Low CO2: Symptoms, Causes and What It Does to Brain Blood Flow

Does CO2 Cause Vasodilation or Vasoconstriction in the Brain?

CO2 is a vasodilator in the brain. When CO2 rises, brain blood flow increases to wash it out of brain tissue, and when CO2 falls, the brain's arteries constrict and flow drops (Ainslie, 2009).

Our team explains it through metabolism. Working cells give off CO2 as they make energy, and the pH change it causes signals nearby vessels to open and pull in fresh blood. The brain runs on the same signal, so a busier region that makes more CO2 calls for more blood to flush it out and bring in oxygen.

How Much Does CO2 Change Cerebral Blood Flow?

Near normal levels, flow velocity in the middle cerebral artery changes about 3.5% for each mmHg of arterial CO2, a figure Immink and colleagues cite as background for healthy adults (Immink, 2006). In a rebreathing study of healthy adults, the response traced an S-shaped curve. The curve was centered near 36 mmHg, close to normal resting CO2, and flattened toward a low-CO2 minimum. Above a higher threshold, roughly 47 to 51 mmHg depending on oxygen level, velocity rose about 5.5% per mmHg (Battisti-Charbonney, 2011).

Both numbers describe velocity from transcranial Doppler, which measures how fast blood moves through the artery and stands in for flow. As an illustration only, a 5 mmHg fall from 40 mmHg on the 3.5% slope would come to roughly a 17% drop in velocity, and the real response flattens as CO2 gets lower.

1. Breaths get faster and deeper, so exhaled CO2 falls from 40 to 35 mmHg.
2. The brain's arteries narrow, and blood moves through them more slowly.
3. In this illustration, flow velocity falls by about 17% on the near-normal slope, and the curve shows the response flattening at lower CO2.

Why Does Low CO2 Cause Vasoconstriction?

The narrowing protects the brain's own CO2 level. With less blood moving through, less CO2 washes out of brain tissue, which slows the fall in tissue CO2 (Ainslie, 2009). The price is less blood reaching the brain.

What Happens When CO2 and Blood Pressure Pull in Different Directions?

Our team describes 2 reflexes acting on the same brain arteries at once. Autoregulation responds to pressure, relaxing the vessels when blood pressure falls so you don't pass out, and CO2 reactivity responds to CO2. When you stand, those 2 can collide, because falling pressure in the head calls for wider arteries while falling CO2 calls for narrower ones.

In the clinic's model, the brain settles that conflict with a hierarchy. It protects pressure-driven flow first and turns down its sensitivity to CO2, the way your ears turn down their sensitivity at a loud concert and leave normal voices sounding faint afterward. One small study measured the slope between arterial CO2 and flow velocity in 6 healthy adults: 1.6 cm/s per mmHg lying down and 1.3 during head-up tilt (Jordan, 2000). Those were healthy adults, so the study can't show that this happens in POTS. On a tilt, our team reads the CO2 and blood flow traces together to work out how much of a blood flow drop comes from CO2 and how much from a reflex that's falling short.

What Are the Symptoms of Low CO2 Levels in Blood?

Lightheadedness has the clearest data. In 22 people with orthostatic intolerance on a 10-minute tilt, the intensity of lightheadedness was proportional to the fall in brain blood flow velocity and end-tidal CO2 (Coelho, 2024).

Our team sees a wider pattern that patients describe in similar words. A common one is air hunger, the feeling that you can't get a full breath even though a pulse oximeter reads normal. Some people also feel wired or shaky inside, sometimes tremulous, and notice anxiety climbing as if a panic attack were starting. In our team's experience, fast breathing can also bring tingling in the hands or face.

Several of these overlap with panic. When people with panic disorder and healthy controls breathed in CO2 or hyperventilated room air, the symptoms that set the groups apart were fear, confusion, dizziness, twitching or trembling, and shortness of breath (Goetz, 2001). That study shows how breathing challenges bring on those sensations in panic disorder, and it doesn't test whether low CO2 produces them. Some breathing-related symptoms have other sources, and hyperventilation syndrome includes symptoms tied to low CO2 along with others that aren't (Boulding, 2016).

What Causes Low CO2?

Can Breathing Too Fast or Too Deep Lower CO2?

Breathing off CO2 faster than your cells make it lowers the level in your blood. Our team describes hyperventilation in 2 forms: tachypnea, which is fast, shallow breathing, and hyperpnea, which is breathing too deeply. Researchers have measured over-breathing on a tilt in POTS, calling it postural hyperventilation (Stewart, 2018) and hypocapnic hyperpnea, a rise in breathing that drives end-tidal CO2 down (Del Pozzi, 2014). In our team's experience, the deep form is the one people overlook.

Is Low CO2 the Cause or the Compensation?

Our team sorts low CO2 into 2 patterns. In the first, breathing changes first, CO2 falls and the brain's arteries narrow. One patient our team worked with showed low CO2 on a tilt. Her posture was hunched and her breaths were shallow, so in that one case the breathing mechanics came first. In the second pattern, brain blood flow drops first and breathing changes in response. This sorting is the clinic's model.

