Carbon Dioxide: One of the Most Important Metrics for Brain Blood Flow

If you've had a tilt table test and walked away with more questions than answers, there's a good chance the test wasn't measuring enough. But beyond the test itself, there's something most clinics aren't tracking at all: the gas your body uses to regulate blood flow to your brain.

It sounds technical. It is. But understanding it might be the thing that finally explains why you feel the way you do when you stand up — and what we can actually do about it.

What Does CO2 Have to Do with Brain Blood Flow?

Most people think of CO2 as something your body just gets rid of when you exhale. But it's actually doing an important job before it leaves. It's one of the key signals your body uses to tell blood vessels when to open up and deliver more blood.

When an area of your body is working hard, it produces more CO2. The same thing happens in your brain. When your neurons need more oxygen, CO2 is the messenger that tells nearby blood vessels to open up and deliver it — which directly controls how much blood is getting to your brain.

If we're trying to solve for issues related to cerebral blood flow, we have to measure one of the key components involved in regulating it.

How We Measure CO2 on a Tilt Table Test

We use a small nasal cannula (a thin tube that sits just under your nose) connected to a device called a capnograph. As you breathe, it measures the concentration of CO2 at the very end of each exhale. That final breath is the most accurate reflection of what's circulating in your bloodstream — called End Tidal CO2, or ETCO2.

You could get a CO2 reading from a blood draw, but that's a single snapshot frozen in time. What we need is a continuous feed — something that shows us how CO2 shifts as you move from lying flat to standing upright. That's what this method allows us to do.

Why Low CO2 Requires a Different Treatment Depending on the Cause

There are two very different ways CO2 can become a problem, and they look similar on the surface but require completely different approaches.

The first is when low CO2 is the primary problem. Some patients have CO2 levels that are already low just lying flat, before we even tilt them. When we raise these patients up, the ETCO2 levels drop further. For these patients it can come down to mechanics: their posture may be so hunched that they can't take a full breath, only breathing in short, shallow cycles, which can cause someone to chronically blow off too much CO2 without even realizing it.

The second scenario is the reverse. The brain starts losing blood flow first, and the body alters breathing in response, trying to compensate for what's happening upstream. In this case, the CO2 change is a consequence, not a cause.

How Does the Brain Prioritize Blood Flow When CO2 and Pressure Conflict?

Your brain runs two reflexes in the same blood vessel simultaneously. One responds to blood pressure — if pressure drops, vessels dilate to let more blood in. The other responds to CO2 levels — if CO2 drops, vessels constrict slightly. Normally these work in harmony. But under stress, they can pull in opposite directions.

If your cerebral blood flow is dropping and your CO2 is also dropping at the same time, your brain faces a conflict: dilate to keep blood flowing, or constrict in response to low CO2? It can't do both.

So your brain makes a choice. It prioritizes keeping you upright and conscious over maintaining perfect CO2 balance. To do that, it actually dials down its sensitivity to CO2 — essentially ignoring that signal so the blood vessels can stay open.

Think of it like being at a loud concert. At first the noise is overwhelming, but your brain adapts and dampens your hearing sensitivity. Then you step outside and you're yelling at your friends because your baseline has shifted. The same thing happens here. Your brain recalibrates its sensitivity to CO2 in order to survive the moment — but that recalibration comes with its own set of symptoms.

What Low CO2 Actually Feels Like

Low CO2 is often misread as anxiety or panic.

Low CO2 typically shows up as air hunger: that sensation of not being able to get a deep enough breath even though nothing is physically blocking your airway. You might put on a pulse oximeter and see normal oxygen levels, which makes the feeling even more confusing. You feel like you're suffocating, but the numbers say you're fine.

It also tends to produce a wired, tremulous feeling — a low-level internal shakiness, elevated anxiety, or that on-edge sensation you might recognize as the early edge of a panic attack.

This is why so many patients get told they're just anxious. The symptoms genuinely overlap.

Patients may be guided toward breathwork when they feel symptomatic. But if low CO2 is part of what's driving your symptoms, certain breathing exercises can actually make things worse. Breathing too deeply blows off CO2 just as effectively as breathing too fast. If you notice that breathwork leaves you feeling more lightheaded or tingly rather than better, that's a signal — and a reason why we don't provide universal treatment advice.

How Capnography Is Used in POTS Rehabilitation

After using capnography as a diagnostic tool, we also use it during rehabilitation.

When someone with dysautonomia tries to exercise, there is a very real fear of crashing. By monitoring CO2 in real time, we can keep someone working within a range where their brain is actually getting what it needs. We can see when things start to slide before symptoms appear, and adjust prior to a crash.

Capnography is most powerful when it's combined with everything else we're measuring. CO2 levels alone won't tell you the full story. But CO2 levels alongside Transcranial Doppler, heart rate, blood pressure, and oxygen levels — that's when the picture comes into focus.

Want to understand what's driving your symptoms? Learn more about how our diagnostic process works or schedule a free consultation.

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