Sinus bradycardia on an EKG

A 6-second lead II strip of sinus bradycardia on ECG (EKG) paper at 50 beats per minute, drawn by the ECG rhythm simulator’s model, with the two definitions of its rate limit 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 sinus bradycardia 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 heart rate is sinus bradycardia on an EKG?

Below 60 beats per minute by the conventional limit, or below 50 in the 2018 ACC/AHA/HRS guideline, with an upright P wave before every QRS, the same PR in every beat and equal RR intervals. Nothing else changes from normal sinus rhythm: the sinus node fires more slowly.

Defining criteria

Sinus rhythm slower than 60 beats per minute, the conventional limit. The simulator allows 30 to 59. (simulator model)
A sinus rate below 50 beats per minute, in the guideline’s definition. (Kusumoto et al. 2018)
A P wave before every QRS, a PR of 200 ms or less and equal RR intervals, as in normal sinus rhythm. (simulator model)

Rates and intervals on this strip

Rates and intervals of the sinus bradycardia strip above, from the simulator’s model
Ventricular rate	50 bpm
Atrial rate	50 per minute
P waves conducted	Every one
PR interval	160 ms
QRS duration	80 ms
QT interval	438 ms
QTc (Bazett)	400 ms
RR interval	1200 ms
Regularity	Regular
Six-second count	5 QRS × 10 = 50 bpm

Measuring the strip

Each RR interval is 1.2 s, 30 small squares or 6 large ones, so the paper gives 1500 / 30 = 50 beats per minute. The PR is 160 ms, 4 small squares, and the QRS 80 ms, 2 small squares.

The QT stretches with the cycle: 438 ms here against 358 ms at 75 beats per minute, while Bazett’s correction, QTc = QT / √RR, gives 400 ms for both. A long QT on a slow strip is not a long QTc until it has been corrected.

This strip, at exactly 50, meets the conventional definition and misses the guideline’s by one beat per minute, so a report should say which definition it uses. Resting rates in the 50s are common in fit adults and during sleep.

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 sinus bradycardia, and the value the strip above uses
Setting	Allowed	This strip
Sinus rate	30 to 59 bpm	50 bpm
PR interval	120 to 200 ms	160 ms
QRS duration	60 to 160 ms	80 ms

Telling it apart

A slow regular ventricular rate with twice as many P waves as QRS complexes is 2:1 AV block: look for the P wave between each pair of beats.
P waves that march through the QRS complexes at a faster rate of their own point to complete heart block.
From 60 to 100 the same rhythm 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.