CEREBRAL AUTOREGULATION IS A NONLINEAR TYPE CONTROL SYSTEM

Abstract

Introduction. In neonates, low frequency (LF) cerebral blood flow velocity oscillations (CBFV-O) are commonly attributed to an underdampened immature cerebral autoregulation (AR). Peri-intraventricular hemorrhage (PIVH) is linked to this ‘instability’. In contrast to linear type control systems, nonlinear type systems express a regular periodicity as a fundamental part of their stable function. Aims. To classify the AR, and to identify factors possibly responsible for PIVF by investigating the relationship between CBFV-O, heart rate variability (HR-V), and intermittent positive pressure ventilation (IPPV). Methods. In 5 preterm neonates (GA 26 to 30 w) we serially Doppler-traced arterial CBFV continuously for 12 min every 3 to 7 days between days 1 and 49 of life. Another 5 preterms (GA 26 to 35 w) were traced sporadically. The time series of both CBFV and HR were subjected to spectral analysis. Results. 46/47 tracings showed LF CBFV-0 (p<0.0001, one sample z-test). For HR, cycling was observed in only 15/47 tracings. LF cycling in HR was not a prerequisite for LF CBFV-O. All patients with < 30 % of total power in the LF band of CBFV-O were on the ventilator. Three of these patients demonstrated a shift of spectral power from LF to a frequency equal or harmonic to the ventilator rate in the sense of entrainment. The range of entrainment encompassed 12 to 25/min stimuli. Conclusion. In analogy to thermo- and blood pressure regulation, CBFVO and entrainment classify the AR as a nonlinear system. HR-V has no direct impact on CBFV-O, whereas respiration acts on both HR-V and CBFV-O. We discuss that periodic high-amplitude stimuli (i.e. IPPV in RDS) may challenge the regulatory capacity of the CBF control system as reflected by entrainment. While fragile vascular matrix elements, severe central blood pressure changes, and high pCO2 are prerequisites, entrainment of CBFV-O might be one key event for cerebral damage.