A heartbeat is an electrical wave that sweeps through the heart in a fixed order. The ECG is simply a recording of that wave from the body surface.

Every heartbeat starts as an electrical impulse, and the heart has its own built-in wiring to carry it. The signal follows the same path in the same order every time: it begins in the sinoatrial (SA) node in the right atrium and spreads across the atria, then pauses briefly at the AV node so the atria can finish emptying into the ventricles. From there it travels down the bundle of His, through the right and left bundle branches, and into the Purkinje fibers, which fire both ventricles almost at once. Because that sequence is so orderly, each step leaves its own fingerprint on the ECG, a distinct wave that appears in the same order on every beat.1

The SA node fires fastest, so it normally sets the pace, a rhythm we call "sinus rhythm." If it ever fails, slower backup pacemakers further down the pathway can step in to keep the heart beating.

1. SA node: the pacemakerSpontaneously fires ~60–100×/min in the right atrium. Sets the heart rate.
2. Atria depolarize → P waveThe impulse spreads across both atria, producing the P wave.
3. AV node: the gatekeeperDeliberately delays the impulse (~0.1 s) so the atria can finish emptying. This pause is the flat PR segment.
4. His bundle & bundle branchesThe signal speeds into the ventricular septum via the right and left bundle branches.
5. Purkinje fibers → QRSFast fibers fire both ventricles almost simultaneously, the sharp QRS complex.

Each part of the beat, what it is, and exactly where it sits on a real Lead II tracing. Colours match the Segment Lab; normal values follow the AHA/ACCF/HRS standardization series.134

ECG paper is standardised worldwide, so every box means the same thing. Time runs left-to-right; voltage runs up-and-down.1

Now read it yourself. Click two points on the strip to drop a caliper; it reads the span in seconds across and millivolts up. Count the boxes between your points to see where the numbers come from.

On standard paper the gap between two R waves is the rate, count the large boxes between them and divide into 300.1 Tap a rate below to see exactly what it looks like on the grid.

The same checklist works on every strip. Run it in order and the diagnosis usually names itself.

1. Rate  Fast, slow, or normal?
   60–100 bpm is normal. < 60 = bradycardia, > 100 = tachycardia.
2. Regularity  Are the R–R intervals even?
   Regular, regularly-irregular, or irregularly-irregular (think AFib).
3. P waves  What is the atrial activity?
   Upright, uniform P = sinus origin. Sawtooth = flutter. Flat or fibrillatory = AFib or junctional.
4. PR interval  Constant and 120–200 ms?
   Applies only when P waves are present. Long or lengthening PR points to AV block.
5. P : QRS relationship  Does every P conduct?
   Dropped or dissociated beats reveal the type of block.

Now try it yourself. Work each strip through the checklist under the monitor; pick the finding that fits to move on, and the rhythm is named only at the end.

The basics in action.67 Each card maps the rhythm back to the checklist above. See them live in the ecg_heartSimulator.