What does the QRS complex on an ECG represent, what is its typical duration, and what do its constituent waves signify?

• Its duration is typically 0.06-0.1 second.
• The Q wave is a small, normally negative deflection caused by depolarization of the interventricular septum.
• The R wave is a large, positive wave (about 10 mm or 1 mV amplitude) resulting from depolarization of the apex and ventricular wall, proceeding from the endocardium to the epicardium.
• The S wave is a negative deflection due to depolarization of the postero-basal part of the left ventricle and the pulmonary conus.

The QRS complex has a much larger amplitude than the P wave because the ventricular muscle mass is greater than that of the atria. Its duration is similar to or shorter than the P wave due to the rapid propagation of excitation by the Purkinje system. Abnormalities in the QRS complex can indicate conditions like ventricular hypertrophy, infarction, extrasystoles, bundle branch block, or electrolyte disturbances.