Effect of carbon dioxide on cerebral blood flow velocity in preterm infants during postnatal transition.
High arterial carbon dioxide (PaCO2 ) and cerebral reperfusion are associated with peri/intraventricular haemorrhage. Our aim was to study the relationship between PaCO2 and cerebral blood flow (CBF) in preterm infants during postnatal transition. We prospectively studied ≤30 weeks' gestation haemodynamically stable preterm infants during the first three postnatal days (n = 21; gestational age 25.8 ± 1.4 weeks). We measured middle cerebral artery mean flow velocity (MCA-MV) as a surrogate for CBF at the time of blood gas analysis. We obtained 78 PaCO2 -MCA-MV data pairs. The expected positive linear relationship between PaCO2 and MCA-MV was absent on the first postnatal day, equivocal on the second and present on the third. Using piecewise bilinear regression models, we identified PaCO2 breakpoints at 52.7 and 51.0 mmHg for postnatal days two and three, respectively. In haemodynamically stable preterm neonates, the expected positive linear relationship between PaCO2 and CBF may be absent on postnatal day one. On postnatal day three, and possibly day two, a PaCO2 threshold exists for this relationship, above which CBF becomes reactive to PaCO2 . We speculate that the enhanced CBF response to PaCO2 above the threshold contributes to the reperfusion injury and partly explains the association between hypercapnia and peri/intraventricular haemorrhage.