EMS Registry ReadyExample 1 · lead II · 6 seconds
- Rate
- 83/min8 QRS complexes in 6 seconds × 10 = 80/min
- Rhythm
- Speeds up and slows down in a cycle
- P waves
- Upright in II, uniform
- P to QRS
- 1:1, a P before every QRS
- PR
- 0.15 s
- QRS
- 0.07 s, narrow
Sinus arrhythmia on an ECG strip
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.
What to look for
Sinus rhythm that speeds up and slows down with breathing. The P-P interval stretches and shortens gradually, and the longest and shortest differ by more than 0.12 s (3 small boxes). Every P is the same sinus P, and the PR stays constant. Irregular doesn't mean abnormal here: it's normal, especially in young people.
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 2 · lead II · 6 seconds
- Rate
- 52/min5 QRS complexes in 6 seconds × 10 = 50/min
- Rhythm
- Speeds up and slows down in a cycle
- P waves
- Upright in II, uniform
- P to QRS
- 1:1, a P before every QRS
- PR
- 0.12 s
- QRS
- 0.10 s, narrow
Example 3 · lead II · 6 seconds
- Rate
- 76/min7 QRS complexes in 6 seconds × 10 = 70/min
- Rhythm
- Speeds up and slows down in a cycle
- P waves
- Upright in II, uniform
- P to QRS
- 1:1, a P before every QRS
- PR
- 0.16 s
- QRS
- 0.10 s, narrow
Don't confuse it with
- Second-degree AV block, type I (Wenckebach)
The PR lengthens beat by beat until a P wave is not conducted, then resets: grouped beating.
- Sinus rhythm with PACs
Sinus rhythm with early beats that start with an early P wave of a different shape, then a narrow QRS.
- Atrial fibrillation
Irregularly irregular, with no P waves: only a fibrillating baseline.
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