Cardiac Conduction System and Understanding ECG, Animation.

The cardiac conduction system includes the sinoatrial (SA) node, the natural pacemaker initiating heartbeats; the atrioventricular (AV) node, which serves as an electrical gateway to the ventricles; the AV bundle (bundle of His); and Purkinje fibers, which spread impulses throughout the ventricular myocardium.

The SA node, located in the right atrium, is the heart's natural pacemaker. It initiates all heartbeats, determines heart rate, and its electrical impulses cause the atria to contract.

The AV node, located in the right atrium, acts as an electrical gateway to the ventricles. It delays electrical impulses, ensuring the atria fully eject blood into the ventricles before ventricular contraction.

Signals from the AV node are passed to the atrioventricular bundle (bundle of His), which divides into right and left bundle branches. These branches conduct impulses towards the heart's apex, then to Purkinje fibers that spread throughout the ventricular myocardium.

The electrical activities of the heart can be recorded as an electrocardiogram (ECG or EKG). An ECG is a composite recording of action potentials from the nodes and myocardium, where each wave or segment corresponds to a specific event in the cardiac electrical cycle.

When the atria are full of blood, the SA node fires, spreading electrical signals throughout the atria, causing them to depolarize. This atrial depolarization is represented by the P wave on the ECG. Atrial contraction (systole) begins about 100 milliseconds after the P wave.

The P-Q segment represents the time taken for signals to travel from the SA node to the AV node. The QRS complex marks the firing of the AV node and represents ventricular depolarization.

The Q wave corresponds to depolarization of the interventricular septum, the R wave is from depolarization of the main ventricular mass, and the S wave represents the final phase of ventricular depolarization at the base of the heart. Atrial repolarization also occurs during this time but is obscured by the QRS complex.

The S-T segment reflects the plateau in the myocardial action potential, during which the ventricles contract and pump blood. The T wave represents ventricular repolarization, occurring just before ventricular relaxation (diastole), and then the cycle repeats.