Introduction
An ECG is a recording of the heart's electrical activity, projected through the body to electrodes on the skin. Each heartbeat produces a sequence of waves separated by segments and bounded by intervals. The shape, size, timing, and direction of these features tell you what the heart is doing — and what's wrong with it.
This lesson covers the foundational building blocks: P wave, QRS complex, T wave, the PR and QT intervals, and the PR and ST segments. By the end, you will be able to identify each feature on a normal sinus rhythm strip, measure intervals correctly, and understand what each represents physiologically.
The P wave
The P wave represents atrial depolarisation — the electrical activation that spreads from the sinoatrial (SA) node across the right and left atria. The SA node sits at the top of the right atrium, and the depolarisation wave moves leftward and downward, which is why the P wave is upright in leads I and II in normal sinus rhythm.
A normal P wave is small (less than 2.5 mm tall and less than 110 ms wide), smoothly rounded, and precedes every QRS complex. Its presence tells you the rhythm originates from the atria; its shape tells you whether the SA node or another atrial focus is firing.
The QRS complex
The QRS complex represents ventricular depolarisation — the rapid spread of electrical activity through the His-Purkinje system and into the ventricular myocardium. Because the ventricles have far more muscle mass than the atria, the QRS deflection is much larger than the P wave.
By convention, the first downward deflection after the P wave is called Q, the first upward deflection is R, and the downward deflection following R is S. Not every complex has all three components; lead V1, for example, often has a tiny R followed by a deep S (rS pattern), while V6 typically has a tall R with a small S.
A normal QRS lasts less than 100 ms (less than 2.5 small boxes). When the QRS is wider than 120 ms, the activation is no longer occurring through the normal conduction pathway — possibilities include bundle branch block, ventricular rhythm, pre-excitation (WPW), hyperkalaemia, or sodium-channel blocker effect.
The T wave
The T wave represents ventricular repolarisation — the recovery of the ventricles after depolarisation. Repolarisation is a slower process than depolarisation, which is why the T wave is broader and lower-amplitude than the QRS.
Normal T waves are upright in leads I, II, V3 through V6, and inverted in aVR (always) and often in V1. Asymmetric in shape — the upstroke is more gradual than the downstroke — and roughly one-third the height of the R wave that precedes it.
Intervals: PR, QRS, QT
Three intervals matter clinically. They measure the time taken for different parts of the cardiac electrical cycle and are central to identifying conduction abnormalities, drug effects, and electrolyte disturbances.
| Interval | From | To | Normal | If abnormal |
|---|---|---|---|---|
| PR interval | Start of P | Start of QRS | 120–200 ms | Prolonged: 1° AV block · Short with delta wave: WPW |
| QRS duration | Start of QRS | End of QRS | <100 ms | ≥120 ms: BBB, ventricular rhythm, hyperkalaemia, drug toxicity |
| QT interval | Start of QRS | End of T | QTc <450 ms (m), <460 ms (f) | >500 ms substantially raises torsades risk |
PR interval — measured from the start of the P wave to the start of the QRS — represents conduction from atria through the AV node and His-Purkinje system to the ventricles. Prolonged PR (greater than 200 ms) is first-degree AV block; very short PR with a slurred QRS upstroke is pre-excitation (WPW).
QRS duration — from start to end of QRS — measures ventricular depolarisation time. Normal less than 100 ms.
QT interval — start of QRS to end of T — measures total ventricular depolarisation plus repolarisation. The QT shortens at faster heart rates, so it's corrected using Bazett's formula (QTc = QT divided by the square root of RR in seconds). QTc longer than 460 ms (women) or 450 ms (men) is prolonged; longer than 500 ms substantially raises torsades risk.
Segments: PR and ST
Segments are the flat baseline portions between waves. They serve as the visual reference for detecting elevation or depression elsewhere in the trace.
The PR segment — between the end of the P wave and the start of QRS — is the electrical baseline reference. Use it (not the TP segment, not the start of the strip) to judge whether ST is elevated or depressed.
The ST segment — between the end of QRS (the J point) and the start of T — is the most clinically charged piece of the ECG. ST elevation in a coronary territory is a STEMI until proven otherwise; ST depression suggests subendocardial ischaemia, posterior MI (mirror-image), or non-coronary causes.
Summary & next steps
You now have the vocabulary to describe a normal ECG: P, Q, R, S, T waves, PR and ST segments, and PR, QRS, QT intervals. You know what each represents physiologically and what abnormal values suggest.
The next lesson — Heart rate and rhythm interpretation — applies these concepts to recognising sinus rhythm, sinus tachycardia and bradycardia, and the irregularly irregular rhythm of atrial fibrillation. After that, axis determination, then chamber enlargement, then ischaemia patterns.