Hypovolemic POTS: Is Low Blood Volume the Whole Story?

POTS affects an estimated 0.1% to 1% of the US population, and about 90% of cases are female (Chung, 2026). Hypovolemic POTS is one of the 3 types of POTS usually described, along with neuropathic and hyperadrenergic, and many people have features of more than one.

A 2026 hypothesis-driven review argues that in many patients the fast heart rate on standing is a compensation, with an intact baroreflex driving the heart faster "to defend cardiac output and cerebral perfusion when effective stroke volume or venous return is reduced" (Chopra, 2026). That matches the clinic's view: POTS is the rescue for brain blood flow succeeding, and orthostatic hypotension is the same rescue failing.

Hypovolemic vs Hyperadrenergic POTS: What's the Physiological Difference?

The difference is where the strain starts. In hypovolemic POTS, the proposed mechanism is that less blood comes back to the heart on standing because there's less volume to work with, so each beat moves less blood and the heart rate climbs to cover the gap. In hyperadrenergic POTS, the sympathetic drive itself runs high, and standing norepinephrine is often above 600 pg/mL (Raj, 2013).

The 2 often overlap. Raj's review notes that high norepinephrine in POTS is most commonly secondary to a partial dysautonomia or low blood volume (Raj, 2013). Hyperadrenergic POTS has its own page.

Does POTS Cause Low Blood Volume?

Many, but not all, people with POTS run low on blood volume (Raj, 2013). In a study of 15 POTS patients and 14 healthy controls, the POTS group had a plasma volume deficit of 334 mL against 10 mL in controls. Their renin didn't rise to make up for it, and aldosterone was paradoxically low, 190 against 380 pmol/L (Raj, 2005).

The deficit shows up in some patients more than others. Among 23 POTS patients grouped by blood flow, only the 10 in the low-flow group had lower blood volume than controls, 2.14 against 2.76 L/m², and the other subgroups didn't (Stewart, 2006). In 21 women with POTS measured by carbon monoxide rebreathing, blood volume averaged 64.5 mL/kg against 76.8 in healthy controls, about 14% below expected (Kulapatana, 2025).

All 3 studies are small, and they measured volume at one point in time, so they can't show whether low volume causes POTS or comes along with it. None of them reports what share of everyone with POTS runs low.

1. Low volume: a measured deficit, such as about 334 mL of plasma (Raj, 2005), means less blood returns up the veins when you stand, so the heart fills less.
2. Poor management: in the clinic's view, volume is often near normal, and the shift of blood into the belly and legs is what goes uncorrected, because the brain's signals to the vessels, heart and kidneys don't keep up.
3. In the clinic's view, the heart rate climbs in both to keep blood reaching the brain.

What Are the Symptoms of Hypovolemic POTS?

Common POTS symptoms include lightheadedness, palpitations, fatigue, brain fog, exercise intolerance, nausea, bloating, constipation, diarrhea and sleep disturbance (Chung, 2026). Low blood volume is the usual explanation offered for these symptoms.

In the clinic's view, symptoms alone can't tell you whether low volume is driving yours, and a subtype label based on symptoms stays a guess until something is measured.

How Is Low Blood Volume in POTS Tested?

What Tests Measure Blood Volume in POTS?

Blood volume is measured with tracer methods. Raj's group used albumin labeled with iodine-131 (Raj, 2013), Stewart's group used dye dilution (Stewart, 2006), and Kulapatana's group used carbon monoxide rebreathing with segmental impedance (Kulapatana, 2025). None of these is a routine blood draw, and none of these 3 studies sets a cutoff for diagnosing hypovolemic POTS.

What Is the Renin-Aldosterone Paradox in POTS?

Low volume should raise renin and aldosterone, the hormones that lead the kidneys to hold onto sodium and water, yet in 15 POTS patients they didn't rise (Raj, 2005). Our team reads that as a sign that "maybe this is not an absolute blood volume problem," and the full argument is in why salt and water aren't enough.

What Does the Clinic Measure Instead of Blood Volume?

Our team measures what the volume is supposed to accomplish, which is blood reaching the brain when you stand. In the clinic's view, blood pressure in the body and blood pressure in the brain are "two different systems," and the cerebral autoregulation reflexes that protect brain blood flow are "reflexes that you can measure." On a tilt table test, our chiropractic neurologists track brain blood flow with transcranial Doppler alongside heart rate, blood pressure and breath CO2. The 3D tour shows that testing step by step.

