Early evidence: hypocapnia and cerebral hypoperfusion during tilt

This 1998 paper in Stroke is one of the earliest published demonstrations that orthostatic intolerance involves more than heart rate and blood pressure. Novak and colleagues tilted patients with orthostatic intolerance and monitored middle cerebral artery blood flow velocity alongside CO₂. What they found was that cerebral blood vessels constricted during tilt, blood flow to the brain fell, and CO₂ dropped in the process, creating a self-perpetuating cycle of hypoperfusion.

The researchers also demonstrated that the symptoms and the physiology could be partially reversed by having patients breathe CO₂, which restored cerebral vessel tone and improved blood flow. That is a direct, mechanism-based intervention demonstrating that CO₂ is not a passive bystander in this process. It is an active regulator of cerebral perfusion.

What makes this paper worth highlighting is not just what it found but when it was published. This methodology has been available and validated since the 1990s. The research showing that CO₂ and cerebral blood flow matter during orthostatic challenge is not new. It has simply not been adopted into standard clinical practice. Patients who are being evaluated with a 10-minute stand test and an arm cuff in 2025 are being evaluated with a protocol that ignores evidence that is more than 25 years old.

This is a standards failure, not a knowledge gap. The knowledge has been there for decades.