First-degree AV block on an EKG

A 6-second lead II strip of first-degree AV block on ECG (EKG) paper, drawn by the ECG rhythm simulator’s model with a PR of 280 ms at 75 beats per minute, with the guideline criterion and every interval measured from it.

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.

Try it: Open first-degree 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.

What is the PR interval in first-degree heart block?

Longer than 200 ms, one large square at 25 mm/s, and the same in every beat. Every P wave is still followed by a QRS, so no beat is dropped and the rhythm stays regular at the sinus rate. The 2018 ACC/AHA/HRS guideline notes that it is more accurately called AV delay, since no P wave is blocked.

Defining criteria

Every P wave conducted, one QRS for each, with a PR interval longer than 200 ms. (Kusumoto et al. 2018)
The same PR in every beat, so the RR intervals stay equal at the sinus rate. (simulator model)
The simulator allows a PR of 210 to 400 ms, and moves a PR from the normal range to 280 ms rather than to the edge, so the block is plain to see. (simulator model)

Rates and intervals on this strip

Rates and intervals of the first-degree AV block strip above, from the simulator’s model
Ventricular rate	75 bpm
Atrial rate	75 per minute
P waves conducted	Every one
PR interval	280 ms
QRS duration	80 ms
QT interval	358 ms
QTc (Bazett)	400 ms
RR interval	800 ms
Regularity	Regular
Six-second count	7 QRS × 10 = 70 bpm

Measuring the strip

The PR runs 80 ms past the limit: 280 ms is 7 small squares where 200 ms is 5. The RR interval is still 800 ms, so the rate is the sinus rate, 75 beats per minute.

A long PR pushes each P wave back toward the T wave before it. At 280 ms they never meet at the rates the simulator allows, but at its longest PR, 400 ms, a sinus rate above about 81 puts each P wave on the T wave ahead of it, where it is easy to miss.

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 first-degree AV block, and the value the strip above uses
Setting	Allowed	This strip
Sinus rate	40 to 100 bpm	75 bpm
PR interval	210 to 400 ms	280 ms
QRS duration	60 to 160 ms	80 ms

Telling it apart

If the PR lengthens from beat to beat until a P wave is dropped, the rhythm is Mobitz type 1; a long PR that never changes and never drops a beat is first-degree block.
A PR of 120 to 200 ms with everything else the same is normal sinus rhythm.

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.