Published studies include findings that fit each direction in different groups. In 11 POTS patients, brain blood flow velocity fell about 17 seconds before breathing picked up and end-tidal CO2 fell (Del Pozzi, 2014). In other groups, over-breathing drove the problem. Hyperventilation was the main cause of brain vasoconstriction on standing in orthostatic intolerance (Novak, 1998), and one variant of POTS followed from postural hyperventilation (Stewart, 2018). A 2024 study of 125 POTS and 127 HYCH patients found reduced orthostatic flow velocity and end-tidal CO2 in both groups (Novak, 2024). The pattern without a fast heart rate has its own name, hypocapnic cerebral hypoperfusion (HYCH).

Can Brain Blood Flow Fall Without Low CO2?

Brain blood flow can fall on standing while CO2 stays normal. In young POTS patients with normal CO2, mean flow velocity fell 19.5% during tilt against 10.3% in controls, and autoregulation was less effective in the POTS group (Ocon, 2009). In healthy adults, the CO2 drop on standing was too small to explain the whole fall in velocity (Immink, 2006). In 535 women with ME/CFS, CO2 reactivity matched healthy controls, and end-tidal CO2 explained little of the change in flow (van Campen, 2023).

Low CO2 is one measurable route to low brain blood flow. The other routes are covered in causes of cerebral hypoperfusion.

Can Anxiety Cause Low CO2 Levels in Blood?

Hyperventilation lowers CO2 (Boulding, 2016; Novak, 1998), and in our team's experience anxiety often comes with fast or deep breathing, which is how anxiety can lower CO2. In 41 patients with panic disorder, breathing training guided by a capnometer moved patients from low to normal CO2, and panic severity improved about as much with that training as with cognitive training (Meuret, 2010). Both approaches improved panic, so the study can't say the CO2 change was the reason.

In our team's experience, people with low CO2 on a tilt are often told they're just anxious. Our team finds the sensations of low CO2 and anxiety overlap enough that they're hard to pull apart without measuring breath CO2 and brain blood flow. For the wider comparison, see POTS vs anxiety.

What Does Low CO2 on a Blood Test Mean?

On a routine blood test, the CO2 result mostly reflects bicarbonate, because most of the body's carbon dioxide is carried in that form. A low value can point to metabolic acidosis, diabetic ketoacidosis, Addison disease, shock or respiratory alkalosis from a breathing problem such as hyperventilation. A result outside the normal range doesn't always mean you have a condition that needs treatment (MedlinePlus). If your low CO2 came from a blood panel such as a BMP, take it to the doctor who ordered it. End-tidal CO2, the measure our team uses on a tilt, is the CO2 at the end of each exhaled breath, read live on a capnograph.

How Is Low CO2 Measured on a Tilt Table Test?

A nasal cannula carries each exhaled breath to a capnograph, which reads end-tidal CO2 while transcranial Doppler tracks flow velocity in the middle cerebral artery. Heart rate and blood pressure run alongside them across a 10-minute tilt table test (Novak, 2018; Coelho, 2024). The clinic prefers this to a blood draw, because a single blood measurement gives one value at one moment and a continuous trace shows what CO2 does as your position changes.

End-tidal CO2 tracks arterial CO2 closely while you stay in one position. On standing, it overstated the fall in 7 healthy adults, dropping 4.8 mmHg while arterial CO2 dropped 3.0 (Immink, 2006). That's one reason our team reads it beside brain blood flow, breathing rate and depth, oxygen, blood pressure and heart rate.

End-tidal CO2 also won't catch every case. Some people have a fast heart rate and falling brain blood flow with no change in CO2 (Ocon, 2009), so the clinic never relies on capnography alone.

How Do You Treat Low Carbon Dioxide in Blood?

In lab tests, raising CO2 reversed some effects of over-breathing. In a study of 58 POTS patients, adding CO2 through a rebreathing setup brought heart rate, blood pressure, cardiac output and vascular resistance in the over-breathing group back to control levels (Stewart, 2018). In orthostatic intolerance, CO2 rebreathing raised low CO2 back up (Novak, 1998). Both were short lab tests, so they show what CO2 does and don't test a treatment. Over a course of treatment, capnometer-guided breathing training corrected low CO2 in panic disorder (Meuret, 2010). Which approach fits in dysautonomia depends on why CO2 is low in the first place, and our team covers that in the hypocapnic cerebral hypoperfusion (HYCH) guide.

Our team uses capnography in rehab as well. Watching end-tidal CO2 beside brain blood flow lets our chiropractic neurologists keep exercise inside a CO2 range that favors blood flow to the brain, and call a session when the readings start to slip. That's the clinic's practice, and this page cites no outcome study for it.

Why Can Deep Breathing Make Low CO2 Worse?

When people feel off, many reach for deep breaths to calm down. Big breaths blow off CO2 too. In POTS, spontaneous over-breathing on standing has been measured lowering end-tidal CO2 (Del Pozzi, 2014; Stewart, 2018), and in our team's experience a deliberate deep breath often makes the lightheadedness worse. If deep breathing leaves you more lightheaded or tingly, slow your breathing down instead of making it bigger. In our team's experience, general breathwork advice backfires for some people with low CO2 for the same reason, which is why our team measures CO2 before building any breathing work into a plan.

Low CO2 and Brain Blood Flow FAQ

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. Medication decisions should be made with your prescribing physician.

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