QRS complex, a series of waveforms on an electrocardiogram that represents both normal and abnormal depolarization of ventricular muscle cells. It is composed of Q, R, and S waves: a Q wave is the negative deflection before the first R wave, an R wave is any positive deflection, and an S wave is the negative deflection after an R wave. If there is no R wave, the totally negative complex is designated QS. A combination of uppercase and lowercase letters is used to describe the amplitude of each wave. Some variations of the QRS complex are qR, QR, qRs, rS, RS, and rSR. Also called QRS wave.
