Conduction of the Cardiac Impulse

A segment of a bundle of canine Purkinje fibers was encased in agar made up in Tyrode solution containing elevated K + (47 m M ) to depress the excitability of the segment. At a degree of depression producing total block of conduction in each direction through the depressed segment, exciting both ends of the bundle yielded full excitatory responses in the center of the depressed area. These responses arose with a long latency after excitation of the normal ends outside the depressed segment. Summation occurred over a long interval of relative timing of excitation of the ends. When excitation of one end alone produced a response in the center of the depressed segment, that response sometimes could be inhibited by stimulating the other end. In another preparation a branch emerged from the center of the depressed segment. Summation of excitation in the center of the depressed area resulted in conduction of activity out the branch at an interval appropriate to re-excite the entire heart. When one end of the preparation could excite the branch, stimulation of the other end sometimes inhibited excitation of the branch. The results are important as part of the explanation of phenomena associated with re-entrant arrhythmias.