Impact of hypercapnia on neurovascular coupling in the fetal sheep and Newborn lamb.

Background: Neurovascular coupling (NVC) leads to increased cerebral blood flow (CBF) and cerebral oxygenation upon increased neural activity. Hypercapnia (HC) increases baseline CBF and attenuates NVC-induced CBF response in the adult brain. HC is common in preterm infants undergoing intensive care, but effects of HC on NVC in the immature brain remain unknown. We examined the impact of HC on NVC in the fetal sheep and neonatal lamb, using somatosensory evoked potential (SEP) after median nerve stimulation measured by EEG as an index of increased cortical activity, and cerebral oxy- and deoxyhaemoglobin (oxyHb and deoxyHb) measured by near infrared spectroscopy (NIRS). Method(s): Under isoflurane anaesthesia, EEG and NIRS electrodes were positioned bilaterally over the somatosensory cortex of fetal sheep (128-132 days gestation; n = 6) and 4-8 day old neonatal lambs (n = 6). Trains of median nerve electrical stimuli evoked a SEP, and oxyHb and deoxyHb were recorded before, during and after HC. Result(s): PaCO2 [mean (SD)] before, during and after HC was 48.2(5.6), 69.6(5.5) and 52.7(6.9)mmHg, respectively, in fetal sheep, and 39.5(3.2), 64.6(6.3) and 41.6(4.4)mmHg, respectively, in newborn lambs. During HC, 5 of the 6 fetal sheep showed higher oxyHb in the contralateral cortex after median nerve stimulation compared to before HC [mean oxyHb (During HC-before HC): 21.0 muM.cm]. Four of the 6 newborn lambs showed higher oxyHb in contralateral cortex [mean oxyHb (During HC-before HC): 10.5 muM.cm]. Conclusion(s): Cerebral oxygenation after somatosensory stimulation is increased with HC in fetal sheep and newborn lambs, in contrast to previous studies in the adult brain.