CEREBRAL AUTOREGULATION OF PRETERM NEONATES - A NONLINEAR CONTROL-SYSTEM

The low frequency cerebral blood flow velocity (CBFV) oscillations in neonates are commonly attributed to an under-dampened immature linear type cerebral autoregulation, and the 'instability' is regarded as cau sative for peri-intraventricular haemorrhage/periventricular leukomala cia. In contrast, oscillations susceptible to frequency entrainment ar e a fundamental part of the stable function of non-linear control syst ems. To classify the autoregulation an observational study was done on the relationship between CBFV oscillations, heart rate variability, a nd artificial ventilation. In 10 preterm neonates (gestational age 26 to 35 weeks) we serially Doppler traced arterial CBFV continuously for 12 minutes between days 1 and 49 of life. The individual time series of CBFV and heart rate were subjected to spectral analysis. Forty six of 47 tracings showed significant low frequency CBFV oscillations. Low frequency heart rate oscillations were not a prerequisite thereof. Ah patients with <30% of total power in the low frequency band of CBFV o scillations were on the ventilator. Three of them demonstrated a shift of spectral power from low frequency to a frequency equal or harmonic to the ventilator rate indicating entrainment. The findings of CBFV o scillations combined with entrainment classify the autoregulation as a nonlinear system. It is suggested that entrainment by periodic high a mplitude stimuli might challenge the regulatory capacity to its limits thus increasing the risk for cerebral damage.