Cardiac Conduction System and Understanding ECG, Animation.

The cardiac conduction system  consists of the following components: - The sinoatrial node, or SA node, located  in the right atrium near the entrance of the superior vena cava. This is the natural  pacemaker of the heart. It initiates all heartbeat and determines heart rate.

Electrical  impulses from the SA node spread throughout both atria and stimulate them to contract. - The atrioventricular node, or AV node, located on the other side of the right  atrium, near the AV valve. The AV node serves as electrical gateway to the ventricles. 

It delays the passage of electrical impulses to the ventricles. This delay is to ensure that  the atria have ejected all the blood into the ventricles before the ventricles contract. - The AV node receives signals from the SA node and passes them onto the atrioventricular bundle - AV bundle or bundle of His.

- This bundle is divided into right and left bundle branches which conduct the impulses  toward the apex of the heart. The signals are then passed onto Purkinje fibers, turning upward and  spreading throughout the ventricular myocardium. Electrical activities of the heart can be  recorded in the form of electrocardiogram, ECG or EKG.

An ECG is a composite recording  of all the action potentials produced by the nodes and the cells of the myocardium. Each  wave or segment of the ECG corresponds to a certain event of the cardiac electrical cycle. When the atria are full of blood, the SA node fires, electrical signals spread throughout  the atria and cause them to depolarize.

This is represented by the P wave on the ECG. Atrial  contraction, or atrial systole starts about 100 milliseconds after the P wave begins. The P-Q segment represents the time the signals travel from the SA node to the AV node.

The QRS complex marks the firing of the AV node and represents ventricular depolarization: - Q wave corresponds to depolarization of the interventricular septum. - R wave is produced by depolarization of the main mass of the ventricles. - S wave represents the last phase of ventricular depolarization at the base of the heart.

- Atrial repolarization also occurs during this time but the signal is  obscured by the large QRS complex. The S-T segment reflects the plateau in  the myocardial action potential. This is when the ventricles contract and pump blood.

The T wave represents ventricular  repolarization immediately before ventricular relaxation, or ventricular diastole. The cycle repeats itself with every heartbeat.