A significant number of people with postural orthostatic tachycardia syndrome (POTS) have measurably less blood circulating through their bodies than healthy individuals, and this shortfall is a central driver of their symptoms. Research has found that female patients with POTS carry roughly 14% less total blood volume than predicted for their body size, with plasma volume deficits even steeper at around 16%.1PubMed Central. Blood volume deficit in postural orthostatic tachycardia syndrome assessed by semiautomated carbon monoxide rebreathing Understanding this low-volume component of POTS changes how the condition is managed, because many of the most effective strategies aim squarely at restoring what’s missing.
How Much Blood Volume Is Actually Missing
The blood volume deficit in POTS patients is not subtle. One study comparing female POTS patients to healthy women found that patients had plasma volumes averaging about 39.5 mL per kilogram of body weight, compared to 48.2 mL/kg in controls, and total blood volumes of roughly 64.5 mL/kg versus 76.8 mL/kg.1PubMed Central. Blood volume deficit in postural orthostatic tachycardia syndrome assessed by semiautomated carbon monoxide rebreathing That gap translates to patients running on noticeably less circulating fluid than their bodies were designed for. Red blood cell volume was also reduced, though the plasma component took the bigger hit.
Earlier research from a group studying the renin-aldosterone system found a similar picture. POTS patients had a plasma volume deficit of about 334 mL and a red blood cell volume deficit of about 356 mL compared to controls, adding up to a total blood volume shortfall of roughly 689 mL.2PubMed. Renin-aldosterone paradox and perturbed blood volume regulation underlying postural tachycardia syndrome To put that in perspective, that’s more than a pint of blood missing from circulation. Some of the earliest investigations described patients whose total blood volume reached only about 73% of normal values, a condition researchers labeled “idiopathic hypovolemia” because they couldn’t find an obvious cause.3The American Journal of the Medical Sciences. Blood Volume Perturbations in the Postural Tachycardia Syndrome
The practical effect of this deficit is straightforward. When you stand up, gravity pulls blood into your legs and abdomen. A healthy body compensates by tightening blood vessels and adjusting heart rate to keep blood flowing to the brain. If you’re starting with significantly less blood to work with, that compensation becomes much harder. Your heart has to beat faster to push the reduced volume upward, which is exactly why POTS patients experience that dramatic heart rate spike on standing.
Why Blood Volume Runs Low
The reasons behind the low blood volume in POTS aren’t fully settled, but researchers have identified several contributing mechanisms. One of the most studied involves the renin-aldosterone system, the hormonal pathway that tells your kidneys to hold onto sodium and water. In healthy people, when blood volume drops, the kidneys release renin, which triggers a chain reaction ending with aldosterone telling the kidneys to retain fluid. In many POTS patients, this system doesn’t work properly. A study measuring renin activity found it was dramatically lower in POTS patients compared to healthy controls, even though aldosterone levels were similar between the two groups.4PubMed Central. Evidence for Impaired Renin Activity in Postural Orthostatic Tachycardia Syndrome Earlier research described this as a “renin-aldosterone paradox,” where patients have low blood volume that should trigger more renin release, but the expected hormonal correction doesn’t happen.2PubMed. Renin-aldosterone paradox and perturbed blood volume regulation underlying postural tachycardia syndrome
Deconditioning is another piece of the puzzle. Researchers have noted strong parallels between what happens physiologically in POTS patients and what happens in healthy people after prolonged bed rest or spaceflight.5PubMed Central. POTS versus deconditioning: the same or different? In one bed rest study, just two weeks of lying flat caused a 17% drop in plasma volume, a 12% reduction in the amount of blood the heart pumped with each beat, and a 24% decrease in how long subjects could tolerate standing upright.6PubMed. Cardiac atrophy after bed-rest deconditioning: a nonneural mechanism for orthostatic intolerance This creates a vicious cycle: POTS symptoms make patients less active, inactivity shrinks blood volume further, and the reduced blood volume worsens POTS symptoms.
