Mobitz type 2 AV block on an EKG

A 6-second lead II strip of second-degree AV block, Mobitz type 2, on ECG (EKG) paper with 4:3 conduction, drawn by the ECG rhythm simulator’s model, with the guideline criteria and the pause across the dropped beat.

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 2 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.

What does Mobitz type 2 look like on an EKG?

Every conducted beat has the same PR interval, before and after a P wave that suddenly fails to reach the ventricles, with no lengthening of the PR to warn of it. The block is usually below the AV node, in the His-Purkinje system, often with a wide QRS, and it can progress to complete heart block.

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 2: a single P wave periodically not conducted, with constant PR intervals before and after it, 2:1 block excluded. (Kusumoto et al. 2018)
The simulator allows P wave rates of 40 to 99 and groups of 3:2, 4:3, 5:4 or 6:5. The PR may be 120 to 280 ms. (simulator model)

Rates and intervals on this strip

Rates and intervals of the Mobitz type 2 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 ms
QRS duration	80 ms
QT interval	413 ms
QTc (Bazett)	400 ms
RR interval	800 to 1600 ms
Regularity	Regularly irregular
Six-second count	6 QRS × 10 = 60 bpm

Measuring the group

The PR is 160 ms in every conducted beat, so the conducted beats come every 800 ms, the PP interval. The dropped P wave leaves a gap of exactly 2 × 800 = 1600 ms, and 3 QRS complexes for every 4 P waves give 75 × 3 / 4 = 56.25 beats per minute.

The 2018 guideline recommends a permanent pacemaker for acquired Mobitz type 2 with no reversible or physiologic cause, whether or not it causes symptoms. Widen the QRS to 120 ms or more in the simulator to see the wide QRS it most often comes with.

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 2, and the value the strip above uses
Setting	Allowed	This strip
Sinus rate	40 to 99 bpm	75 bpm
PR interval	120 to 280 ms	160 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 1 the PR lengthens before the dropped P wave and the first PR after it is the shortest.
2:1 AV block has no two conducted PR intervals in a row to compare, so it is excluded from Mobitz type 2.
When no P wave reaches the ventricles at all, the rhythm is complete heart 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.