Abstract
To study the influence of sympathetic ingrowth on sinus node (SAN) function, 2 groups of puppies underwent electrophysiologic studies. Eleven puppies (20 studies, ages 3-17w, m 9.0) underwent chemical sympathectomy by 6 hydroxydopamine (60H). Twelve puppies (22 studies, ages 3-25w, m 10.9) were controls (C). Testing was repeated after atropine (A) and atropine and propranolol (A+P).Resting cycle length (CL) was shortened by A, in both C (p<.001) and 60H (p<.001). After A, differences of C1, sinoatrial conduction time (SACT), and corrected sinus node recovery time (CSNRT) between C and 60H were N.S. In C, CSNRT correlated with age (r=.61, p<.02); in 60H it did not.In C, addition of P to A lengthened Cl 71 ± 71 ms (p<.001) SACT 9 ± 10 ms (p<.01) and CSNRT 33 ± 51 ms (p<.05). In 60H, addition of P to A lengthened Cl 71 ± 31 ms (p<.001) and CSNRT 27 ± 36 ms (p<.01). Change in SACT was N.S.We conclude: (1) SAN function responds to beta blockade with and without chemical sympathectomy. (2) SAN automaticity following vagal blockade appears to decrease with age. This relationship is abolished by sympathectomy, and may be due to developing cardiac sympathetic innervation.
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