Cardiac Depolarization and Repolarization and Mean Instantaneous Vectors
The cardiac impulse originates in the sinus node and starts the process of atrial depolarization by lowering the resistance of the cell membrane, allowing neutralization or reversal of certain dipoles. This leaves an electric-wave front, an accession wave, which is preceded by positive forces and followed by negative ones. Normally, this wave is initiated at the sinoauricular (S-A) node (see Plate 2-17). Early during atrial depolarization, however, the wave spreads toward the foot and A-V node. Toward the end of atrial depolarization, the accession wave is directed toward the left atrium and left arm. The early atrial depolarization wave may be represented as a vector, the length of which indicates the magnitude (strength) of the voltage generated by the accession wave. The late atrial depolarization voltage is represented by a second vector, the length of which is a measure of the voltage generated at this time.
If the heads of these vectors are connected with their points of origin, a loop is formed; this is the P loop of the vectorcardiogram (VCG). The P loop is seen in the frontal plane.
A mean P vector can be determined from the instan-taneous vectors 1 and 2 by using the parallelogram law. To derive the mean vector from two instantaneous vectors, a parallelogram is drawn. The instantaneous vectors are drawn as originating from a common point of origin E. The parallelogram is completed by drawing a line from each arrowhead, parallel to the opposite vector. The mean vector is an arrow connecting E with the opposite angle of the parallelogram. The mean vector indicates the average direction taken by the atrial accession wave, and its magnitude as the wave travels over the atria.
One can analyze the mean atrial-depolarization vector against the Einthoven triangle reference frame to predict the type of P waves that will appear in leads I, II, and III. Projecting the mean vector against the reference line of lead I creates a projected vector, the length of which is proportional to the amplitude of the P wave in that lead. The direction of the wave (up or down) is determined by the direction of the projected atrial vector with respect to the polarity of the reference line. The direction of the P wave will be upward (positive) when the projected vector points in the same direction as the reference arrow for that lead and downward (negative) when the opposite relationship exists.
Just before atrial depolarization is complete, depolarization of the A-V node begins. However, the nodal depolarization process is of such low magnitude that the ECG instrument is unable to detect these changes, and it is not until the interventricular septum is invaded that a QRS complex begins. Normally, there is a time interval from the end of the P wave to the beginning of the QRS complex (P-R segment), which is usually opposite in direction to the P wave and is a result of atrial repolarization.
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.
The second electric movement of significance 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.
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.
END OF DEPOLARIZATION FOLLOWED BY REPOLARIZATION
Late Left Ventricular Depolarization
The fourth or late instantaneous vector (electric move- ment) exists toward the base of the left ventricle and occurs just before the end of the ventricular depolariza- tion 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.
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 base- line in all leads; it is during this time that the S-T segment is written.
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.
|END OF DEPOLARIZATON FOLLOWED BY REPOLARIZATION|
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.