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 then 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 ms 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.