As the heart undergoes depolarization and repolarization, the electrical currents that are generated spread not only within the heart, but also throughout the body.
This electrical activity generated by the heart can be measured by an array of electrodes placed on the body surface. The recorded tracing is called an electrocardiogram (ECG, or EKG).
A typical ECG tracing of the cardiac cycle (heartbeat) consists of a P wave, a PR segment, a QRS complex, and a T wave:
- P wave - During normal atrial depolarization, the main electrical vector is directed from the sinoatrial (SA) node towards the AV node and spreads from the right atrium to the left atrium.
- PR segment - During the PR segment, the electrical wave moves from the AV node to the Bundle of His to the bundle branches and then to the Purkinje fibers. This electrical activity does not produce a contraction directly and is not seen on the ECG. PQ interval is the time between the beginning of atrial depolarisation and the beginning of ventricular depolarisation.
- QRS complex - The QRS complex reflects the rapid depolarization of the right and left ventricles. The ventricles have a large muscle mass compared to the atria, so the QRS complex usually has a much larger amplitude than the P-wave.
- ST segment - The ST segment is the portion of the tracing falling between the QRS complex and the T wave. During this time, the ventricle is contracting, but no electricity is flowing.
- T wave - The T wave represents the repolarization of the ventricles, whereby the cardiac muscle is prepared for the next cycle of the ECG.