The Basics of ECG Interpretation

Start here if the ECG is completely new to you. This page builds the mental model — what the tracing is, what each wave means, and the vocabulary — so the step-by-step method makes sense.

What is an ECG, really?

An ECG is a voltage-versus-time graph of the heart's electrical activity, picked up by electrodes on the skin. Each time the heart's cells depolarize and repolarize, that electrical wave is recorded as a deflection. The ECG does not show blood flow or pumping directly — it shows the electrical events that trigger them.

Depolarization and repolarization

Heart cells sit at a negative resting charge. When stimulated they depolarize (charge shifts positive) and contract, then repolarize (return to rest). Depolarization moving toward a lead makes an upward deflection; away from it, a downward one. That single rule explains most of an ECG's shape.

What each wave means

Wave	Event
P wave	Atria depolarize (and contract)
QRS complex	Ventricles depolarize (and contract)
T wave	Ventricles repolarize (recover)

Atrial repolarization happens too, but it is hidden inside the much larger QRS.

Reading the grid

Standard ECG paper runs at 25 mm/s. Horizontally, one small box = 0.04 s and one large box = 0.20 s. Vertically, 10 mm = 1 mV. Time and voltage are both measured in boxes — that is all the maths you need to start.

Key terms you'll hear

• Lead — a particular view of the heart's electrical activity (there are 12 on a standard ECG).
• Interval / segment — the timed gaps between waves (PR, QRS, QT).
• Sinus rhythm — the normal rhythm, driven by the SA node.
• Ectopic — a beat from somewhere other than the SA node.

Where to go next

Now that the concepts make sense, learn the reading routine in how to read an ECG, then the full method in the ECG interpretation guide. Keep the cheat sheet handy.

Frequently asked questions

What are the basics of ECG interpretation?

An ECG records the heart's electrical activity as P (atrial depolarization), QRS (ventricular depolarization), and T (ventricular repolarization) waves on a grid where a small box is 0.04 s and 10 mm equals 1 mV.

What do the P, QRS, and T waves mean?

The P wave is atrial depolarization, the QRS is ventricular depolarization, and the T wave is ventricular repolarization.

Is ECG interpretation hard to learn?

No — with a systematic method and practice it becomes routine. Start with the concepts, then learn the five-step reading approach.

What does one small box on ECG paper represent?

0.04 seconds in time; a large box (five small boxes) is 0.20 seconds.