Third-degree (complete) heart block on an EKG

A 6-second lead II strip of third-degree AV block, complete heart block, on ECG (EKG) paper, drawn by the ECG rhythm simulator’s model with P waves at 75 per minute and a junctional escape at 44, with the guideline criterion and every P to QRS gap 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 complete heart 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.

How do you recognize complete heart block on an EKG?

The P waves and QRS complexes each keep their own regular rate with no link between them, so the gap from P wave to QRS changes every beat and the P waves outnumber the QRS complexes. An escape rhythm drives the ventricles: junctional at 40 to 60 per minute, or ventricular at 20 to 40.

Defining criteria

No evidence of conduction from the atria to the ventricles. (Kusumoto et al. 2018)
In the model a junctional escape keeps the QRS width set for the rhythm, 80 ms here, and a ventricular escape has a QRS of 140 ms and a T wave pointing the other way. (simulator model)
The simulator holds the atrial rate at 65 to 110, above the fastest escape rate, so the P waves always outpace the QRS complexes. (simulator model)

Rates and intervals on this strip

Rates and intervals of the complete heart block strip above, from the simulator’s model
Ventricular rate	44 bpm
Atrial rate	75 per minute
P waves conducted	None
PR interval	None, no P wave is conducted
QRS duration	80 ms
QT interval	467 ms
QTc (Bazett)	400 ms
RR interval	1364 ms
Regularity	Regular
Six-second count	4 QRS × 10 = 40 bpm

Marching out the P waves

From the last P wave to each QRS on the strip the gap is 420, 184, 747 and 511 ms, never the same twice, which is what no conduction looks like. The P waves keep coming every 800 ms straight through the beats: 3 of the 8 on this strip start inside a QRS complex or its T wave, where only calipers set from the visible ones find them.

The escape rate is 44 rather than a round 45 for a reason. 75 and 45 share a factor, so with those rates the pattern would repeat every 4 seconds and the P waves would land at only 5 points of the escape cycle, which can look like a fixed link between them. 75 and 44 share none, so the pattern takes 60 seconds to repeat and the P waves drift through every part of the cycle.

The 2018 guideline recommends a permanent pacemaker for acquired third-degree AV block with no reversible or physiologic cause, whether or not it causes symptoms.

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 complete heart block, and the value the strip above uses
Setting	Allowed	This strip
Atrial (sinus) rate	65 to 110 bpm	75 bpm
Escape rate, junctional	40 to 60 bpm	44 bpm
QRS duration	60 to 160 ms	80 ms
Escape rate, ventricular	20 to 40 bpm	Not drawn
Escape rhythm	Junctional or ventricular	Junctional

Telling it apart

In 2:1 AV block every conducted beat keeps the same PR; in complete heart block the gap changes from beat to beat.
Two or more P waves in a row not conducted, with conduction at other times, is advanced or high-grade AV block, which the guideline defines separately and the simulator does not draw.
A slow rhythm with one P wave before each QRS at a fixed PR is sinus bradycardia.

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.