Each bump is a step you already know
An electrocardiogram (ECG) is simply a recording of the heart’s electrical waves picked up from the skin. The good news: you have already met everything it shows. The famous wiggle is just the conduction system story drawn out as voltage over time, beat by beat.
- The P wave is the atria depolarizing — the SA-node spark spreading across the top chambers.
- The flat PR interval is the AV node pause — the deliberate AV delay you met in guide 2.
- The tall QRS complex is the ventricles depolarizing — the big squeeze firing through the Purkinje fibers.
- The T wave is the ventricles repolarizing — the cells recovering for the next beat.
Counting the rate off the trace
Standard ECG paper moves at a fixed speed, so distance across the page is time. Each large box is 0.2 seconds, so five large boxes make exactly one second and 300 large boxes make one minute. That gives a quick way to read the heart rate: count the large boxes between two beats and divide that number into 300.
Quick ECG rate (regular rhythm) -------------------------------- Rule: rate (bpm) = 300 / (number of large boxes between two QRS complexes) Worked example: Count from one QRS to the next = 4 large boxes rate = 300 / 4 = 75 bpm -> normal More examples: 3 large boxes -> 300 / 3 = 100 bpm (upper end of normal) 5 large boxes -> 300 / 5 = 60 bpm (lower end of normal) 6 large boxes -> 300 / 6 = 50 bpm (mild bradycardia) Reference band for a resting adult: about 60-100 bpm. (Educational example, not a diagnosis.)
This is where the whole track comes together. The SA node you met first sets the spacing between beats; the AV node pause shows up as the flat stretch before each QRS; the action potential and its long recovery are written into the shape and width of the waves. The ECG is not a new topic — it is the picture of everything you have already learned, and that is why, with the wiring in mind, the squiggles finally read like a sentence.