• MutchW Alan CWACFrom the Department of Anesthesiology, Perioperative and Pain Medicine, University of Manitoba (WACM), Canada North Concussion Network, Winnipeg, Manitoba (WACM), Department of Anesthesiology and Pain Medicine, University of Toronto (JD), and James Duffin.
• From the Department of Anesthesiology, Perioperative and Pain Medicine, University of Manitoba (WACM), Canada North Concussion Network, Winnipeg, Manitoba (WACM), Department of Anesthesiology and Pain Medicine, University of Toronto (JD), Thornhill Medical (JD) and Department of Physiology, University of Toronto, Toronto, Ontario, Canada (JD).
• Eur J Anaesthesiol. 2022 Sep 1; 39 (9): 774784774-784.

BackgroundRegional cerebrovascular reactivity (rCVR) is highly variable in the human brain as measured by blood oxygenation level-dependent (BOLD) MRI to changes in both end-tidal CO 2 and O 2 .ObjectivesWe examined awake participants under carefully controlled end-tidal gas concentrations to assess how regional CVR changes may present with end-tidal gas changes seen commonly with anaesthesia.DesignObservational study.SettingTertiary care centre, Winnipeg, Canada. The imaging for the study occurred in 2019.SubjectsTwelve healthy adult subjects.InterventionsCerebral BOLD response was studied under two end-tidal gas paradigms. First end-tidal oxygen (ETO 2 ) maintained stable whereas ETCO 2 increased incrementally from hypocapnia to hypercapnia (CO 2 ramp); second ETCO 2 maintained stable whereas ETO 2 increased from normoxia to hyperoxia (O 2 ramp). BOLD images were modeled with end-tidal gas sequences split into two equal segments to examine regional CVR.Main Outcome MeasuresThe voxel distribution comparing hypocapnia to mild hypercapnia and mild hyperoxia (mean F I O 2  = 0.3) to marked hyperoxia (mean F I O 2  = 0.7) were compared in a paired fashion ( P  < 0.005 to reach threshold for voxel display). Additionally, type analysis was conducted on CO 2 ramp data. This stratifies the BOLD response to the CO 2 ramp into four categories of CVR slope based on segmentation (type A; +/+slope: normal response, type B +/-, type C -/-: intracranial steal, type D -/+.) Types B to D represent altered responses to the CO 2 stimulus.ResultsDifferential regional responsiveness was seen for both end-tidal gases. Hypocapnic regional CVR was more marked than hypercapnic CVR in 0.3% of voxels examined ( P  < 0.005, paired comparison); the converse occurred in 2.3% of voxels. For O 2 , mild hyperoxia had more marked CVR in 0.2% of voxels compared with greater hyperoxia; the converse occurred in 0.5% of voxels. All subjects had altered regional CO 2 response based on Type Analysis ranging from 4 ± 2 to 7 ± 3% of voxels.ConclusionIn awake subjects, regional differences and abnormalities in CVR were observed with changes in end-tidal gases common during the conduct of anaesthesia. On the basis of these findings, consideration could be given to minimising regional CVR fluctuations in patients-at-risk of neurological complications by tighter control of end-tidal gases near the individual's resting values.Copyright © 2022 European Society of Anaesthesiology and Intensive Care. Unauthorized reproduction of this article is prohibited.

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