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.