Hypocapnic Cerebral Hypoperfusion (HYCH): How Low CO2 Cuts Blood Flow to the Brain

If you feel foggy, dizzy and short of air every time you stand, and your tilt test showed a heart rate rise under 30, you may have been told your results were normal. They may well have been normal for the 2 numbers a standard tilt test watches. In hypocapnic cerebral hypoperfusion, the problem sits in 2 numbers it doesn't watch: the blood flow reaching your brain and the carbon dioxide in your breath.

Broken down, hypo means low, capnic refers to carbon dioxide, cerebral means the brain and hypoperfusion means too little blood flow, so together it describes low CO2 leaving your brain short of blood. Peter Novak's group at Brigham and Women's Hospital in Boston described it as a marker of orthostatic intolerance in 2018 (Novak, 2018) and later compared it head to head with POTS (Novak, 2024).

What Does Hypocapnic Mean?

Hypocapnic means having too little carbon dioxide in the blood, a state called hypocapnia. CO2 is more than a waste gas. Your cells make it as they burn fuel, and the amount in your blood is one of the main signals your brain's blood vessels use to decide how wide to open: when part of the brain works harder, it makes more CO2, and the vessels there widen to bring in fresh blood.

In the clinic, CO2 is measured breath by breath with a capnograph, a small tube under the nose that reads the CO2 at the end of each exhaled breath, called end-tidal CO2. In Novak's 2024 study, healthy people lying down averaged about 38 mmHg, and HYCH is defined by an upright reading under 30 mmHg (Novak, 2024). A blood test can measure CO2 too, but it captures a single moment, and what matters here is what CO2 does as your position changes.

Does Low CO2 Cause Vasoconstriction in the Brain?

Yes: blood flow in the brain is highly sensitive to the CO2 in your arteries: more CO2 widens the brain's vessels to wash it out, and less CO2 makes them constrict, which reduces blood flow (Ainslie, 2009). That's why over-breathing can leave you lightheaded within a minute. In a 2026 study, 30 experienced breathwork practitioners lost about 45% of their grey matter blood flow during a session of fast breathing (Schmitz, 2026). There's more on how carbon dioxide controls blood flow to the brain.

The brain's arteries answer to 2 reflexes at once. Autoregulation responds to pressure, relaxing the vessels when pressure falls so more blood gets in, while CO2 reactivity responds to carbon dioxide. Standing, the 2 can pull in opposite directions, with falling pressure asking the vessels to open and falling CO2 asking them to tighten, and a healthy brain settles that conflict in favor of keeping you upright. In HYCH, blood pressure holds steady and blood flow to the brain falls anyway, which Novak's 2018 study traced to the vessel-tightening effect of low CO2 (Novak, 2018).

HYCH vs POTS: The Same Drop in Blood Flow, a Different Heart Rate

In 2024, Novak's team went through the records of 3,327 people tested for orthostatic intolerance and compared 127 with HYCH, 125 with POTS and 42 healthy controls (Novak, 2024). The 2 patient groups matched on almost everything: how long they'd been sick, the share of young women, their other diagnoses, their symptoms, their norepinephrine levels lying and standing, their inflammatory markers and how often they had small fiber neuropathy. Both lost close to a third of their brain blood flow speed upright, and both breathed out low CO2.

Brain blood flow CO2 Heart rate

Upright, brain blood flow speed fell about 29% in HYCH and 32% in POTS, against 9% in healthy controls.

Group averages from Novak, 2024 (127 HYCH, 125 POTS, 42 controls).

Where they differed was heart rate, and the authors concluded that HYCH and POTS represent a spectrum of the same disorder, with reduced blood flow to the brain as the key feature that unites them, and that the faster heart in POTS is driven by the brain, which they described as overcompensating for the challenge of standing (Novak, 2024). The 30-beat cutoff sorts people by how hard the heart responds, while the problem underneath, a brain that loses blood flow upright, is the same in both. That's why a rise under 30 doesn't settle the question when you feel awful upright, the same point made on the page about why your heart races when you stand.

The other diagnoses that came with each group were close as well, and none of the differences was statistically significant:

Diagnosis	HYCH	POTS
Central sensitization	88%	88%
Anxiety	68%	76%
Depression	63%	67%
Headaches	63%	64%
Irritable bowel syndrome	47%	27%
Hypermobile Ehlers-Danlos	19%	15%
Fibromyalgia	16%	15%
ME/CFS	14%	15%
Mast cell activation	13%	10%

Novak, 2024, Table 1 and survey results.

Hypocapnic Cerebral Hypoperfusion Symptoms

The symptoms come in 2 layers. The first comes from the brain getting too little blood: lightheadedness, dizziness, nearly fainting, trouble thinking and fatigue, all worse standing or sitting up for long stretches and better lying down. The low blood flow can also set off a stress response, with palpitations, anxiety, nausea, chest pain and headaches (Novak, 2024). The second layer comes from low CO2 itself:

• Air hunger, the feeling that you can't get a full breath, even though a pulse oximeter reads normal.
• Tingling in the hands or face.
• Feeling shaky or wired, with a tremor inside or one you can see.
• A rise in anxiety that can feel like the start of a panic attack.

