2:1 AV block on an EKG

A 6-second lead II strip of 2:1 AV block on ECG (EKG) paper, drawn by the ECG rhythm simulator’s model at 75 P waves per minute, with the guideline criteria and why the strip cannot say which type of second-degree block it is.

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 2:1 AV block 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.

Is 2:1 AV block Mobitz type 1 or type 2?

Neither can be called from the strip alone. With every second P wave dropped there are never two conducted PR intervals in a row, so the PR cannot be seen to lengthen or to stay constant. The 2018 ACC/AHA/HRS guideline defines 2:1 block on its own and leaves it out of Mobitz type 2.

Defining criteria

P waves at a constant rate below 100 per minute, every other P wave conducted to the ventricles. (Kusumoto et al. 2018)
The guideline’s Mobitz type 2 definition excludes 2:1 block, so it stands as a category of its own. (Kusumoto et al. 2018)
The simulator draws the block in the AV node when the QRS is narrow and below it when the QRS is 120 ms or wider, and allows P wave rates of 40 to 99. (simulator model)

Rates and intervals on this strip

Rates and intervals of the 2:1 AV block strip above, from the simulator’s model
Ventricular rate	37.5 bpm
Atrial rate	75 per minute
P waves conducted	1 of every 2
PR interval	160 ms
QRS duration	80 ms
QT interval	506 ms
QTc (Bazett)	400 ms
RR interval	1600 ms
Regularity	Regular
Six-second count	4 QRS × 10 = 40 bpm

Measuring the strip

The ventricular rate is half the atrial rate, 75 / 2 = 37.5 beats per minute, and every RR interval is two PP intervals, 2 × 800 = 1600 ms. Every conducted beat has the same PR, 160 ms, so the ventricular rhythm is regular.

The dropped P wave starts 134 ms after the T wave ends, alone on the baseline between two beats. With a faster atrial rate and a longer PR it moves into the T wave, where the strip can pass for a slow sinus rhythm until the P waves are marched out with calipers.

A narrow QRS points to a block in the AV node and a wide one to the conduction system below it, which is how the simulator’s ladder decides where to draw the block. Telling them apart with certainty needs more than this strip, such as a longer recording that catches two conducted beats in a row.

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 2:1 AV block, 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

Telling it apart

If two conducted beats appear in a row and the PR lengthens before the drop, the block is Mobitz type 1; if the PR stays the same it is Mobitz type 2.
A slow regular rhythm with one P wave per QRS is sinus bradycardia: in 2:1 block a second P wave sits between each pair of beats.
If the gap from P wave to QRS changes from beat to beat, no P wave is conducted and 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.