Sir.—Using the Doppler pulsatility index technique, Daven et al1 have demonstrated the cerebral vasodilatory response to hypoxia and hypercarbia in premature infants. This is an important contribution to the understanding of perinatal hemodynamic events, and it confirms the observations of covariation of Paco2 and apparent cerebral blood flow (CBF) by Cooke et al2 with the occlusion plethysmographic method.
In our studies of distressed neonates, my colleagues and I could not detect any clear relation between Pao2 or Paco2 and CBF. This is probably because of the small number of patients and the many variables. We did, however, see a significant proportionality between CBF and arterial BP that could not be accounted for by differences in gas tension, indicating pressure-passive perfusion (ie, absence of autoregulation). This interpretation was supported by repeated studies at different arterial BPs in two cases.3
Animal studies are, of course,
Lou HC. 'Lost Autoregulation Hypothesis' and Cerebral Hemodynamic Events in Neonates. Am J Dis Child. 1983;137(12):1211. doi:10.1001/archpedi.1983.02140380069026