Transcript for:
Cardiac Conduction System and ECG

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.