrives, which is why the term block is misleading and why some authors prefer to call it delay.
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Where the delay actually sits
A narrow QRS points at the node itself, which is the usual case and the benign one. A wide QRS raises the possibility of delay in the His-Purkinje system instead, which behaves quite differently: nodal delay is stable and often reversible, while infranodal delay sits on a spectrum with the blocks that do drop beats.
When a long PR stops being harmless
Past roughly 300 ms the atria start contracting against valves that have already been closed by the previous ventricular systole. The atrial contribution to filling is lost and left atrial pressure rises, which can produce breathlessness and fatigue that look like heart failure and are sometimes labelled as such. The syndrome is uncommon, it needs a PR interval well beyond the diagnostic threshold, and it is the one situation in which first-degree block is itself worth treating.
It is not always a fixed finding
A long PR that appears only at rest and normalises with exercise is vagal, because exercise withdraws vagal tone and speeds nodal conduction. A long PR that worsens with exercise is the opposite and suggests disease below the node, because a faster atrial rate gives diseased tissue less time to recover.
What it predicts
In isolation, in a structurally normal heart, very little. It gains meaning in company: alongside bifascicular block, or in a patient with syncope, it becomes part of an argument about the whole conduction system rather than a finding on its own.
Clinical impact
For most people this is an incidental finding that needs a sentence of explanation and nothing else. It matters when it is very long, when the QRS is wide, or when it is keeping company with other conduction disease.
Management
Usually nothing at all
An asymptomatic long PR with a narrow QRS in a structurally normal heart needs no treatment and no follow-up.
Review the drug list
Beta-blockers, calcium-channel blockers, digoxin and many antiarrhythmics all lengthen the PR. Withdrawing one is worthwhile only if the interval is causing a problem.
Look harder when the QRS is wide
Delay below the node keeps different company. A wide QRS with a long PR, especially with syncope, deserves a cardiology opinion rather than reassurance.
Consider pacing only in the rare symptomatic case
A PR well past 300 ms causing demonstrable symptoms of lost atrial contribution is the one indication, and it is uncommon enough to warrant specialist confirmation.
Differential
Identical in every respect except the PR interval. Measure it rather than eyeballing it: 200 ms is one large square, and the difference between 190 and 220 is not obvious by eye.
The PR lengthens here too, but progressively, and it ends in a dropped beat. A first-degree block that creeps is a Wenckebach cycle you have not watched for long enough.
Fixed PR as here, but beats are dropped without warning. The PR being constant is common to both; whether a QRS ever goes missing is what separates them.
Also no visible sinus P before the QRS in some presentations, and easily confused if the P is hard to find. Here every P is upright, on time, and in front.
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