The electrical signals recorded on an electrocardiogram (ECG) occur before the mechanical processes of contraction and relaxation during the cardiac cycle.
A cardiac action potential originates in the SA node and spreads throughout the atria and the AV node in approximately 0.03 seconds. This results in the P wave in an ECG and triggers atrial contraction. The action potential is then briefly slowed at the AV node, allowing the atria to contract and fill the ventricles with blood before ventricular systole begins.
Following the delay, the action potential is rapidly propagated through the AV bundle (Bundle of His), then into the right and left bundle branches, and finally through the Purkinje fibers into the ventricles. This results in the QRS complex seen in the ECG, which signals the depolarization of the ventricles. Concurrently, atrial repolarization occurs, although this is usually obscured in an ECG by the more prominent QRS complex.
Ventricular contraction begins soon after the QRS complex appears, pushing blood upwards toward the semilunar valves. Ventricular repolarization follows, starting at the apex and spreading throughout the ventricular myocardium, and is marked by the T wave in an ECG.
Finally, ventricular diastole begins, allowing the ventricles to relax. Following a brief period where all cardiac fibers are relaxed, the cycle resumes with the appearance of another P wave.
