EMS Registry ReadyExample 1 · lead II · 6 seconds
- Rate
- 122/min12 QRS complexes in 6 seconds × 10 = 120/min
- Rhythm
- Regular
- P waves
- Sinus: upright in II, biphasic in V1, one before every QRS
- P to QRS
- 1:1, a P before every QRS
- PR
- 0.16 s
- QRS
- 0.09 s, narrow
Pericardial effusion (low voltage, electrical alternans) on an ECG strip
Sinus tachycardia, small QRS complexes everywhere (low voltage), and QRS complexes that alternate tall, short, tall as the heart swings in the fluid.
What to look for
Fluid around the heart muffles every complex, so the QRS is small in every lead: under 5 mm in the limb leads, or under 10 mm across the chest. In a large effusion the heart swings back and forth inside it, and the QRS height alternates beat to beat, tall, short, tall. Add the compensatory sinus tachycardia and you have the triad. In a hypotensive patient that's tamponade until proven otherwise: think trauma, dissection, cancer or renal failure, and get them to someone who can drain it. Low voltage alone has other causes too, like obesity and emphysema, without the alternans.
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
- 119/min12 QRS complexes in 6 seconds × 10 = 120/min
- Rhythm
- Regular
- P waves
- Sinus: upright in II, biphasic in V1, one before every QRS
- P to QRS
- 1:1, a P before every QRS
- PR
- 0.15 s
- QRS
- 0.09 s, narrow
Example 3 · lead II · 6 seconds
- Rate
- 108/min11 QRS complexes in 6 seconds × 10 = 110/min
- Rhythm
- Regular
- P waves
- Sinus: upright in II, biphasic in V1, one before every QRS
- P to QRS
- 1:1, a P before every QRS
- PR
- 0.12 s
- QRS
- 0.09 s, narrow
Don't confuse it with
- Sinus tachycardia
Sinus rhythm over 100/min: regular, with an upright P before every QRS.
- Normal 12-lead ECG
Sinus rhythm, a normal axis, normal intervals, R waves growing from V1 to V5, and no ST elevation or depression.
- Supraventricular tachycardia (AVNRT)
Regular, narrow and 150-250/min, with no visible P waves.
- Acute pericarditis
Widespread, modest, concave ST elevation with PR depression, and the only ST depression in aVR (and V1), where the PR is elevated instead.
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