FREQUENCY-DEPENDENCE OF CEREBROVASCULAR IMPEDANCE IN PRETERM NEONATES- A DIFFERENT VIEW ON CRITICAL CLOSING PRESSURE

The nonproportional relationship between instantaneous arterial blood pressure (BP) and cerebral blood flow velocity (CBFv) is well explaine d by the concept of critical closing pressure (CCP). We aimed to deter mine the frequency response of the neonatal cerebrovascular system, an d to establish the exact mathematical relationship between cerebrovasc ular impedance and CCP under physiologic conditions. In 10 preterm neo nates (gestational age, 25-32 weeks; birth weight, 685-1,730 g; age 1- 7 days) we Doppler-traced CBFv of the internal carotid artery. Blood p ressure was traced simultaneously. Critical closing pressure was graph ically determined. Cerebrovascular impedance was calculated as the squ are root of the ratio of the corresponding peaks in the power spectra of BP and CBFv at zero frequency, and at heart rate (H) and harmonics (xH). Uniformly, the impedance between H and 3H (2 to 6 Hz) was reduce d about fivefold, compared with the impedance at zero frequency. The c erebrovascular system behaves like a high-pass filter, leading to a re duction of the DC (direct current) component of CBFv (analogous to cur rent) relative to that of the driving force BP (analogous to voltage). The frequency response of cerebrovascular impedance reflects the rati o of CCP and DC BP. A mathematical derivation of this relationship is given matching the observed results, Thus, both the CCP and the impeda nce approach are valid.