Response of cerebral blood flow to changes in PCO2 in fetal, newborn, and adult sheep.

Developmental effects on the response of cerebral blood flow (Qc) and cerebral O2 consumption (CMRO2) to changes in CO2 tension were assessed in unanesthetized fetal, newborn, and adult sheep. Blood flow was measured using the radioactive microsphere technique. CMRO2 was calculated as the product of Qc and the difference in O2 content between arterial and sagittal sinus blood (CaO2 -- CVO2). The response of Qc to changes in arterial CO2 tension increased from fetus [3.53 +/- 0.56 ml.100 g-1.min-1.mmHg PaCO2(-1) (SE)] to newborn (5.16 +/- 0.59) to adult (6.20 +/- 0.63). Only the fetal-adult difference was significant (P less than 0.05). It has been suggested that developmental differences in CO2 responsiveness of cerebral blood flow are the result of differences in CMRO2. We corrected for differences in CMRO2 by looking at the response to CO2 of the variable 1/(CaO2--CVO2). According to the Fick principle 1/(CaO2--CVO2) = Qc/CMRO2, i.e., blood flow per unit O2 consumption. The fetal response was not significantly different from the newborn, but the adult was significantly different from both (P less than 0.05). Thus the difference in CO2 response of cerebral blood flow between fetus and adult cannot be explained by differences in CMRO2.