The interplay of CO2, inhalation anesthetics, cerebral autoregulation of blood flow has been a delicate dance that I've been aware of for (checks calendar) 45 years.
Gelb, A. W., & Bayona, N. A. (2005). "Anesthesia and cerebral blood flow." Current Opinion in Anesthesiology, 18(6), 537-542.
Laffey, J. G., & Kavanagh, B. P. (2002). "Hypocapnia." The New England Journal of Medicine, 347(1), 43-53.
Eintrei, C., & Varnauskas, E. (1993). "Effects of isoflurane, halothane, and enflurane on cerebral blood flow and metabolism." Anesthesiology, 78(5), 976-982.
In most instances, you want to maintain normocapnia (normal CO2) levels during anesthesia. In some cases, where there is brain swelling, hypocapnia is indicated to reduce cerebral blood flow and therefore reduce swelling.
The effect of anesthetic gasses on CBF autoregulation has been studied for a long time.
What's new is that there are now intravenous drugs that can minimize that change.
Bottom line - these are physiologically-interesting studies, but have very little practical effect in the majority of cases.
From the original article:
Editor’s Perspective
What We Already Know about This Topic
Cerebrospinal fluid flow via the perivascular glymphatic clearance pathway is important to maintain central nervous system homeostasis
General anesthetics and carbon dioxide have major influence on cerebral vasomotor tone and, therefore, can influence glymphatic cerebrospinal fluid dynamics
The combined influence of general anesthetics and varying carbon dioxide levels on cerebrospinal fluid dynamics is unexplored
What This Article Tells Us That Is New
In ventilated rats under physiologic carbon dioxide levels, ketamine–dexmedetomidine anesthesia led to cerebral vasoconstriction and concomitant increases in cerebrospinal fluid space size, whereas isoflurane anesthesia had opposite effects
In hypercapnic conditions, ketamine–dexmedetomidine anesthesia preserved the total intracranial cerebrospinal fluid volume, and hyperventilation-induced hypocapnia did not change cerebrospinal fluid volume under isoflurane anesthesia
These observations suggest that general anesthetics are potent modulators of cerebrospinal fluid space and can overcome the influence of carbon dioxide on cerebrospinal fluid dynamics
I italicized the potentially important one.
Now, having said that, I've hyperventilated patients with increased intracranial pressure at the request of the surgeon. I have no idea what the blood flow to the brain was, but I can attest that both he and I saw the brain shrink. So there's that.
Other things to note:
The CO2 in this rat study was 80 mmHg. That's more than twice normal CO2 in a human, and a rat. If I had a patient with a CO2 that high, there would be other signs that things are not going well. Similarly a CO2 of 20 is pretty profound hyperventilation. Most humans would have to breathe into a paper bag to avoid passing out at that level.
Interesting, but I wonder how practically important it is.
