Formation of the major deflections on the ECG

The electrical impulse is initiated in the sinus node and then activates the right and left atria, generating the P wave. Impulse conduction through the atrioventricular (AV) node and bundle of His, which are small structures, does not generate any ECG activity; this period of "electrical silence" is the PR interval. The first part of the ventricle to be depolarized is the left side of the interventricular septum, producing a small septal Q wave, followed by depolarization of the remainder of the ventricular myocardium, generating the full QRS complex. The T wave represents ventricular repolarization. A U wave may be present, representing repolarization of the His Purkinje system.