• MutchW Alan CWAChttp://orcid.org/0000-0001-7174-7839Department of Anesthesiology, Perioperative and Pain Medicine, University of Manitoba, 2nd Floor, Harry Medovy House, 671 William Ave., Winnipeg, MB, R3E 0Z2, Canada. wacmutch@shaw.ca.Canada North Concus and James Duffin.
• Department of Anesthesiology, Perioperative and Pain Medicine, University of Manitoba, 2nd Floor, Harry Medovy House, 671 William Ave., Winnipeg, MB, R3E 0Z2, Canada. wacmutch@shaw.ca.
• Can J Anaesth. 2021 Oct 1; 68 (10): 1497-1506.

PurposeAnesthesia is associated with alterations in end-tidal (ET) respiratory gases from the awake state. These alterations result in marked vasoactive changes in regional cerebral blood flow (rCBF). Altered regional cerebrovascular reactivity (rCVR) is linked to neurologic dysfunction. We examined these differences in reactivity from prior work by focusing on the ratio of vasoconstriction with hyperoxia/hypocapnia (HO/hc):vasodilation with hypercapnia (HC) using magnetic resonance imaging pseudo-continuous arterial spin labelling (pCASL) to measure rCBF and compare rCVR The distribution and magnitude of these ratios could provide insights into rCBF during clinical anesthesia and inform future research into the origins of postoperative delirium (POD).MethodsTen healthy subjects underwent cerebral blood flow (CBF) studies using pCASL with computer-controlled delivery of ET gases to assess flow effects of hyperoxia, hypercapnia, and hyperoxia/hypocapnia as part of a larger study into cerebrovascular reactivity. The vasoconstrictor stimulus was compared with the vasodilator stimulus by the ratio HO/hc:HC.ResultsHyperoxia minimally decreased whole brain CBF by - 0.6%/100 mm Hg increase in ETO2. Hypercapnia increased CBF by +4.6%/mm Hg carbon dioxide (CO2) and with HO/hc CBF decreased by - 5.1%/mm Hg CO2. The brain exhibited markedly different rCVR-regional HO/hc:HC ratios varied from 7.2:1 (greater response to vasoconstriction) to 0.49:1 (greater response to vasodilation). Many of the ratios greater than 1, where vasoconstriction predominated, were seen in regions associated with memory, cognition, and executive function, including the entorhinal cortex, hippocampus, parahippocampus, and dorsolateral prefrontal cortex.ConclusionsIn awake humans, marked rCBF changes occurred with alterations in ET respiratory gases common under anesthesia. Such heterogeneous reactivity may be relevant to future studies to identify those at risk of POD.© 2021. Canadian Anesthesiologists' Society.

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