That said, deconditioning doesn’t fully explain the blood volume deficit. A pooled analysis of bed rest studies found that while plasma volume dropped by an average of about 367 mL after bed rest, and cardiopulmonary fitness declined significantly, the changes in standing blood pressure didn’t correlate neatly with the degree of plasma volume loss.7Hypertens Res. Cardiopulmonary deconditioning and plasma volume loss are not sufficient to provoke orthostatic hypertension This suggests that while deconditioning makes things worse, something else is typically going on at the same time in POTS patients.
Connective Tissue Disorders and Venous Pooling
People with Ehlers-Danlos syndrome (EDS) and other connective tissue disorders turn up in POTS clinics at unusually high rates, and hypovolemia is part of the reason. The theory is that stretchy, overly compliant blood vessels allow too much blood to pool in the legs and pelvis when standing, effectively reducing the volume available to return to the heart. Researchers have proposed that abnormal connective tissue in dependent blood vessels permits excessive distension under ordinary gravitational pressure, leading to increased venous pooling and its downstream effects on heart rate and blood pressure.8PubMed. Orthostatic intolerance and chronic fatigue syndrome associated with Ehlers-Danlos syndrome
Beyond generalized vessel stretchiness, EDS patients are also at higher risk for specific anatomical problems that compress veins in the pelvis and abdomen, conditions like May-Thurner syndrome and Nutcracker syndrome. A case series found that EDS-related connective tissue abnormalities and the resulting increased risk of vein compression appear to interact together to produce more severe POTS symptoms.9PubMed Central. May-Thurner syndrome in patients with postural orthostatic tachycardia syndrome and Ehlers-Danlos syndrome: a case series For patients in this group, the blood volume problem isn’t just about having less fluid in the system; it’s also about that fluid being trapped in the wrong places.
Subtypes Overlap More Than They Divide
It’s tempting to think of hypovolemic POTS as a clean, separate category, but that’s misleading. POTS has been described in terms of at least five pathophysiological subtypes: neuropathic, hypovolemic, primary hyperadrenergic, joint-hypermobility-related, and immune-related.10PubMed. Postural Orthostatic Tachycardia Syndrome: Prevalence, Pathophysiology, and Management But the framework explicitly highlights that these subtypes overlap rather than existing as distinct entities. A given patient might have low blood volume, some degree of nerve damage affecting the blood vessels in their legs, and a hyperadrenergic tendency all at once. This matters for management because addressing only one mechanism while ignoring others may leave symptoms partially uncontrolled.
Tilt table testing can sometimes offer clues about which mechanisms dominate. Research comparing different hemodynamic responses during tilt testing found that POTS patients whose heart rate rose between 30 and 39 beats per minute showed patterns consistent with increased venous pooling, while those with a rise of 40 or more beats per minute lost that expected relationship between heart rate and stroke volume, suggesting a more hyperadrenergic response.11PubMed Central. Two Different Hemodynamic Responses in ME/CFS Patients with Postural Orthostatic Tachycardia Syndrome During Head-Up Tilt Testing The real-world implication is that two people who both meet the diagnostic criteria for POTS might need quite different treatment approaches depending on which mechanisms are driving their symptoms.
The Diagnostic Challenge
POTS is typically diagnosed when standing or tilt testing produces a heart rate increase of 30 beats per minute or more (40 bpm in adolescents) within ten minutes, without a major drop in blood pressure. But identifying the hypovolemic component specifically requires more than a standard tilt test. Formal blood volume measurement, which involves injecting a tracer into the bloodstream and measuring how it distributes, is available at some specialized centers but remains far from routine clinical practice.
