In any stage of partial block, pressure on the vagus nerve in the neck produces certain specific changes. (Fig. 52.) Robinson and Draper[13] have found qualitative differences in the two vagi. The right vagus sends most of its fibers to the s-a node (Fig. 53) and has a more evident influence on the rate and force of the cardiac contractions. The majority of fibers from the left vagus are distributed to the A-V node so that its most evident action is upon the conductivity of the impulse. Pressure then on the right vagus will have a tendency to slow the whole heart. Pressure on the left vagus will have a tendency to prolong the P-R interval until even complete block occurs. Even when the heart block is complete, stimulation of the accelerator nerve, as a rule, increases the rate of both auricles and ventricles.
Fig. 52.—Influence of mechanical pressure on the right vagus nerve. (After Eyster and Evans.)
Fig. 53.—Schematic distribution of right and left vagus. (After Hart.)
If the block is functional, depending upon some temporary overstimulation of the vagus nerve, atropin, which paralyzes the endings of the vagus, will naturally lift the block. If the block is due to some actual lesion of the bundle of His, such as fibrosis, gumma, or other lesion, then atropin will have no influence to terminate the block. In this manner we are able to distinguish between functional and organic heart block.