Strawman redux: management of end-tidal gases in patients at risk of perioperative neurocognitive disorder
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CORRESPONDENCE
Strawman redux: management of end-tidal gases in patients at risk of perioperative neurocognitive disorder W. Alan C. Mutch, MD
. Rene´e M. El-Gabalawy, PhD . M. Ruth Graham, MD
Received: 6 May 2020 / Revised: 7 July 2020 / Accepted: 9 July 2020 Ó Canadian Anesthesiologists’ Society 2020
To the Editor, In a previous published commentary in the Journal1 entitled ‘‘Anesthesia and postoperative delirium: the agent is a strawman—the problem is CO2’’, we proposed that the conduct of anesthesia and, in particular, end-tidal (ET) carbon dioxide (CO2) management may be an important contributor to cognitive dysfunction after surgery, now termed perioperative neurocognitive disorder (POND).2 Subsequent work by our group and others has confirmed an association between intraoperative hypocapnia and one of the sub-categories of POND—postoperative delirium (POD).3,4 Not considered in these publications was the potential influence of the other ET respiratory gases, specifically O2 or potential synergistic interactions between ETCO2 and ETO2 as additional contributing factors to the development of POD, although periods of both hypocapnia and hyperoxia are commonly observed during mechanical ventilation in the course of most routine anesthetics. We have recently examined the combined effects of hyperoxia and hypocapnia on cerebral blood flow (CBF).5 Using pseudo-continuous arterial spin labelling, we have identified marked, dynamic changes in CBF during tightly W. A. C. Mutch, MD (&) M. Ruth Graham, MD Department of Anesthesiology, Perioperative and Pain Medicine, Max Rady College of Medicine, Rady Faculty of Health Sciences, University of Manitoba, Winnipeg, MB, Canada e-mail: [email protected] R. M. El-Gabalawy, PhD Department of Anesthesiology, Perioperative and Pain Medicine, Max Rady College of Medicine, Rady Faculty of Health Sciences, University of Manitoba, Winnipeg, MB, Canada Department of Clinical Health Psychology, Max Rady College of Medicine, Rady Faculty of Health Sciences, University of Manitoba, Winnipeg, MB, Canada
controlled changes in ETCO2 and ETO2 that are observed commonly during anesthesia. That study also showed the independent effect of hyperoxia at normocapnic values on regional decreases in CBF. Additionally, a synergistic decrement in CBF is described with combined hyperoxia and hypocapnia. Further analysis of these data in Figure (A–C) shows the change in CBF in a single subject under different test conditions. These conditions were (A) baseline (normoxia/normocapnia) and (B) spontaneous hyperventilation to lower the ETCO2 to 5 mmHg below baseline in the presence of hyperoxia (ETO2 = 400 mmHg). The difference map (image 2 - image 1) is presented in (C) and decreasing CBF is represented as a colour change from blue to purple. A marked global decrement with combined hyperoxia and hypocapnia is evident with regional grey matter flow frequently decreasing by approximately 50%. As such changes in ET respiratory gases are routinely observed during the course of standard anesthetic care,
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