The electrocardiogram (ECG) measures the electrical activity of the heart. The ECG has three characteristic waves: the P wave, QRS complex, and T wave. The P wave of the ECG is due to contraction of the atria (atrial systole). The QRS complex of the ECG marks ventricular systole. The T wave of the ECG is due to ventricular relaxation (diastole).
Two intervals are of special interest. The P-R segment represents the time for conduction through the AV node and AV bundle. The R-R interval is used to measure heart rate.
Making a Difference in the World 