Time-order of ventricular activation for premature beats in sheep and dogs

¹ Department of Veterinary Physiology and Pharmacology, College of Veterinary Medicine, Ohio State University, Columbus, Ohio

Average durations for the sinus QRS complex were 0.035 seconds for the sheep and 0.041 seconds for the dog. The average prolongation of premature beats was 145% in the sheep and 210% in the dog. Spatially, the mean QRS vector for both species was directed coplaner to the median sagittal plane—the sheep being minus 120 degrees at a magnitude of 0.8 mv, and the dog being at 88 degrees and a magnitude of 1.4 mv. In both species, premature beats from left ventricular focuses were directed cephalically; and from right ventricular focuses, caudally. It is difficult to explain why premature beats from identical focuses in sheep and dog hearts have nearly identical mean spatial QRS vectors, yet the prolongation and magnitude is so much greater in the dog whose heart is less than one-half the size of the sheep's. Of the possible explanations for the data: (a) that the activation for premature beats is all muscle fiber to muscle fiber; (b) that the impulse quickly re-enters into the Purkinje system; or (c) that a combination of the two exists, the last is most tenable based upon anatomic studies describing the more complete penetration of the Purkinje fibers into the epicardium of sheep.