Even the basic diagnostic test has nuances that affect accuracy. A study comparing head-up tilt testing to active standing found that tilt produced larger heart rate increases than simply standing up, with the difference growing over time: at five minutes, tilt provoked an average increase of 38 bpm compared to 33 bpm during standing, and by thirty minutes the gap widened to 51 versus 38 bpm.12PubMed Central. Diagnosing Postural Tachycardia Syndrome: Comparison of Tilt Test versus Standing Hemodynamics This means someone might not meet diagnostic thresholds with a simple in-office standing test but would during formal tilt testing, or vice versa.
In practice, many clinicians infer a hypovolemic contribution based on clinical features: the patient’s symptoms improve dramatically with fluid loading, they feel worst in the morning after overnight dehydration, their symptoms worsen in heat, and they respond well to salt and volume-expanding strategies. A formal blood volume measurement, when available, can confirm suspicions and help guide treatment intensity, but the absence of that test doesn’t mean the hypovolemic component should be ignored.
Salt, Fluids, and Oral Rehydration
Increasing salt and fluid intake is the first line of treatment for hypovolemic POTS, and the evidence behind it is more concrete than you might expect for something so simple. A controlled study published in the Journal of the American College of Cardiology compared POTS patients on a high-sodium diet (about 10 grams of sodium per day) to the same patients on a low-sodium diet. During the low-salt phase, total blood volume, plasma volume, and red blood cell volume were all significantly below what was predicted for each patient’s body size. The high-sodium diet reduced the plasma volume deficit from about 11% to less than 1%, based almost entirely on expanding the plasma component.13PubMed Central. Effect of High Dietary Sodium Intake in Patients with Postural Tachycardia Syndrome That’s a striking correction of one of the core problems.
For patients who have trouble hitting high salt targets through food alone, oral rehydration solutions (the electrolyte-rich drinks originally developed for dehydration in developing countries) offer another route. A pediatric study found that oral rehydration solution significantly improved orthostatic tolerance in children with POTS, and interestingly, the oral solution worked better than plain intravenous saline in that population.14PubMed Central. The Benefits of Oral Rehydration on Orthostatic Intolerance in Children with POTS The likely explanation is that the sodium-glucose cotransport mechanism in the gut absorbs fluid more efficiently when both sodium and glucose are present, leading to better plasma volume expansion than water or saline alone.
The common advice is to aim for about 2 to 3 liters of fluid daily and 8 to 12 grams of sodium, though these numbers vary depending on the clinician and the patient’s kidney and heart health. For people used to hearing that salt is bad, this can feel counterintuitive. But the normal public health advice about limiting sodium is aimed at people whose blood pressure tends to run high. In hypovolemic POTS, the problem is the opposite: the body isn’t holding onto enough fluid, and salt helps fix that.
Intravenous Saline for Severe Cases
When oral strategies fall short, some patients with severe, medication-resistant POTS receive intermittent intravenous saline infusions. A study of this approach found that patients receiving an average of about 1.5 liters of saline roughly every 11 days experienced significant improvements in both their orthostatic symptom scores and their overall quality-of-life measures.15PubMed. Effects of intermittent intravenous saline infusions in patients with medication-refractory postural tachycardia syndrome For patients who can barely function, these infusions can be transformative in the short term.
However, IV saline is generally reserved for refractory cases. Repeated IV access carries risks including infection, blood clots, and vein damage, particularly for patients who need long-term port access. Clinical guidance from specialists in autonomic disorders frames ongoing intravenous hydration as an option for severe cases that have failed other treatments, with careful attention to the risk-benefit balance.16Autonomic Neuroscience. Oral and intravenous hydration in the treatment of orthostatic hypotension and postural tachycardia syndrome Most clinicians will push hard to optimize oral fluid and salt intake, compression garments, medications, and exercise before recommending regular IV infusions.
