Cerebral electrical impedance: do indices derived from it provide information on cerebral blood flow in the neonate?

Cerebral electrical impedance was compared with simultaneous estimations of cerebral blood flow (CBF infinity) using the 133Xe clearance technique on 28 occasions in 9 neonates who had changes in PaCO2 and hence presumably cerebral blood flow between measurements. Various indices derived from the impedance waveform which have been reported in the adult to correlate with blood flow were analysed for their relationship with CBF infinity. Percentage change of the two variables was calculated and three indices showed a significant correlation using linear regression. They were peak amplitude (delta Zp), delta Zp times heart rate (delta Zp x HR), and backslope projection. The regression equation for the index with the closest correlation was delta Zp x HR = 0.57CBF infinity + 3.4 with r = 0.68 (p = 0.003) and standard error of the estimate = 29%. Some cerebral electrical impedance indices did correlate with cerebral blood flow changes under the conditions of this study and provide accurate information on the direction of changes but because of the wide confidence limits cannot be used for quantification of cerebral blood flow changes.