QRS complex

The QRS complex is the tall, spiky part of each heartbeat on the ECG — the bold stroke that most people picture when they think of a heart tracing. It records the instant the heart's main pumping chambers, the ventricles, are electrically activated, just before they contract to push blood out to the lungs and body.

Its name comes from its possible parts: an initial downward Q wave, a tall upward R wave, and a following downward S wave; not every beat shows all three. Because the ventricles carry far more muscle than the atria, their activation produces a much larger deflection than the P wave. A normal QRS is narrow, completing in well under a tenth of a second, which reflects how rapidly the specialized conduction fibers spread the impulse.

The width and shape of the QRS carry a lot of meaning. A wide QRS suggests the impulse is travelling slowly or by an abnormal route — as in a bundle branch block or a beat arising in the ventricles themselves. Changes in QRS height can hint at thickened or enlarged ventricular muscle. Reading the QRS is central to nearly every ECG interpretation.