CARDIAC DEPOLARIZATION AND REPOLARIZATION AND MEAN INSTANTANEOUS VECTORS

CARDIAC DEPOLARIZATION AND REPOLARIZATION AND MEAN INSTANTANEOUS VECTORS

PROGRESSION OF DEPOLARIZATION

• Atrial Depolarization and Mean Vectors.

• Septal Depolarization.

• Apical Depolarization.

• Left Ventricular Depolarization.

END OF DEPOLARIZATION FOLLOW BY REPOLARIZATION

• Late Left Ventricular Depolarization

• Ventricles Depolarized

• Ventricular Repolarization

• Ventricles Repolarized

1. Progression of depolarization:

- Atrial Depolarization and Mean Vectors: the first important electric-movement in septal depolarization normally begins at the left side of the septum, moves to the right, and results from the entry of bundle of His branches into the septum at a higher level on the left than the right. The septal left-to-right movement is important because it writes the normal septal Q wave in leads I, aVL, and V6. If the first electric movement is analyzed (using Einthoven reference frame), it is evident that a Q wave will initiate the QRS complex in leads I and II and an R wave in lead III.

- Septal Depolarization: The second electric movement of signifiance is apical depolarization, which follows the early depolarization of the right ventricle. Projection of the second instantaneous vector onto the Einthoven triangle indicates that leads I, II, and III will develop R waves at this time.
- Apical Depolarization: Depolarization of the right ventricle occurs quickly and is completed early because of the thinness of this structure compared to that of the left ventricle. The third significant electric movement is toward the lateral  wall of the left ventricle. At this time the amplitude of the R waves is increased in leads I and II, and S waves  appear in lead III. The forces at this time are strong because there are no counterforces from the right ventricle and the LV muscle mass is thick.

2. End of Depolarization Followed By Repolarization:

- Left Ventricular Depolarization: The fourth or late instantaneous vector (electric movement) exists toward the base of the left ventricle and occurs just before the end of the ventricular depolarization process. This force results in a deepening of the S waves in lead III and an accentuation of the amplitude of the R waves in leads I and II.
-Ventricles Depolarized: When the dipoles are removed or reversed, with no potential differences on the body as a result of electric changes affecting the heart, the heart is in the depolarized state. The myocardium is in a refractory condition during this period, and a myocardial stimulus will fail to elicit a contraction. Since there are no voltage differences, the ECG trace returns to the baseline in all leads; it is during this time that the S-T segment is written.
-Ventricular Repolarization: Repolarization of the ventricles is a complex process in which a vector appears opposite the wave of  depolarization. As a result, development of positive  (upward) T waves is shown in the standard leads I and II. The normal direction of T waves in lead III is variable. ( mechanism by The putative midmyocardial cells ( M cells) appear to have the longest action potential duration across the myocardial wall, largely attributed to their weaker IKs current but stronger late INa and Na+-Ca2+ exchanger currents )

-Ventricles Repolarized: Finally, each cell of the myocardium becomes repolarized, with a preponderance of negative charges inside the cell and positive charges outside. The heart is now  ready for its next stimulation and contraction. The heart muscle is thus in a receptive state, and a stimulus  will elicit a contraction. Now the trace is isoelectric because there are no net potential differences on the body surface.

References.
1. Vol 8, The Netter Collection of Medical Illustrations - Cardiovascular System, 2 edition.
2.Clinical Arrhythmology and Electrophysiology. ...