Atrioventricular transmission in young mammals

The hearts of young anesthetized goats, pigs and puppies were exposed for electrical stimulation and recording. Bipolar electrodes were attached to the right atrium and right ventricle. The sinus node was crushed. The heart was driven by stimuli applied to the atrial appendage. At intervals, one or two atrial ‘premature’ responses were induced to assess the time relations of A-V transmission. In all experiments with goats and pigs the earliest possible premature responses, on transmission to the ventricle, yielded electrograms of bizarre configuration. In some preparations multiple responses and ventricular fibrillation were repeatedly induced by early re-excitation of the atrium. Studies of the transmission times of premature responses in the normal and retrograde directions suggested that the refractory period of the A-V transmission system was shorter than that of the ventricle. It is postulated that early premature atrial responses reached the ventricle during its vulnerable period.