Correlation between ECG and Cardiac Cycle

The P wave signifies the spread of cardiac action potential from the sinoatrial node to the atrial contractile cells. This electrical signal occurs approximately 0.1 seconds before atrial systole begins. The P wave marks the initiation of atrial contraction during the cardiac cycle.

The QRS complex appears approximately 0.2 seconds after the P wave and indicates that cardiac potential has reached the ventricular contractile cells, moving down the septum to the apex, then upward. This electrical event signals the depolarization of the ventricles and triggers ventricular systole, which spans the entire S-T interval on the ECG.

Electrical signals on the ECG precede mechanical processes of contraction and relaxation. The P wave appears before atrial contraction, the QRS complex before ventricular contraction, and the T wave before ventricular relaxation. This delay allows the conduction system of the heart to coordinate the precise sequence of chamber contractions needed for effective blood pumping.

The T wave emerges approximately 0.4 seconds after the P wave and signifies ventricular repolarization. Shortly after the T wave appears, the ventricles begin to relax. By 0.6 seconds post-P wave, the ventricles are completely relaxed, marking the end of ventricular systole and the beginning of ventricular diastole.

The action potential is briefly slowed at the AV node, allowing the atria to contract and fill the ventricles with blood before ventricular systole begins. This delay ensures proper coordination between atrial and ventricular contractions. Following this delay, the action potential rapidly propagates through the AV bundle and Purkinje fibers into the ventricles.

After the T wave, the ventricles relax completely by 0.6 seconds. The next 0.2 seconds mark a brief period where both the atria and ventricles are relaxed, representing a time of complete cardiac rest. Another P wave appears at 0.8 seconds, initiating atrial contraction and restarting the cardiac cycle.

Atrial repolarization occurs concurrently with ventricular depolarization following the QRS complex. However, atrial repolarization is usually obscured in an ECG by the more prominent QRS complex. This overlap means the atrial repolarization wave is typically not visible as a distinct feature on standard ECG recordings.