Second-degree AV block, type I (Wenckebach) on an ECG strip

The PR lengthens beat by beat until a P wave is not conducted, then resets: grouped beating.

What to look for

The PR gets longer beat by beat until a P wave doesn't conduct, then the PR resets. The biggest jump comes between the first and second beats, so the R-R intervals shorten through each group. The pause holding the dropped beat is shorter than two P-P intervals. Put together, you get grouped beating.

Examples

Each strip is 6 seconds of lead II, drawn to scale (25 mm a second, 10 mm a millivolt), so you can count boxes the way you would on paper. Each one is a different example, measured below it.

Example 1 · lead II · 6 seconds

Rate: 48/min5 QRS complexes in 6 seconds × 10 = 50/min
Rhythm: Regularly irregular: a pattern that repeats
P waves: Upright in II, uniform
P to QRS: 4:3, grouped; one P in each group is not conducted
PR: Lengthens 0.24 s, then 0.29 s, then 0.32 s, then a P is not conducted
QRS: 0.09 s, narrow

Example 2 · lead II · 6 seconds

Rate: 61/min4 QRS complexes in 6 seconds × 10 = 40/min
Rhythm: Regularly irregular: a pattern that repeats
P waves: Upright in II, uniform
P to QRS: 5:4, grouped; one P in each group is not conducted
PR: Lengthens 0.12 s, then 0.21 s, then 0.24 s, then 0.25 s, then a P is not conducted
QRS: 0.10 s, narrow

Example 3 · lead II · 6 seconds

Rate: 70/min8 QRS complexes in 6 seconds × 10 = 80/min
Rhythm: Regularly irregular: a pattern that repeats
P waves: Upright in II, uniform
P to QRS: 4:3, grouped; one P in each group is not conducted
PR: Lengthens 0.22 s, then 0.31 s, then 0.35 s, then a P is not conducted
QRS: 0.07 s, narrow

Don't confuse it with

Second-degree AV block, type II
A constant PR, then a P wave that suddenly isn't conducted. The QRS is often wide.
First-degree AV block
Sinus rhythm with every P conducted, but a constant PR over 0.20 s.
Sinus arrhythmia
Sinus P waves and a constant PR, but the rate speeds up and slows down with breathing: the P-P varies by more than 0.12 s.

Want a new strip every time? The ECG Drill in the app draws a fresh, measured example on every try and steers you toward the rhythms you miss. It's part of the Paramedic level, and you can try 5 strips a day free.

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Where the criteria come from

LITFL: AV block 2nd-degree Mobitz I Wenckebach phenomenon

Second-degree AV block, type I (Wenckebach) on an ECG strip

What to look for

Examples

Don't confuse it with

Where the criteria come from

Others in the AV blocks group