Correlation between ECG and Cardiac Cycle

The electrical signals recorded on an electrocardiogram or ECG occur before the mechanical processes of systole and diastole during the cardiac cycle.

The P wave signifies the spread of cardiac action potential from the sinoatrial node to the atrial contractile cells.

Approximately 0.1 seconds post the P wave, atrial systole begins, lasting until the P-Q interval.

About 0.2 seconds after the P wave, the QRS complex shows up, indicating that the cardiac potential has reached the ventricular contractile cells, moving down the septum to the apex, then upwards.

The ventricular systole begins after the QRS complex, spanning the entire S-T interval. Simultaneously, the atria return to diastole.

Next, 0.4 seconds after the P wave, the T wave emerges, signifying ventricular repolarization. Shortly after the T wave, the ventricles begin to relax.

By 0.6 seconds, the ventricles are completely relaxed. The next 0.2 seconds mark a brief period where both the atria and ventricles are relaxed.

Another P wave appears at 0.8 seconds, initiating atrial contraction and restarting the cycle.