One heartbeat, three waves
A single normal beat draws three landmarks in a fixed order. The P wave is the small first hump: it is the atria depolarizing as the signal spreads out from the sinoatrial node. The QRS complex is the tall, narrow spike that follows: it is the much larger ventricles depolarizing, which is why it dwarfs the P wave. Last comes the T wave, a gentle rounded hump: it is the ventricles recovering, or repolarizing, getting ready for the next beat.
The intervals are stopwatches
Between the waves lie intervals, and each one times a specific journey. The PR interval runs from the start of the P wave to the start of the QRS. It measures how long the signal takes to cross from the atria, pause inside the AV node, and reach the ventricles — the built-in delay that lets the atria finish filling the ventricles before they squeeze. Normal is about 0.12 to 0.20 seconds (three to five small squares).
The QRS duration is the width of that tall spike, and it tells you how fast the ventricles depolarize. Normally the signal races down dedicated wiring and finishes in under 0.12 seconds (three small squares), so the QRS is narrow. A wide QRS means the signal had to take a slow, off-road path — a clue we return to later. The QT interval runs from the start of the QRS to the end of the T wave: it is the whole electrical event, depolarization plus full recovery.
Naming the QRS letters
The letters Q, R and S each name a specific part of the complex. The first downward deflection is the Q; the first upward one is the R; the downward deflection after the R is the S. Not every complex has all three — some have only an R, some have a Q and R. The naming sounds fussy, but it pays off in the ischemia guide, where an abnormal Q wave carries a precise meaning.
- Find the P wave — small hump, atria firing.
- Measure the PR interval — P start to QRS start (0.12–0.20 s).
- Measure the QRS width — the tall spike (<0.12 s is narrow).
- Find the T wave and note the QT span (QRS start to T end).