Is POTS a Blood Volume Issue? Our Team's View

In our team's view, low volume is rarely the whole problem in POTS, because the body defends its volume closely. Volume receptors in the heart's right atrium detect how much blood is arriving, and our team describes them keeping it "regulated within a fairly narrow range." Against that precision, the clinic says a drop of 10%, 20% or 30% "may not be the main factor." The measured deficits above, about 13% to 14%, sit at the low end of that range, so a deficit can be real and still not explain the whole picture.

Standing moves blood even in healthy people. About 500 mL shifts from the chest to the lower abdomen, buttocks and legs (Raj, 2013), and our team puts the normal range at 300 to 800 mL. Everyone's regulator has to handle that shift each time they stand. In POTS, the clinic sees "poor blood volume management, where the brain cannot efficiently control the vascular, cardiac, and kidney systems."

That's why the clinic's approach centers on the regulator, the brain's control of the blood vessels, heart and kidneys. Having too little blood and having enough blood in the wrong place are compared in absolute vs relative blood volume, and where the blood collects is covered in blood pooling in POTS.

How Is Hypovolemic POTS Treated?

Standard care aims to improve how much blood fills the heart, with more fluid and sodium and lower-body compression garments (Chung, 2026), and sometimes medication. Each helps some people, and each has a ceiling.

Salt and Fluids for Low Blood Volume: What the Studies Show

Salt has some evidence of short-term benefit. A 2022 review found salt supplementation improved symptoms, plasma volume and orthostatic responses in POTS over about 3 months (Williams, 2022). In a crossover study of 14 POTS patients, 6 days of a high-salt diet raised total blood volume and plasma volume and lowered upright heart rate compared with a low-salt diet (Garland, 2021). Both are small and short, so they don't show how long a benefit lasts.

The evidence for fluids is thinner. A 2022 review called the published evidence for hydration "relatively weak" and noted that no randomized clinical trials had been done. Because some patients can't drink enough, it described a role for IV volume loading in severe cases (Snapper, 2022). Our team covers IV fluids in IV saline for dysautonomia, and amounts in how much salt and water helps.

Hypovolemic POTS Medications: What They Do and Where They Stop

Desmopressin lowered standing heart rate in a trial of 30 POTS patients. One 0.2 mg dose brought it to 101.9 beats per minute against 109.2 on placebo and eased symptoms at 2 hours, with no change in standing blood pressure (Coffin, 2012). That trial tested a single dose, and the authors wrote that its safety "would need to be examined before it can be recommended for routine treatment."

Fludrocortisone is often used when low volume is known or strongly suspected, because it helps the body hold sodium, which should expand plasma volume. Its side effects can include low potassium, worse headaches, acne and swelling from fluid retention (Raj, 2013). A 2020 systematic review of POTS medications found "some favorable effect" in individual studies and "a paucity of high-quality data" (Hasan, 2020), and a Cochrane review called the evidence in orthostatic hypotension "very uncertain" (Veazie, 2021).

Across POTS treatments, a 2025 review found 21 randomized trials with 750 patients between 2000 and 2023 and concluded that "many small trials" have tested different treatments (Kwok, 2025). In the clinic's view, these medications help by adding volume, and their ceiling is that they leave the brain's management of that volume unchanged. Medication decisions belong with your prescribing doctor.

Why Does Salt Loading Stop Working?

Even on the high-salt diet, upright heart rate in POTS stayed well above healthy controls, a median of 117 against 85 beats per minute (Garland, 2021).

Our team's explanation is that salt loading is "a giant sledgehammer" aimed at volume when the body already regulates its salt closely: "your body's not dumb. It knows how much salt it wants to have with like a high level of precision." In our team's experience, more salt and more fluid sometimes change nothing.

If plain water hasn't raised your blood volume, why drinking more water stops working covers where it goes. For the full plan, see POTS treatment.

Where Do You Start If Salt and Fluids Haven't Helped?

Start by measuring whether blood reaches your brain when you stand, since that's what the extra volume was meant to fix.

Hypovolemic POTS 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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