That overlap is why so many people with low CO2 hear that they're just anxious. Low CO2 produces many of the same sensations as anxiety, and anxiety can speed breathing, so the 2 can feed each other. Novak's study only counted people whose low blood flow and low CO2 lasted 4 minutes or longer on the tilt, to rule out the short-lived changes anxiety can cause when the table first tips up (Novak, 2024). If you've been told it's all nerves, telling POTS apart from anxiety starts with measuring.

Why CO2 Drops When You Stand

Even healthy people breathe out a little less CO2 standing. In one study it fell from about 40 to 36 mmHg after 150 seconds upright, mostly because each breath got a little bigger, with smaller contributions from less blood being pumped through the lungs (Gisolf, 2004). In HYCH and POTS the drop is far larger, and it can start in 2 places.

Sometimes breathing starts it: fast, shallow breathing blows off CO2, and so do breaths that are too big, which is easy to fall into when you feel bad and try to calm yourself with deep breaths. Posture can lock in a shallow pattern too, as when a hunched upper body limits how far the ribs can move. If deep breathing leaves you more lightheaded or tingly, your breaths are probably too big, and slowing them down helps.

Sometimes blood flow starts it: in a 2023 study of young people with POTS, those who became hypocapnic upright had a larger fall in brain blood flow on standing, and the fall came before the CO2 dropped; the authors proposed that the fall itself set off the over-breathing (Stewart, 2023). The same research group reports that about half of people with POTS are hypocapnic when upright.

And sometimes breathing isn't the whole story: in Novak's 2024 study, breathing rate upright was about the same in HYCH, POTS and healthy controls, around 16 breaths a minute, and every patient in the subset who had invasive exercise testing showed preload failure, meaning too little blood returning to the heart (Novak, 2024). When less blood returns to the heart, less flows through the lungs, and some air sacs get air without much blood passing them, so the breath you exhale can carry less CO2 than your blood does. A low reading on a capnograph isn't always proof of over-breathing. Researchers who measured CO2 in the arteries during tilt found the drop wasn't explained by breathing alone and pointed to a mismatch between air and blood in the lungs or a shift in the body's CO2 stores (Serrador, 2006), and the 2024 study named venous pooling and the low CO2 that comes with it as a likely physical basis for both HYCH and POTS.

How Hypocapnic Cerebral Hypoperfusion Is Diagnosed

A standard tilt test can't see HYCH, because it only watches heart rate and arm blood pressure, and in HYCH both stay in range. Seeing it takes 2 more measurements during the tilt:

• Blood flow to the brain: transcranial Doppler tracks blood flow speed in the brain's main arteries in real time. In Novak's lab, a normal drop at 3 minutes upright is up to about 11% (Novak, 2024).
• CO2: capnography reads end-tidal CO2 with every breath. HYCH is defined by a reading under 30 mmHg upright, together with the drop in blood flow, no POTS-level rise in heart rate and no fall in blood pressure (Novak, 2024).

Then the order matters: did CO2 fall first, or blood flow? Breathing rate and depth, oxygen, blood pressure and heart rate get read against both, which tells our team whether low CO2 is the cause or a response, and how much of the drop in blood flow it explains.

CO2 alone doesn't catch everyone, because some people lose blood flow to the brain upright with normal CO2 and a normal heart rate, a pattern Novak called orthostatic cerebral hypoperfusion syndrome (OCHOS), where flow speed fell about 24% on tilt (Novak, 2016). That's why capnography is read alongside Doppler. It's also why buying a capnograph to test yourself at home tells you less than it seems to: a CO2 reading without blood flow, blood pressure and breathing measured with it can't show whether low CO2 is the cause or the response.

Hypocapnic Cerebral Hypoperfusion Treatment

Treatment depends on where the loop starts, which is why the testing comes first.

• When breathing starts it, the work begins with how you breathe: the posture and rib mechanics behind a shallow pattern, and slowing breaths that have gotten too big, so CO2 holds steady without you having to think about it.
• When blood flow starts it, breathing work alone won't hold. Your brain has to keep its own blood flow up when you're upright, so upright tolerance gets built a little at a time, starting lying down and moving up as the measurements allow.
• During that work, capnography becomes a gauge. Exercise can be kept in the CO2 range that helps the brain's vessels open, which trains them to respond to CO2 again and shows when to stop before symptoms flare.

Medication has a place: in Novak's study, both groups used heart rate and blood pressure medications at similar rates, and the authors note that people with POTS generally benefit from medication that slows the heart (Novak, 2024). Medication can make standing easier while this work happens, and it doesn't retrain the reflexes that set CO2 and blood flow to the brain in the first place. Changes to it belong with your prescriber. For the wider picture of why blood flow to the brain falls, see cerebral hypoperfusion and its causes.

Hypocapnic Cerebral Hypoperfusion: FAQ

Keep Reading

• Cerebral hypoperfusion: why your brain isn't getting enough blood
• Carbon dioxide and brain blood flow
• OCHOS: when brain blood flow drops with normal CO2
• Transcranial Doppler for POTS: what tilt tables miss
• How we evaluate and treat POTS

Dr. Keiser is a board-certified chiropractic neurologist (DC, DACNB, FABBIR), not a medical doctor (MD/DO). This content is for educational purposes and does not constitute medical advice. It is not a substitute for professional medical evaluation, diagnosis, or treatment. Always consult a qualified healthcare provider about your specific situation.

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