ECG rhythms AV blocks
Second-degree AV block with 2:1 conduction ECG
Second-degree AV block with 2:1 conduction; the type cannot be determined from this strip. Every second P wave is blocked. Without two consecutive conducted beats, Mobitz I and II cannot be told apart.
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 | 42/min |
|---|---|
| PR | 158 ms |
| QRS | 84 ms |
| QT | 425 ms |
| QTc (Bazett) | 356 ms |
| QTc (Fridericia) | 377 ms |
What it is
Second-degree AV block in which every second P wave is not conducted: two P waves for each QRS, so the ventricular rate is half the atrial rate. The blocked P wave often sits on or near the T wave. Because no two conducted beats come in a row, the strip cannot show whether the PR lengthens before a dropped beat, so it cannot be classified as Mobitz I (usually in the AV node) or Mobitz II (usually below it).
How to read this strip
| Rate | Ventricular rate half the atrial rate. |
|---|---|
| Rhythm | Regular. |
| P waves | Regular; every second one not conducted. |
| PR | Constant on conducted beats (within a few ms). |
| QRS | Narrow. |
| QT | Normal for the slow rate. |
Causes
- At the AV node: high vagal tone, drugs that slow AV conduction (beta-blockers, non-dihydropyridine calcium channel blockers, digoxin), inferior myocardial infarction.
- Below the AV node: degenerative conduction system disease, anterior myocardial infarction, after cardiac surgery or transcatheter aortic valve replacement, infiltrative disease (sarcoidosis, amyloidosis).
- Hyperkalemia, Lyme carditis, myocarditis.
What to consider
- A wide QRS (bundle branch block) or a normal PR on the conducted beats favors block below the AV node; a narrow QRS or a very long PR favors the AV node, but neither proves it.
- Look for two conducted beats in a row on a longer recording or at another rate: a lengthening PR before the drop means Mobitz I, a constant PR means Mobitz II.
- Assess symptoms and perfusion. Unstable bradycardia follows the adult bradycardia algorithm with transcutaneous pacing ready: atropine helps block in the AV node but can worsen block below it by speeding the atrial rate. If block below the AV node is likely, do not rely on atropine: go to pacing or a chronotropic infusion with expert help.
- Review drugs and potassium, and record a 12-lead ECG for ischemia.
- Without a reversible cause, 2:1 block below the AV node generally needs a permanent pacemaker even without symptoms, as does 2:1 block at any level that causes symptoms (2018 ACC/AHA/HRS); exercise testing or an electrophysiology study can locate the level when it is unclear.
- At very fast atrial rates, as in atrial flutter, 2:1 conduction is the expected AV node response, not AV block; atrial tachycardia with block should prompt a check for digoxin toxicity.
Key point
In 2:1 AV block, say "2:1 block" rather than guessing the Mobitz type. A narrow QRS favors block in the AV node, but it is not proof: a longer recording with two conducted beats in a row can show the type, and if 2:1 persists, the response to exercise or atropine can help locate the level.
Not to be confused with
- Second-degree AV block, Mobitz I — Mobitz I is proven by a PR that lengthens over consecutive conducted beats. With 2:1 there are never two in a row.
- Second-degree AV block, Mobitz II — A constant PR is expected with 2:1 because each conducted beat follows a dropped one; it does not prove Mobitz II.
- Sinus bradycardia — A second P wave sits between each pair of QRS complexes, often near the T wave. The atrial rate is twice the ventricular rate.