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Reading Rhythm on the ECG: Fast, Slow, and Irregular

The ECG is how we actually see a rhythm. Learn the few marks that matter — P, QRS, the intervals — and a simple method to count the rate and judge whether a beat is normal.

What the squiggles mean

An electrocardiogram (ECG) records the heart's electrical activity from the skin and draws it as a repeating squiggle. Each normal beat has three landmarks. The P wave is the small bump made when the atria depolarize — the SA node's signal spreading. The QRS complex is the tall spike made when the ventricles depolarize and contract. And the T wave is the gentle hump as the ventricles recover. For rhythm, your eye lives mostly on the P waves and the QRS complexes.

The spacing between marks carries meaning too. The PR interval — from the start of the P wave to the start of the QRS — measures how long the signal takes to cross the AV node. A long PR means the gateway is dragging. The QT interval tracks how long the ventricles take to fire and reset; an abnormally long one is a clue to a dangerous condition you will meet in guide 5.

Counting the rate

ECG paper runs at a fixed speed, so distance equals time. Each large square is 0.2 seconds; five large squares make one second. The quickest bedside trick is to count the large squares between two QRS spikes and divide 300 by that number. If QRS spikes sit one big square apart, the rate is 300; two squares, 150; three, 100; four, 75; five, 60; six, 50.

Rate from a rhythm strip — the "300 rule"

Large squares between two QRS spikes  ->  heart rate
  1 square   ->  300 / 1 = 300 bpm
  2 squares  ->  300 / 2 = 150 bpm
  3 squares  ->  300 / 3 = 100 bpm
  4 squares  ->  300 / 4 =  75 bpm
  5 squares  ->  300 / 5 =  60 bpm
  6 squares  ->  300 / 6 =  50 bpm

Irregular rhythm? Count QRS complexes on a 6-second strip
and multiply by 10.
  Example: 8 QRS in 6 s  ->  8 x 10 = 80 bpm

Normal resting rate = 60-100 bpm
  > 100 = tachycardia (fast)
  <  60 = bradycardia (slow)
The bedside method for turning ECG spacing into a heart rate.

A four-question routine

  1. Rate? Use the 300 rule or the 6-second method. Above 100 is tachycardia; below 60 is bradycardia.
  2. Regular? Are the QRS spikes evenly spaced, or do the gaps vary?
  3. P waves? Is there a P before every QRS, and a QRS after every P? That pairing is the signature of sinus rhythm.
  4. QRS width? A narrow QRS means the beat used the normal wiring (from above the ventricles); a wide QRS often means it came from the ventricles themselves.