ECG rhythms AV blocks
First-degree AV block ECG
Sinus rhythm with first-degree AV block. Every P wave conducts, but the PR is prolonged beyond 200 ms and stays the same on every beat.
A 30-second recording of the heart model, played in real time. Run live starts the engine itself with this rhythm.
Computed by the TracingsMD heart model (Junkers engine): simulated for education, not a patient recording and not for diagnosis.
Measured on this recording
| Ventricular rate | 64/min |
|---|---|
| PR | 238 ms |
| QRS | 84 ms |
| QT | 370 ms |
| QTc (Bazett) | 382 ms |
| QTc (Fridericia) | 378 ms |
What it is
Every atrial impulse reaches the ventricles, but late: the PR interval is longer than 200 ms and stays the same on every beat. The delay is usually in the AV node.
How to read this strip
| Rate | 60–100/min. |
|---|---|
| Rhythm | Regular. |
| P waves | Upright, one before every QRS. |
| PR | Prolonged (over 200 ms) and constant. |
| QRS | Narrow. |
| QT | Normal. |
Causes
- Normal variant: athletes, high vagal tone.
- Drugs: beta-blockers, non-dihydropyridine calcium channel blockers (diltiazem, verapamil), digoxin, amiodarone.
- Age-related degeneration of the conduction system.
- Inferior myocardial infarction.
- Myocarditis, Lyme disease, rheumatic fever, infiltrative disease (sarcoidosis, amyloidosis).
- Endocarditis with an aortic root abscess (new PR prolongation is a warning sign).
- Neuromuscular disease (for example myotonic dystrophy).
- Hyperkalemia.
What to consider
- Usually benign and silent; the block itself needs no treatment.
- A very long PR (around 300 ms or more) can cause symptoms similar to pacemaker syndrome.
- Review AV-nodal blocking drugs; first-degree block together with a bundle branch block suggests more extensive (His–Purkinje) conduction disease.
- In Lyme carditis, a PR of 300 ms or more signals a high risk of complete heart block.
- Repeat ECGs can show progression, especially after new medications.
Key point
First-degree AV block is a delay, not a block: every P wave still conducts.
Not to be confused with
- Normal sinus rhythm — The PR interval is longer than 200 ms.
- Second-degree AV block, Mobitz I — The PR does not lengthen from beat to beat and no P wave is dropped.
- Junctional rhythm (simulator) — Each QRS is preceded by an upright sinus P wave.