Developmental changes in impulse conduction in the canine heart.

We studied the age-related changes that occur in conduction in Purkinje fiber bundles (PF) from adult and 8-wk-old dogs. As PF are stretched, conduction velocity increases to a maximum when bundle length is approximately 1.5 times control. Conduction velocity in adult PF was significantly faster than that in 8-wk PF at all bundle lengths, the maximum being 2.32 +/- 0.45 m/s (mean +/- SD) in adult and 1.56 +/- 0.59 m/s in 8-wk-old dogs. At greater than 1.5 times control length, conduction velocity decreased, as did resting potential, action potential amplitude, and upstroke velocity. At 1.5 times control length, the ratio of PF to collagen in the bundles increased, and structural changes consistent with both unbuckling and unfolding of the cell membranes were seen. At greater degrees of stretch, cleft size diminished, and intracellular edema was noted. In sum, structural changes in cardiac PF can explain the changes in conduction velocity that occur with stretch. At all degrees of stretch, however, velocity is faster in PF from adult than from young dogs.