The Form of the Electrocardiogram. III. Opposed Potential Differences

Abstract

At the onset of ventricular systole the excitation process is spreading within the ventricular muscle in many different directions simultaneously. Many of the electrical forces, or potential differences, produced are opposed by forces opposite in direction, and their effects are consequently neutralized. Other potential differences are not opposed; it is these potential differences and these alone which have an effect upon the electrocardiogram.

Consider the simple strip of muscle AB (Fig. 1) immersed in a large body of physiological saline solution or other similar conducting material. Place one electrode at R, and a second electrode at S and connect these electrodes to a string galvanometer in such a way that relative negativity at R will cause an upward deflection when the electro gram of the muscle is recorded. Given that RX equals SX (Fig. 1). Stimulate this muscle at X; given that XA equals XY. It is obvious that the excitation process will spread toward A and toward Y simultaneously; that the potential differences produced by the spread in one direction will be completely neutralized by the spread in the opposite direction and that no deflection will result until that part of the muscle lying between A and Y has completed its period of activation. Such deflections as do occur will result from activation of that part of the muscle lying between Y and B.

If instead of the strip of muscle AB we have a circular sheet of muscle and stimulate this muscle at its center, the center being equidistant from R and S, the excitation process will travel radially in all directions and since no unopposed potential differences will be produced no deflections whatsoever will occur. The same result will be obtained from a spherical shell of muscle if all points on the interior surface of the shell are stimulated at the same instant.