Fetal cerebral blood flow, electrocorticographic activity, and oxygenation: Responses to acute hypoxia

Arterial blood gases are critical in regulation of cerebral blood flow (CBF) and cerebral metabolic rate for O₂ (CMRO₂). However, the relation of these variables to cortical tissue (t), and electrocorticographic (ECoG) activity (high voltage low frequency, HVLF, versus low voltage high frequency, LVHF), are not well defined. In the fetus, we tested the hypothesis that ECoG pattern is associated closely with cerebral oxygenation. In fetal sheep (n = 8) with laser Doppler flowmeter, fluorescent O₂ probe and ECoG electrodes, we measured laser Doppler CBF (LD-CBF), t, ECoG and spectral edge frequency-90 (SEF 90) in response to 40 min isocapnic hypoxia. In the normoxic fetus, LD-CBF and CMRO₂ correlated highly with ECoG state. With a shift from HVLF to LVHF, t decreased followed by increased LD-CBF (18%) and CMRO₂ (13%). With acute hypoxia (P_aO2 = 12 ± 1 Torr), t decreased to ∼3 Torr, LD-CBF increased 48 ± 10%, ECoG shifted to chiefly the HVLF state, SEF 90 decreased ∼15%, and CMRO₂ decreased ∼20% (P < 0.05 for each). For the normoxic fetus, CBF was closely related to ECoG state, but this association was less evident during acute hypoxia. We speculate that, in the otherwise stressed fetus, acute hypoxia may further compromise cerebral oxygenation.