Which ventricle depolarizes first

After another 20 milliseconds, the mean vector is pointing toward the left arm and anterior chest as the free wall of the ventricle depolarizes from the endocardial to the epicardial surface vector 3, Panel C. This vector records a small positive voltage in lead II.

Finally, the last regions to depolarize results in vector 4 Panel D , which produces a slight negative deflection S wave of the QRS.

The shape of the QRS complex is different for each of the limb leads because each of the leads will "see" the sequence of depolarization vectors from a different perspective see axial reference system. There is no Q wave because septal depolarization is not directed away from the lead see ECG rules.

The R wave is very positive because early ventricular depolarization is largely directed toward this lead. The S wave is also present because the terminal depolarization of the upper wall of the left ventricle is directed away from aV F.

In general, the atrioventricular node is located in the so-called floor of the right atrium, over the muscular part of the interventricular septum, inferior to the membranous septum: i.

Following atrioventricular nodal excitation, the slow pathway conducts impulses to the His bundle, indicated by a longer interval between atrial and His activation. Currently, there is interest in the ability to place pacing leads to preferentially activate the bundle of His; in such approaches, ultrasound or other imaging modalities are used to map the electrical characteristic His potentials to position the pacing leads [20].

After leaving the bundle of His, the normal wave of cardiac depolarization spreads first to both the left and right bundle branches; these pathways rapidly and simultaneously carry depolarization to the apical regions of both the left and right ventricles see Figure 1. Finally, the signal broadly travels through the remainder of the Purkinje fibers and ventricular myocardial depolarization spreads. In certain pathological conditions, direct accessory connections from the atrioventricular node and the penetrating portion of the bundle of His to the ventricular myocardium have been described [21].

Yet, the function and prevalence of these connections, termed Mahaim fibers, is poorly understood. A rare bundle of Kent, an additional aberrant pathway when present, exists between the atria and ventricles and is associated with the clinical manifestation of ventricular tachycardias also known as Wolff-Parkinson-White syndrome.

Therapeutically, this accessory pathway is electrically identified and then commonly ablated as a curative procedure. The left bundle branch splits into fascicles as it travels down the left side of the ventricular septum just below the endocardium. Its fascicles extend for a distance of 5 to 15 mm, fanning out over the left ventricle.

Importantly, typically about midway to the apex of the left ventricle, the left bundle separates into two major divisions, the anterior and posterior branches or fascicles. These divisions extend to the base of each papillary muscle as well as the adjacent myocardium.

In contrast, the right bundle branch continues inferiorly, as if it were a continuation of the bundle of His, traveling along the right side of the muscular interventricular septum. This bundle branch runs proximally, just beneath the endocardium, and its course runs slightly inferior to the septal papillary muscle of the tricuspid valve before dividing into fibers that spread throughout the right ventricle.

The complex network of conducting fibers that extends from either the right or left bundle branches is composed of the rapid conduction cells known as Purkinje fibers.

It will identify depolarization of the right ventricle current flowing towards the lead as a positive deflection and left ventricular depolarization current flowing away from the lead as a negative deflection. In normal depolarization, the septum depolarizes before the bundle branches. The septum depolarizes from left towards right. Normal V 1 In a normal V 1 pattern, the QRS begins with a small initial "r" wave, indicating depolarization of the septum towards the lead.

This is followed by simultaneous transmission of the impulse through the right and left bundle branches, producing simultaneous left and right ventricular depolarization. Because the left ventricle sends a stronger impulse, the net sum of the ventricular forces is away from V 1 , producing a downward deflection or S.

The right ventricular depolarization wave upward deflection is buried in the S wave. Because the right bundle is blocked, the impulse then travels towards the left bundle branch only, producing a downward deflection or S wave. Following depolarization of the left ventricle, the right ventricle depolarizes from a "back door" impulse, producing a second R wave in V 1 a second R wave is called "R Prime" and is identified as R'.

The following statements are true:. The following statements are true: One does NOT see any deflection on the ECG during the time that the sinoatrial node is being depolarized. The depolarization of the atrioventricular node and the His-Purkinje system also does not generate any electrical activity that is detectable in the ECG.

One does not necessarily see a Q-wave or an R-wave or an S-wave in each lead that one examines. Indeed, in some individuals with perfectly normal hearts, there is no Q-wave present in any of the three leads I-III. Other normal individuals have no S-waves in any of the three leads.

To continue with the next section: ECG Experiments, click here. Your browser does not support script Cardiovascular Laboratory. Activation of the Heart and the ECG. The drawings to the right in the table below show the main stages of activation of the heart, as well as the ECG recorded in lead II at those stages.

Cardiac axis.