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Sinus rhythm with blocked PACs on an ECG strip

An early P wave on the T wave with no QRS after it, then a pause.

What to look for

A PAC so early it lands on the T wave of the beat before. The conduction system is still refractory, so no QRS follows it. Look for a T wave that's peaked or notched compared with the others, then a pause. The trap is calling it sinus arrest or a dropped beat of AV block.

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
99/min underlyingThe whole strip averages 85/min · 9 QRS complexes in 6 seconds × 10 = 90/min
Rhythm
Regular underneath, with early beats breaking it
P waves
Sinus P waves, plus early P waves of a different shape (the PACs)
P to QRS
1:1 for the sinus beats; each PAC's early P has no QRS after it
PR
0.17 s
QRS
0.07 s, narrow
PACs
2 on this strip
Example 2 · lead II · 6 seconds
II25 mm/s · 10 mm/mV
Rate
75/min underlyingThe whole strip averages 62/min · 6 QRS complexes in 6 seconds × 10 = 60/min
Rhythm
Regular underneath, with early beats breaking it
P waves
Sinus P waves, plus early P waves of a different shape (the PACs)
P to QRS
1:1 for the sinus beats; each PAC's early P has no QRS after it
PR
0.17 s
QRS
0.08 s, narrow
PACs
2 on this strip
Example 3 · lead II · 6 seconds
II25 mm/s · 10 mm/mV
Rate
60/min underlyingThe whole strip averages 60/min · 6 QRS complexes in 6 seconds × 10 = 60/min
Rhythm
Regular underneath, with early beats breaking it
P waves
Sinus P waves, plus early P waves of a different shape (the PACs)
P to QRS
1:1 for the sinus beats; each PAC's early P has no QRS after it
PR
0.12 s
QRS
0.11 s, narrow
PACs
2 on this strip

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 Atrial group