Abstract
Previous studies have suggested that increases in heart rate (HR) may represent the primary mechanism available to the newborn for increasing cardiac output (CO). In this study, we examined the independent effects of HR and afterload changes on CO in newborn lambs. Atrioventricular node ablation was performed in eight newborn lambs (3-10 days of age) by formalin injection, and the right ventricle was paced at a HR of 200 (baseline). Nine studies were performed in young lambs (less than 12 days of age), and nine studies in older lambs (greater than 12 days of age). CO/kg increased progressively with increasing HR. The maximum CO/kg attained was similar for both groups, but this was achieved at a higher HR in the younger lambs. From a baseline HR of 200, the maximum increase in CO/kg attained by increasing HR was 6.7% in the younger lambs, and 8.9% in the older lambs. At lower HR's (70/min), oxygen consumption (VO2) decreased, and systemic vascular resistance (SVR) increased. With a fixed HR of 200, nitroprusside caused an increase in CO/kg of 10.5% in the younger lambs, and 11.8% in the older lambs. Phenylephrine caused a decrease in CO/kg of 19.3% in the younger lambs, and 10.6% in the older lambs. HR above baseline has limited value in increasing CO in newborn lambs. At lower HR's, CO and VO2 fall, and SVR increases. Decreasing afterload produces modest increases in CO. Increasing afterload results in decreases in CO, and these changes were most striking in the younger lambs.
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Shaddy, R., Li, C., Tyndall, M. et al. 139 THE ROLE OF HEART RATE AND AFTERLOAD ON CARDIAC OUTPUT IN NEWBORN LAMBS. Pediatr Res 19, 134 (1985). https://doi.org/10.1203/00006450-198504000-00169
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DOI: https://doi.org/10.1203/00006450-198504000-00169