Mechanisms of beat-to-beat variability in the heart rate of the neonatal lamb. I. Influence of the autonomic nervous system
Zugaib, M.; Forsythe, A.B.; Nuwayhid, B.; Lieb, S.M.; Tabsh, K.; Erkkola, R.; Ushioda, E.; Brinkman, C.R.; Assali, N.S.
American Journal of Obstetrics and Gynecology 138(4): 444-452
1980
ISSN/ISBN: 0002-9378 PMID: 7425003 Document Number: 161296
The contributions of the intrinsic, .beta.-adrenergic and cholinergic systems to the beat-to-beat variability mechanisms of the heart rate were investigated in chronically instrumented, unanesthetized newborn lambs from the 1st-8th wk of neonatal life. During this period of neonatal growth, the resting heart rate decreased spontaneously every week; the decrease was not related to alterations in the sympathetic and parasympathetic tones but rather to changes in heart rate control intrinsic mechanisms. Despite the weekly decrease in the neonatal lamb resting heart rate, the long- and short-term beat-to-beat variabilities did not change significantly. An absence of a significant influence of the heart rate control intrinsic mechanisms on the genesis of beat-to-beat variability is suggested. The results obtained with the various modes of adrenergic blockades and stimulation indicate that the autonomic nervous system contributes significantly to the appearance of beat-to-beat variability. The influences of the adrenergic and cholinergic systems differ from week to week of neonatal growth and the patterns of changes are not the same for the long- and short-term variabilities. The implication of these studies in terms of physiologic and hemodynamic significance of beat-to-beat variability in heart rate is discussed.