Electrical Conductivity System of the Heart Dr.Sisara Bandara Gunaherath MBBS

The heart is the pump responsible for maintaining adequate circulation of oxygenated blood around the vascular network of the body. It is a four-chamber pump, with the right side receiving deoxygenated blood from the body at low pressure and pumping it to the lungs (the pulmonary circulation) and the left side receiving oxygenated blood from the lungs and pumping it at high pressure around the body (the systemic circulation).

“Electrical Circuit” of the Heart

The S-A Node The S-A Node is the most important element in the electrical circuit of the heart. It starts the cardiac cycle by periodically generating action potentials without any external stimulation. (Therefore, it is said to be autorhythmic.) It is also known as the pacemaker of the heart.

Location of SA node In the right atrium Near the opening of the superior vena cava

Conducting Tissues of the Heart Action potentials that originate in the SA node spread to adjacent myocardial cells of the right and left atria through the gap junctions between these cells Since myocardium of the atria is separated from the myocardium of the ventricles by fibrous skeleton of the heart , however the impulse cannot be conducted directly from atria to the ventricles

Therefore specialized conducting tissue ( modified myocardial cells) need for electrical conduction of the heart These specialized myocardial cells form the AV node, Bundle of His ( Atrioventricular bundle) and Purkinje fibers Once the impulse has spread through atria , it passes to the AV node, which located on the inferior portion of the interatrial septum; from there impulse continues through the bundle of His , beginning at the top of the interventricular septum

The bundle of His divides in to right and left bundle branches , which are continuous with the Purkinje fibers with in the ventricular walls Stimulation of Purkinje fibers causes both ventricles to contract simultaneously and eject blood into the pulmonary and systemic circulation.

The A-V Node The most important function of the A-V node is to regulate the timing of the ventricular contraction by delaying the action potentials. The delayed action potentials are spread over the ventricles to cause a contraction.

The Electrical Cycle 1.The S-A Node generates an action potential. 2. The action potential propagates in the atria and causes a contraction. It is also transmitted to the AV Node. 3.The action potential is delayed at the A-V Node.

4.The action potential is transmitted to the ventricles and causes a contraction.

The Electrocardiogram The electrocardiogram (ECG) is a standardized way to measure and display the electrical activity of the heart.

The P Wave: Depolarization of the atria The QRS Complex: Depolarization of the ventricles The T Wave: Repolarization of the ventricles From one P Wave to the next P wave duration equals to a cardiac cycle