Normal sinus rhythm on an EKG

A 6-second lead II strip of normal sinus rhythm on ECG (EKG) paper, drawn by the ECG rhythm simulator’s model at 75 beats per minute, with the criteria that define it 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 normal sinus rhythm 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 normal sinus rhythm look like on an EKG?

An upright P wave in lead II before every QRS complex, each after the same PR interval, a narrow QRS and equal RR intervals, at 60 to 100 beats per minute at rest. Every impulse starts in the sinus node and every one reaches the ventricles, so the P waves and QRS complexes run at one rate.

Defining criteria

Sinus P waves, all of one shape and upright in lead II, each followed by a QRS after the same PR interval. (simulator model)
A sinus rate of 60 to 100 beats per minute, the conventional limits at rest. (simulator model)
A PR interval of 200 ms or less; a longer one is first-degree AV block. The simulator allows 120 to 200 ms. (Kusumoto et al. 2018)
A QRS under 120 ms. At 120 ms or more, the width of a bundle branch block, the simulator names the rhythm sinus rhythm with a wide QRS instead. (Surawicz et al. 2009)
Every RR interval the same length. (simulator model)

Rates and intervals on this strip

Rates and intervals of the normal sinus rhythm strip above, from the simulator’s model
Ventricular rate	75 bpm
Atrial rate	75 per minute
P waves conducted	Every one
PR interval	160 ms
QRS duration	80 ms
QT interval	358 ms
QTc (Bazett)	400 ms
RR interval	800 ms
Regularity	Regular
Six-second count	8 QRS × 10 = 80 bpm

Measuring the strip

Each RR interval is 0.8 s, 20 small squares or 4 large ones, so the paper gives 1500 / 20 = 75 beats per minute. The PR is 160 ms, 4 small squares, and the QRS 80 ms, 2 small squares.

Each T wave ends 358 ms after its QRS starts, and the next P wave begins 282 ms after that: the flat TP segment, the stretch of baseline between one beat and the next.

The six-second count finds 8 QRS complexes, which gives 80, not 75: the method counts whole complexes, so it is good to about 10 beats per minute and is meant for irregular rhythms, where no single RR interval stands for the rate.

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 normal sinus rhythm, and the value the strip above uses
Setting	Allowed	This strip
Sinus rate	60 to 100 bpm	75 bpm
PR interval	120 to 200 ms	160 ms
QRS duration	60 to 160 ms	80 ms

Telling it apart

Below 60 per minute the same rhythm is sinus bradycardia, and above 100 it is sinus tachycardia.
A PR longer than 200 ms with everything else unchanged is first-degree AV block.
An early beat that breaks the run of equal RR intervals is a PAC if it has a P wave and a narrow QRS, or a PVC if it is wide with none.

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
Surawicz B, Childers R, Deal BJ, Gettes LS. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, part III: intraventricular conduction disturbances. Circulation. 2009;119(10):e235-e240. doi:10.1161/CIRCULATIONAHA.108.191095
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.