The QRS complex of an electrocardiogram (ECG) represents depolarization of the ventricles (lower, pumping chambers of the heart). Depolarization is an electrical change within a cell causing a shift in the electrical charge. Depolarization of the ventricles then leads the muscle cells to contract, which in turn causes the ventricles to pump blood. The P-wave on an ECG comes before the QRS complex and represents depolarization of the atria (upper, receiving chambers of the heart). Depolarization of the atria, cause them to contract, which in turn pushes blood into the ventricles. The T-wave is a deflection on the ECG that follows the QRS complex. T-waves represent repolarization of the ventricles. Repolarization essentially means that the electrical changes that occurred during depolarization reset in preparation for the next cycle. The normal waveforms on an ECG are P, QRS, T, which repeat for each beat.