Cerebral autoregulation issues in POTS even when CO₂ is normal

This paper is included here specifically because the emphasis on CO₂ monitoring in the other entries on this list should not create a false binary. CO₂ dysregulation through hyperventilation is one mechanism by which cerebral blood flow can fall during orthostatic stress. It is not the only mechanism.

Ocon and colleagues studied POTS patients who did not hyperventilate and whose CO₂ levels remained normal during tilt. Despite this, these patients showed impaired cerebral blood flow velocity and abnormal cerebral autoregulation during orthostatic challenge. The problem in these patients was not that they were breathing off too much CO₂. The problem was that the dynamic regulatory system responsible for keeping cerebral blood flow stable across changes in blood pressure was not functioning correctly.

Cerebral autoregulation is the brain's ability to maintain relatively constant blood flow despite changes in perfusion pressure. In healthy individuals, when blood pressure drops slightly during standing, cerebral blood vessels dilate to maintain flow. In POTS patients with impaired autoregulation, this compensatory dilation is sluggish or insufficient, and cerebral blood flow falls even when the chemical environment is otherwise normal.

The practical takeaway is that a normal CO₂ reading during an orthostatic evaluation does not rule out a cerebral perfusion problem. Both CO₂ and cerebral blood flow velocity need to be measured, because they reflect different aspects of cerebrovascular regulation. A clinician who sees normal CO₂ and concludes that nothing is wrong at the brain level is missing the autoregulatory dimension entirely.