POSTNATAL MATURATION OF THE RESPONSE OF THE CANINE SINUS NODE TO CRITICALLY TIMED, BRIEF VAGAL-STIMULATION

We have previously demonstrated that vagal phase-response curves (PRC) , which characterize the effects of critically timed, brief vagal stim uli on sinus node automaticity, exhibit a fundamentally different shap e in the canine newborn than in the adult. In this study we analyzed t he changes in sinus cycle length in response to critically timed, brie f vagal stimuli, delivered to the decentralized cervical right and lef t vagosympathetic trunks, in two older age groups: 14 1-mo-old puppies (ages 21-36 d), and eight 2-mo-old puppies (ages 56-62 d). Vagal PRC were constructed by plotting the magnitude (percent change) of the vag al chronotropic response as a function of the phase of the cardiac cyc le at which the vagus nerve was stimulated. At 1 mo of age adult-type PRC were observed, but in only six of the puppies (43%) and only in re sponse to right vagal stimulation. By 2 mo of age adult-type PRC were observed in seven of eight puppies (88%) in response to right vagal st imulation and in three of eight (38%) in response to left vagal stimul ation. Thus, clear developmental changes in the phase dependence of th e vagal chronotropic response can be tracked over the first 2 mo of li fe in the dog.