Development of Baroreflex Control of Heart Rate in Swine

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Abstract
The purpose of this study is to describe the developmental course of arterial baroreflex control of heart rate in swine. Tests of baroreflex function were performed with eight conscious piglets serially over their first 2 mo of life. Systemic blood pressure was raised with phenylephrine (pressor test) and lowered with nitroprusside (depressor test), and stimulus-response curves relating heart rate to mean blood pressure were constructed. Baroreflex sensitivity was determined as the slope of the linear portion of the curve. Baroreflex sensitivity decreased with increasing age. Baroreflex sensitivity was not different between pressor and depressor tests except when the piglets were >52 d old and sensitivity was greater with the depressor test. The heart rates at threshold and saturation, and therefore the heart rate response range, shifted to lower heart rates with increasing age. This shift was more than can be accounted for by the simultaneously decreasing resting heart rate.