Essex, Hiram E.

The speed at which the excitatory process spreads through a certain segment of the myocardial syncytium depends upon the orientation of the fibers in that segment to each other and to the point of origin of excitation. Spread down a strip of fibers of which the long axis is parallel to the long axis of the strip is rapid as compared with spread down a strip in which the fibers run at a right angle to the long axis of the strip. Endocardial activation is rapid, not because of the presence of Purkinje fibers, but because the subendocardial bands of myocardium form a network through which excitation can move rapidly along the long axis of the fibers. Spread across the septum in bundle-branch block and across the free wall of the left ventricle in the normally activated heart is slow because excitation is moving through fibers the long axis of which is prependicular to the advancing wave of excitation.

Calculated therapeutic doses of digitalis and its derivatives did not produce demonstrable microscopic changes in the myocardium of experimental animals. Toxic doses of digitalis (twice a therapeutic dose) induced myocardial lesions in animals. In those animals with experimentally induced mild to moderate hyperthyroidism, calculated therapeutic doses of digitalis did produce myocardial lesions. Toxic doses of digitalis administered to animals with mild to moderate hyperthyroidism caused extensive degenerative changes in the myocardium or promptly killed the animals without associated microscopic lesions.