RECORDING ELECTRICAL POTENTIALS FROM A PARTIALLY DEPOLARIZED MASS OF SYNCYTIAL CARDIAC MUSCLE
Figure 1 shows a syncytial mass of cardiac muscle that has been stimulated at its centralmost point. Before stimulation, all the exteriors of the muscle cells had been positive and the interiors had been negative. For reasons presented in Chapter 5 in the discussion of membrane potentials, as soon as an area of cardiac syncytium becomes depolarized, negative charges leak to the out sides of the depolarized muscle fibers, making this part of the surface electronegative, as represented by the minus signs in Figure 1. The remaining surface of the heart, which is still polarized, is represented by the plus signs. Therefore, a meter connected with its negative terminal on the area of depolarization and its positive terminal on one of the still-polarized areas, as shown to the right in the figure, records positively.
Fig1. Instantaneous potentials develop on the surface of a cardiac muscle mass that has been depolarized in its center.
Two other electrode placements and meter readings are also demonstrated in Figure 1. These placements and readings should be studied carefully, and the reader should be able to explain the causes of the respective meter readings. Because the depolarization spreads in all directions through the heart, the potential differences shown in the figure persist for only a few thousandths of a second, and the actual voltage measurements can be accomplished only with a high-speed recording apparatus.
FLOW OF ELECTRICAL CURRENTS IN THE CHEST AROUND THE HEART
Figure 2 shows the ventricular muscle lying within the chest. Even the lungs, although mostly filled with air, conduct electricity to a surprising extent, and fluids in other tissues surrounding the heart conduct electricity even more easily. Therefore, the heart is actually sus pended in a conductive medium. When one portion of the ventricles depolarizes and therefore becomes electro negative with respect to the remainder, electrical current f lows from the depolarized area to the polarized area in large circuitous routes, as noted in the figure.
Fig2. . Flow of current in the chest around partially depolarized ventricles. A and B are electrodes.
It should be recalled from the discussion of the Purkinje system in Chapter 10 that the cardiac impulse first arrives in the ventricles in the septum and shortly there after spreads to the inside surfaces of the remainder of the ventricles, as shown by the red areas and the negative signs in Figure2. This process provides electronegativity on the insides of the ventricles and electropositivity on the outer walls of the ventricles, with electrical current f lowing through the fluids surrounding the ventricles along elliptical paths, as demonstrated by the curving arrows in the figure. If one algebraically averages all the lines of current flow (the elliptical lines), one finds that the average current flow occurs with negativity toward the base of the heart and with positivity toward the apex.
During most of the remainder of the depolarization process, current also continues to flow in this same direction, while depolarization spreads from the endocardial surface outward through the ventricular muscle mass. Then, immediately before depolarization has completed its course through the ventricles, the average direction of current flow reverses for about 0.01 second, flowing from the ventricular apex toward the base, because the last part of the heart to become depolarized is the outer walls of the ventricles near the base of the heart.
Thus, in normal heart ventricles, current flows from negative to positive primarily in the direction from the base of the heart toward the apex during almost the entire cycle of depolarization, except at the very end. If a meter is connected to electrodes on the surface of the body as shown in Figure2, the electrode nearer the base will be negative, whereas the electrode nearer the apex will be positive, and the recording meter will show positive recording in the ECG.
