Defining HYCH: orthostatic intolerance without tachycardia

This paper by Dr. Peter Novak at Brigham and Women's Hospital defined a measurable condition called Hypocapnic Cerebral Hypoperfusion, or HYCH. The patients in this study had classic symptoms of orthostatic intolerance. They felt terrible standing up. But their heart rate did not rise enough to meet POTS criteria, and their blood pressure did not fall enough to meet orthostatic hypotension criteria. On standard testing, they would pass.

What Novak found when he monitored end-tidal CO₂ and middle cerebral artery blood flow velocity during tilt was that these patients showed dramatic drops in cerebral blood flow velocity, averaging around 22%, driven by CO₂ falling as they stood. The brain was being progressively starved of blood flow while the arm-cuff numbers looked fine. HYCH patients had CO₂ levels averaging 26 mmHg during tilt, compared to about 37 mmHg in healthy controls. That is a meaningful difference with real consequences for how the brain regulates itself.

Critically, HYCH patients and POTS patients showed nearly identical cerebral blood flow drops during tilt. The distinction was that POTS patients compensated with tachycardia and HYCH patients did not. Both groups were experiencing the same underlying problem at the level of cerebral perfusion. The tachycardia in POTS is a compensation. Its absence in HYCH does not mean the problem is less severe. It means a different compensatory pathway was used, or none was available.

The practical implication is one that every patient with unexplained orthostatic symptoms should understand. A tilt table test that does not elevate heart rate or drop blood pressure to threshold levels is not a negative test. It is an incomplete test. You cannot pass or fail a tilt if the measurements being taken are not the ones that matter for your physiology.