VARIABILITY OF HEART RATE, BLOOD PRESSURE & CEREBRAL OXYGENATION: LBW NEONATES

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The broad objective of the study is to characterize the physiologic relationships between variability of heart rate, blood pressure, and cerebral oxygenation in a population of preterm newborns at high risk for severe brain injury. The study aims to identify and describe the physiologic antecedents of brain injury in a way that will facilitate subsequent interventional trials. From comparison of repeated measures during the first week of life, the investigators will characterize the developmental maturation of autonomic nervous system modulation of these factors. Previous experiments have found a close correlation between changes in cerebral oxygenation with changes in blood flow velocity as measured by Doppler ultrasound. Data suggests that correlation between fluctuations in mean arterial blood pressure with fluctations in cerebral oxygenation measured by near infrared spectroscopy can stratify likelihood of finding brain injury, including intraventricular hemorrhage and periventricular leukomalacia. An important aim of the study is to confirm and clarify this apparent relation between blood pressure and cerebral oxygenation. Measurement, characterization, and monitoring of unstable cerebral oxygen delivery, and more importantly, its relation to blood pressure and heart rate variability may provide a useful starting point from which to devise neuroprotective clinical strategies. Because blood pressure and heart rate variability are physiologic variables that can be manipulated, the investigators may be able to exploit the relation between these parameters and cerebral oxygenation in a prophylactic manner. Specific Aims: Fourier analysis will be used to quantify variability of cerebral oxygenation. Using coherence functions and transfer functions as previously described, the investigators will evaluate: 1) heart rate variability as a function of blood pressure variability; and 2) variability of cerebral oxygenation as a function of blood pressure variability. The former is a parameter of the baroreflex; the latter of cerebral autoregulation. With respect to transfer functions analysis of the baroreflex and of cerebral autoregulation, blood pressure will be considered the input signal.