Mobitz type 1 (Wenckebach) on an EKG

A 6-second lead II strip of second-degree AV block, Mobitz type 1, on ECG (EKG) paper with 4:3 conduction, drawn by the ECG rhythm simulator’s model, with the guideline criteria and each PR and RR interval measured.

Lead II, 6 seconds at 25 mm/s and 10 mm/mV: each small square is 0.04 s and 0.1 mV, each large square 0.2 s and 0.5 mV, and the step at the left is the 1 mV calibration pulse. Filled triangles mark P waves that reached the ventricles, open triangles those that did not.

Try it: Open Mobitz type 1 in the ECG simulator →

A study tool, not a clinical one

This page exists for learning what defines each rhythm and how it is measured on paper. It is not clinical decision support and must not be used to diagnose or treat anybody.

The strip is a model, not a recording. Every interval holds for the whole minute and every beat has the same shape, where a real rhythm varies with breathing, autonomic tone and drugs, and real complexes vary between people and between leads.
One lead is not a 12-lead ECG. The axis, bundle branch block patterns, ST changes and pre-excitation need all twelve leads, read by somebody trained to read them.
Definitions differ between sources. Guidelines, textbooks and institutions do not all draw the same rate and interval limits, so check which ones your course or institution uses.
Every criterion names its source. If one does not match what you were taught, check it against the source listed below, and tell us if we have it wrong. How the pages are checked is set out in the editorial policy.

How do you recognize Mobitz type 1 on an EKG?

The PR interval lengthens from beat to beat until a P wave is not followed by a QRS, and the first PR after the pause is the shortest of the group. In typical Wenckebach the PR grows by less each beat, so the RR intervals shorten before the pause and the pause is shorter than two PP intervals.

Defining criteria

Second-degree AV block: P waves at a constant rate below 100 per minute, with AV conduction present but not 1:1. (Kusumoto et al. 2018)
Mobitz type 1: a single P wave periodically not conducted, with the PR intervals of the P waves before and after it not constant. (Kusumoto et al. 2018)
In the model the PR grows by the set step on the second beat of a group and by half the step before on each beat after that, so the steps shrink as typical Wenckebach periodicity does. (simulator model)
The simulator allows P wave rates of 40 to 99 and groups of 3:2, 4:3, 5:4 or 6:5. The first PR may be 120 to 280 ms and the first step 20 to 120 ms. (simulator model)

Rates and intervals on this strip

Rates and intervals of the Mobitz type 1 strip above, from the simulator’s model
Ventricular rate	56.25 bpm
Atrial rate	75 per minute
P waves conducted	3 of every 4
PR interval	160 to 280 ms
QRS duration	80 ms
QT interval	413 ms
QTc (Bazett)	400 ms
RR interval	840 to 1480 ms
Regularity	Regularly irregular
Six-second count	6 QRS × 10 = 60 bpm

Measuring the group

On this strip the PRs of a group are 160, 240 and 280 ms, and the P waves come every 800 ms. Each RR interval inside the group is that PP interval plus the step in the PR: 800 + 80 = 880 ms, then 800 + 40 = 840 ms.

Across the dropped P wave the RR interval is two PP intervals less the growth of the PR over the group: 2 × 800 − (280 − 160) = 1480 ms, against 1600 ms for two full cycles. The beats come in groups of 3 with a pause between, the group beating that gives the rhythm away on a long strip.

With 3 QRS complexes for every 4 P waves the ventricular rate is 75 × 3 / 4 = 56.25 beats per minute, the same as Mobitz type 2 at the same settings, so the rate alone never tells the two apart. The model always halves the steps; real Wenckebach groups, long ones especially, often do not, and then the RR intervals fail to shorten.

Settings the simulator allows for this rhythm

Each rhythm limits its settings to the values its definition allows, so the simulator can never draw it as something else.

Settings the ECG simulator allows for Mobitz type 1, and the value the strip above uses
Setting	Allowed	This strip
Sinus rate	40 to 99 bpm	75 bpm
First PR of each group	120 to 280 ms	160 ms
PR lengthening	20 to 120 ms	80 ms
QRS duration	60 to 160 ms	80 ms
Conduction ratio	3:2, 4:3, 5:4 or 6:5	4:3

Telling it apart

In Mobitz type 2 the PR is the same before and after the dropped P wave, and the RR interval across it is exactly two P to P intervals.
When every second P wave is dropped the strip is 2:1 AV block, and the type cannot be read from it.
A long PR that never changes and never drops a beat is first-degree AV block.

Sources

Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay. Circulation. 2019;140(8):e382-e482. doi:10.1161/CIR.0000000000000628
Kligfield P, Gettes LS, Bailey JJ, et al. Recommendations for the Standardization and Interpretation of the Electrocardiogram, part I. Circulation. 2007;115:1306-1324. doi:10.1161/CIRCULATIONAHA.106.180200
McSharry PE, Clifford GD, Tarassenko L, Smith LA. A dynamical model for generating synthetic electrocardiogram signals. IEEE Transactions on Biomedical Engineering. 2003;50(3):289-294. doi:10.1109/TBME.2003.808805
Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353-370.