Developmental changes in action potential duration, refractoriness, and conduction in the canine ventricular conducting system

: The purpose of these studies was to determine the age-related changes that occur in refractoriness and conduction in the canine Purkinje system and ventricle. We used standard microelectrode techniques to study the isolated left and right ventricular specialized conducting system of neonatal and 2-, 5-, and 10-year-old beagles. We mapped the preparations with microelectrodes and bipolar surface electrodes and recorded action potential duration, conduction velocity, and refractoriness. We found five age-related changes. (1) The region of maximum action potential duration in the adult Purkinje system occurred in the distal false tendon near its subendocardial insertion. In neonates, the entire proximal subendocardial Purkinje system and free running false tendon had a uniformly long action potential, which then shortened in the distal subendocardial fibers. (2) Conduction block of responses to induced premature stimuli occurred proximally, near the origin of the false tendon in all groups. (3) The effective refractory period of the ventricular specialized conduction system did not vary among different age groups and was the same for the left and right ventricles. (4) Conduction velocity of basic and premature impulses was significantly lower in neonates and higher in 2-year-olds than in older age groups; (5) values for amplitude and maximum upstroke velocity of phase 0 were reduced in the neonatal and 10-year-old beagles with respect to values for 2- and 5-year-old beagles.