Electrophysiological characteristics of canine atrial plateau fibers.

Right atria from dog hearts were immersed in Tyrode's solution with the endocardial surface of the anterior wall exposed. Glass microelectrodes were used to impale fibers in diSerent anatomical areas of this preparation. Electrical characteristics of atrial plateau fibers located along the caval border of the crista terminalis were compared with those of regular atrial fibers from other sites. The former conducted at a rate (0.803 ±0.110 M/sec) two to three times more rapidly than pathways composed of regular atrial fibers. A period of supernormal excitability was demonstrated for plateau fibers but not for regular fibers. Graded premature responses could be elicited in plateau fibers by both electrical stimulation and propagated action potentials. Duration of the effective refractory period of plateau fibers (196.1 ± 2.5 msec) was longer than that of regular fibers (147.8 ± 5.4 msec). Plateau fibers required a lower level of membrane potential (—62.1 ± 0.8 mv) to generate a propagated action potential than did regular fibers (—65.5 ± 1.3 mv). Regular atrial fibers could respond at faster rates to high frequency stimulation than plateau fibers.