Arterial CO2 pressure changes during hypercapnia are associated with changes in brain parenchymal volume

Abstract

The Monro-Kellie hypothesis (MKH) states that volume changes in any intracranial component (blood, brain tissue, cerebrospinal fluid) should be counterbalanced by a co-occurring opposite change to maintain intracranial pressure within the fixed volume of the cranium. In this feasibility study, we investigate the MKH application to structural magnetic resonance imaging (MRI) in observing compensating intracranial volume changes during hypercapnia, which causes an increase in cerebral blood volume. Seven healthy subjects aged from 24 to 64 years (median 32), 4 males and 3 females, underwent a 3-T three-dimensional T1-weighted MRI under normocapnia and under hypercapnia. Intracranial tissue volumes were computed. According to the MKH, the significant increase in measured brain parenchymal volume (median 6.0 mL; interquartile range 4.5, 8.5; p = 0.016) during hypercapnia co-occurred with a decrease in intracranial cerebrospinal fluid (median-10.0 mL; interquartile range-13.5,-6.5; p = 0.034). These results convey several implications: (i) blood volume changes either caused by disorders, anaesthesia, or medication can affect outcome of brain volumetric studies; (ii) besides probing tissue displacement, this approach may assess the brain cerebrovascular reactivity. Future studies should explore the use of alternative sequences, such as three-dimensional T2-weighted imaging, for improved quantification of hypercapnia-induced volume changes.

Is a Mask That Covers the Mouth and Nose Free from Undesirable Side Effects in Everyday Use and Free of Potential Hazards?

“…In the experiment, there was a significant increase in brain parenchymal volume measurable under increased arterial CO 2 levels ( p < 0.02), with a concomitant decrease in CSF spaces ( p < 0.04), entirely in accordance with the Monroe–Kelly doctrine, according to which the total volume within the skull always remains the same. The authors interpreted the increase in brain volume as an expression of an increase in blood volume due to a CO 2 increase-induced dilation of the cerebral vessels [ 169 ]. The consequences of such equally subthreshold carbon dioxide (CO 2 ) increases even under masks [ 13 , 15 , 18 , 19 , 22 , 23 , 25 ] are unclear for people with pathological changes inside the skull (aneurysms, tumors, etc.)…”

“…The consequences of such equally subthreshold carbon dioxide (CO 2 ) increases even under masks [ 13 , 15 , 18 , 19 , 22 , 23 , 25 ] are unclear for people with pathological changes inside the skull (aneurysms, tumors, etc.) with associated vascular changes [ 27 ] and brain volume shifts [ 169 ] especially due to longer exposure while wearing a mask, but could be of great relevance due to the blood gas-related volume shifts that take place.…”

Subtle vascular and astrocytic changes in the brain of coronavirus disease 2019 (COVID‐19) patients

“…Brain oxygenation and nutrition are both controlled by the basal tonus of the blood vessels, and this in turn is controlled by many mechanisms amongst which are intrinsic innervation from subcortical neurons, extrinsic innervation from the autonomous nervous system and local release of vasoactive substances [ 40 ]. Brain blood vessel diameters, blood flow and brain volume have been shown to vary rapidly in hypercapnic/hypocapnic conditions with increased capillary diameters and brain volumes in hypercapnia [ 41 , 42 ], proving that brain blood vessels, including capillaries, are capable and flexible enough to allow rapid morphological changes in response to stressor events. A myriad of complex intricate factors alters smooth muscle tonus and vessel diameter in COVID‐19 patients.…”

Cerebral Hemodynamics and Intracranial Compliance Impairment in Critically Ill COVID-19 Patients: A Pilot Study

“…We did not specifically evaluate which factors could influence alterations in brain hemodynamics or compliance. Changes in PaCO 2 , mean arterial pressure, pH, sodium, or temperature could also lead to altered brain hemodynamics [48][49][50]. Because of the relatively small sample size and different patterns of TCD and P2/P1 alterations, we could not assess the association of systemic abnormalities with some specific patterns, such as low mCBFV, high eICP, or P2/P1.…”