Effect of Permissive Mild Hypercapnia on Cerebral Vasoreactivity in Infants: A Randomized Controlled Crossover Trial.


PaCO2 is a major regulator of cerebral vascular tone. Hypocapnia can result in poor neonatal neurological outcomes, presumably through decreased cerebral blood flow. The clinical effect of mild hypercapnia is less well known.

Study Summary

  • This study investigates whether mild hypercapnia increases cerebral blood flow by calculating Tissue Oxygenation Index (TOI) and Tissue Haemoglobin Index (THI) using NIRS monitoring.
  • 19 infants aged < 1 year were recruited into a randomised crossover study.
  • Infants randomised to Group N(ormal) were ventilated to an EtCO2 of 5.5kPa, then 6.5kPa, then back to 5.5kPa.
  • Infants randomised to Group H(ypercapnia) were ventilated to an EtCO2 of 6.5kPa, then 5.5kPa, then back to 6.5kPa.


  • The primary outcome of this study found no evidence that mild hypercapnia affected TOI and THI. There was a large scatter in all NIRS parameters.
  • Ancillary analysis found that mean arterial pressure (MAP) did correlate with TOI and THI.


  • Overall, this study suggests that mild hypercapnia is not a strategy for increasing cerebral blood flow.
  • The scatter of NIRS parameters shows its limitations as a single monitor of cerebral perfusion.
  • Controlling MAP could be more important than PaCO2 for cerebral blood flow but the study was not designed to examine this specifically.
  • Further studies are required to investigate whether more hypercapnia increases cerebral blood flow.

Reviewed by M. Tan