EKG rhythms and their criteria

Each of the 12 rhythms below has a page of its own, with a 6-second lead II ECG (EKG) strip drawn from the ECG rhythm simulator’s model, the criteria that define it with their sources, and every rate and interval measured from the strip.

How do you read an EKG rhythm strip?

Ask the same five questions every time: the rate, whether the rhythm is regular, whether a P wave comes before every QRS, how long the PR interval is, and how wide the QRS is. When P waves outnumber QRS complexes, some are not reaching the ventricles, and how the PR behaves around each dropped one tells the blocks apart.

Every rhythm side by side

Each row is the strip on that rhythm’s page, at the settings the ECG rhythm simulator opens it on. Times are in ms.

Rate, regularity, PR, QRS and QT of each rhythm’s strip
Rhythm	Ventricular rate	Atrial rate	Regularity	PR	QRS	QT
Normal sinus rhythm	75	75	regular	160	80	358
Sinus bradycardia	50	50	regular	160	80	438
Sinus tachycardia	120	120	regular	160	80	283
First-degree AV block	75	75	regular	280	80	358
Mobitz type 1 (Wenckebach)	56.25	75	regularly irregular	160 to 280	80	413
Mobitz type 2	56.25	75	regularly irregular	160	80	413
2:1 AV block	37.5	75	regular	160	80	506
Third-degree (complete) heart block	44	75	regular	None conducted	80	467
Atrial fibrillation	100	None	irregularly irregular	No P waves	80	310
Atrial flutter	75	300	regular	Flutter waves	80	358
Premature atrial contractions (PACs)	82.19	82.19	regularly irregular	160	80	342
Premature ventricular contractions (PVCs)	75	75	regularly irregular	160	80, 140	358

Rate from the large squares

At the standard 25 mm/s and 10 mm/mV, a small square is 0.04 s and 0.1 mV, and a large square of five is 0.2 s and 0.5 mV. A minute of paper is 300 large squares, so a regular rhythm’s rate is 300 divided by the large squares from one R wave to the next:

Heart rate for an RR interval of one to six large squares at 25 mm/s
Large squares	RR interval	Rate, bpm
1	0.2 s	300
2	0.4 s	150
3	0.6 s	100
4	0.8 s	75
5	1 s	60
6	1.2 s	50

Between two lines, use the small squares instead: 1500 divided by their number. For an irregular rhythm, count the QRS complexes in 6 seconds and multiply by 10.

Every strip here can be played, measured with calipers and changed in the ECG (EKG) Rhythm and Heart Block Simulator. The QTc Calculator compares Bazett’s correction with others, and the cardiac cycle simulator draws the same beat under its Wiggers diagram.