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

Area of Science:

• Neuroscience
• Medical Imaging
• Physiology

Background:

• The Monro-Kellie hypothesis posits that intracranial volume is fixed, requiring compensatory changes in one compartment for volume shifts in another.
• Hypercapnia, an increase in blood carbon dioxide, is known to increase cerebral blood volume.

Purpose of the Study:

• To investigate the applicability of the Monro-Kellie hypothesis to structural MRI.
• To observe compensatory intracranial volume changes during induced hypercapnia.

Main Methods:

• Seven healthy subjects underwent 3-Tesla, 3D T1-weighted MRI scans under normocapnic and hypercapnic conditions.
• Intracranial tissue and cerebrospinal fluid volumes were quantified.
• Statistical analysis was performed to assess volume changes.

Main Results:

• A significant increase in brain parenchymal volume (median 6.0 mL) was observed during hypercapnia.
• This increase was accompanied by a significant decrease in intracranial cerebrospinal fluid volume (median -10.0 mL).
• The observed volume changes support the Monro-Kellie hypothesis during hypercapnia.

Conclusions:

• The study demonstrates MRI's feasibility in observing Monro-Kellie compensatory mechanisms.
• Findings suggest that blood volume fluctuations can impact brain volumetric studies.
• The approach may offer a method to assess cerebrovascular reactivity.