Premature atrial contractions (PACs) on an EKG

A 6-second lead II strip of sinus rhythm with premature atrial contractions on ECG (EKG) paper, one every fourth beat, drawn by the ECG rhythm simulator’s model, with its coupling interval and the pause that follows 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.

Try it: Open premature atrial contractions 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 a PAC look like on an EKG?

An early P wave with a different shape from the sinus P waves, usually followed by a normal narrow QRS. A PAC usually resets the sinus node, so the pause after it is shorter than compensatory. A PAC every second beat is atrial bigeminy, every third trigeminy and every fourth quadrigeminy.

Defining criteria

In the model the premature P wave is lower and shorter than a sinus one, with a negative tail, and is conducted at the sinus PR to a normal QRS. (simulator model)
A coupling interval of 55 to 90 percent of the sinus cycle in the simulator, plainly early and plainly separate from the next sinus beat. (simulator model)
In the model the premature impulse always resets the sinus node, so the next sinus P wave comes exactly one sinus cycle after it. (simulator model)

Rates and intervals on this strip

Rates and intervals of the premature atrial contractions strip above, from the simulator’s model
Ventricular rate	82.19 bpm
Atrial rate	82.19 per minute
P waves conducted	Every one
PR interval	160 ms
QRS duration	80 ms
QT interval	342 ms
QTc (Bazett)	400 ms
RR interval	520 to 800 ms
Regularity	Regularly irregular
Six-second count	8 QRS × 10 = 80 bpm

Measuring the pause

The PAC comes 65 percent of the way through the sinus cycle, 0.65 × 800 = 520 ms after the P wave before it. The sinus node restarts from it, so the QRS complexes either side of the PAC are 520 + 800 = 1320 ms apart, less than the 1600 ms of two sinus cycles.

At this coupling the PAC’s P wave starts 18 ms after the T wave before it ends. At the shortest coupling the simulator allows, 55 percent, it lands on that T wave, where an early PAC hides, deforming the T wave rather than showing as a P wave of its own.

Because each PAC shortens one cycle and the next is an ordinary sinus cycle, the mean ventricular rate rises above the sinus rate of 75, to 82.19 beats per minute.

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 premature atrial contractions, and the value the strip above uses
Setting	Allowed	This strip
Sinus rate	50 to 100 bpm	75 bpm
PR interval	120 to 200 ms	160 ms
Coupling interval	55 to 90 percent of the sinus cycle	65 percent of the sinus cycle
QRS duration	60 to 160 ms	80 ms
Premature beat	Every second, third, fourth or sixth beat	Every fourth beat

Telling it apart

A PVC is wide, has no P wave of its own, and leaves a full compensatory pause.
Irregular RR intervals with no P waves at all are atrial fibrillation.

Sources

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.