A regular narrow tachycardia over 100/min whose P waves aren't sinus: one abnormal shape (here inverted in II), every P alike, each with a PR of 0.12 s or more and a flat baseline between.
One irritable spot in the atria, outside the sinus node, takes over and fires faster than 100/min. Every P wave comes from that one spot, so they all match each other, but none of them look sinus. Here they're inverted in II, and the PR is still 0.12 s or more. That PR tells you the impulse started in the atria and not the junction: an inverted P with a PR under 0.12 s is junctional. Between each T wave and the next P there's a flat baseline, which flutter never has. With one P shape it isn't MAT, which needs three or more.
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.
Sinus rhythm over 100/min: regular, with an upright P before every QRS.
A regular narrow-complex rhythm over 100/min from the AV junction: no sinus P waves, and the P that is there is inverted in II with a PR under 0.12 s, or comes just after the QRS.
Regular, narrow and 150-250/min, with no visible P waves.
Sawtooth waves at about 300/min, with a QRS after every 2nd, 3rd or 4th wave.
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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