Correlation between an ECG and the Heart
P wave = SA Node
Under normal conditions, electrical activity is spontaneously generated by the SA node, which, under normal conditions is the physiological pacemaker. The electrical impulse is the continued through the right atrium and through Bachmann’s bundle to the left atrium. This stimulating causes the muscle of both the atria to contract (myocardium). The conduction of the electrical impulse throughout both the left and the right atria can be seen as the ‘P wave’ in an ECG. As the electrical activity is spreading throughout the atria, it travels via specialized pathways, known as internodal tracts. These lead from the SA node to the AV node.
PR interval = AV node and Bundles Branches:
The AV node then delays the conduction of this electrical activity through to the ventricles. This delay is important, because if the conduction continued at a continuous rate, the atria and ventricles would contract at the same time. This would leave make the pump (heart) work ineffectively. This delay can be seen on the PR segment on the ECG, before the AV node propagates the electrical activity down the bundle of his and ventricles.
The distal portion of the AV node is known as the bundle of His. The bundle of His splits into two branches in the interventricular septum, the left bundle branch and the right bundle branch. The left bundle branch activates the left ventricle, while the right bundle branch activates the right ventricle. The left bundle branch is short, splitting into the left anterior fascicle and the left posterior fascicle. The left posterior fascicle is relatively short and broad, with dual blood supply, making it particularly resistant to ischemic damage. The left posterior fascicle transmits impulses to the papillary muscles, leading to mitral valve closure. As the left posterior fascicle is shorter and broader than the right, impulses reach the papillary muscles just prior to depolarization, and therefore contraction, of the left ventricle myocardium. This allows pre-tensioning of the chordae tendinae, increasing the resistance to flow through the mitral valve during left ventricular contraction.
QRS complex = Purkinje fibers and ventricular contraction
The two bundle branches taper out to produce numerous purkinje fibers, which stimulate individual groups of myocardial cells to contract.
The spread of electrical activity (depolarization) through the ventricular myocardium produces the QRS complex on the ECG.
T wave = Ventricular repolarization
The final event of the cycle is the repolarization of the ventricles. The PQRS portion of an ECG is influenced by the sympathetic nervous system; where as the T wave is influenced by the parasympathetic nervous system guided by the vagus nerve.