Conduction of the Cardiac Impulse

Depressed excitability and responsiveness were created in excised bundles of canine Purkinje fibers. A segment 8 mm long was depressed by being encased in agar containing 47 m M K + , the ends of the bundle outside the agar remaining normal. Either normal end could be excited through extracellular electrodes. Action potentials were recorded by intracellular microelectrodes at each end and within the depressed segment. Conduction velocity within the depressed segment fell as low as 0.05 m/sec. Abnormalities of impulse transmission through the depressed segment included delay, 2:1 block, higher degrees of block, rate-dependent block, and block showing the Wenckebach phenomenon. Asymmetries of conduction seen included one-way block. Action potentials in the depressed segment were of low amplitude and showed slow upstrokes. Variations in action potential duration occurred in the depressed segment when conduction failed or was very slow and when impulses were dropped. Delay in conduction too great to result simply from a slow upstroke is attributed to summation of excitatory events across regions of block in a syncytium of cells. The results prove that conduction delays great enough to permit re-entry can occur in short segments of Purkinje fibers subjected to high K + .