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Third-degree (complete) AV block on an ECG strip

P waves and QRS complexes each regular but unrelated: no P is conducted, so the PR is all over the place.

What to look for

No P wave gets through. The P waves march along at their own regular rate and the QRS complexes at theirs, with no relationship between them, so the PR varies at random. A ventricular escape is wide at 20-40/min; a junctional escape is narrow at 40-60.

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
II25 mm/s · 10 mm/mV
Rate
45/min4 QRS complexes in 6 seconds × 10 = 40/min
Rhythm
Regular
P waves
Upright in II, uniform
P to QRS
None: P waves and QRS complexes are independent (AV dissociation)
PR
None to measure
QRS
0.10 s, narrow
Example 2 · lead II · 6 seconds
II25 mm/s · 10 mm/mV
Rate
55/min5 QRS complexes in 6 seconds × 10 = 50/min
Rhythm
Regular
P waves
Upright in II, uniform
P to QRS
None: P waves and QRS complexes are independent (AV dissociation)
PR
None to measure
QRS
0.08 s, narrow
Example 3 · lead II · 6 seconds
II25 mm/s · 10 mm/mV
Rate
46/min4 QRS complexes in 6 seconds × 10 = 40/min
Rhythm
Regular
P waves
Upright in II, uniform
P to QRS
None: P waves and QRS complexes are independent (AV dissociation)
PR
None to measure
QRS
0.07 s, narrow

Don't confuse it with

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.

Try the ECG Drill

Where the criteria come from

Others in the AV blocks group