Medications That Target Blood Volume
Several medications used in POTS aim to expand or preserve circulating blood volume. Fludrocortisone, a synthetic mineralocorticoid, is one of the most commonly prescribed. It mimics aldosterone to tell the kidneys to retain sodium and water, directly addressing the hormonal gap many POTS patients have. It’s included in most treatment protocols for pediatric and adult POTS.17PubMed Central. Management of Postural Orthostatic Tachycardia Syndrome in Pediatric Patients: A Clinical Review The main caveats are that it can cause potassium loss and headaches, and blood electrolytes need monitoring.
Desmopressin (DDAVP) takes a different approach. By increasing free water reabsorption in the kidneys, it can acutely expand plasma volume.18PubMed Central. Desmopressin acutely decreases tachycardia and improves symptoms in the Postural Tachycardia Syndrome (POTS) It tends to work faster than fludrocortisone, making it potentially useful as a rescue medication for bad days, but it also carries the risk of dangerously low sodium levels if fluid intake isn’t carefully balanced. This isn’t a “take it and forget it” drug.
Pyridostigmine doesn’t expand blood volume directly but works through a different angle: it enhances the activity of the autonomic nervous system at the ganglia, improving the body’s ability to tighten blood vessels and regulate heart rate on standing. A study of over 200 POTS patients found that about half of those who tolerated the drug showed symptom improvement, along with a statistically meaningful drop in standing heart rate.19PubMed. Pyridostigmine in the treatment of postural orthostatic tachycardia: a single-center experience Separate research confirmed that pyridostigmine reduced standing heart rate by about 10% and improved baroreceptor sensitivity, which is the body’s ability to detect and respond to blood pressure changes.20PubMed. Pyridostigmine in the treatment of orthostatic intolerance Gastrointestinal side effects are the main reason some patients can’t tolerate it.
Other medications commonly used alongside these include midodrine (which tightens blood vessels directly), ivabradine (which slows heart rate without lowering blood pressure), and beta-blockers like metoprolol (which dampen the excessive heart rate response). These don’t expand blood volume per se, but by addressing the downstream effects of hypovolemia, they can make the body’s limited blood supply work more effectively.
Why Compression Garments Work Best on the Belly
Compression garments are a staple recommendation for POTS, but not all compression is created equal. Research comparing different types found that garments compressing the abdomen were more effective than those compressing only the legs at maintaining upright blood pressure and vascular tone. In one study, blood pressure and vascular resistance only improved significantly when abdominal compression was used, either alone or with leg compression. Compression below the thighs alone did not significantly improve blood pressure compared to no compression at all.21PubMed Central. Abdominal Compression as a Treatment for Postural Tachycardia Syndrome
This finding has practical implications. Knee-high compression socks, which are the easiest to buy and the most commonly worn, are probably the least useful option. Waist-high compression garments or abdominal binders target the large venous reservoir in the gut and pelvis, where a substantial portion of blood pools on standing. The tradeoff is comfort: abdominal compression is harder to tolerate, especially in warm weather or for patients with gastrointestinal symptoms (common in POTS). Some patients find military-style abdominal binders or compression shorts more practical than full medical compression stockings.
Exercise as a Long-Term Volume Expander
Exercise is one of the few interventions that can durably increase blood volume rather than just temporarily propping it up. A three-month training program designed specifically for POTS patients started with recumbent exercise like rowing, swimming, and stationary biking (all done in positions that avoid the upright posture that provokes symptoms). Patients began with two to four sessions per week of 30 to 45 minutes and gradually progressed. Upright exercise was added cautiously, usually not until the second or third month. By the end of the program, patients were exercising five to six hours per week and also incorporating resistance training.22PubMed Central. Short-term exercise training improves the cardiovascular response to exercise in the postural orthostatic tachycardia syndrome
The key insight is that exercise training in POTS has to be approached differently than in healthy people. Starting with intense upright activity like running is a recipe for symptom flares and abandonment of the program. The recumbent-first approach avoids the gravitational challenge while still training the cardiovascular system. Over time, the heart muscle regains mass and distensibility, the body produces more red blood cells, and plasma volume expands. These are the same adaptations that happen in any exercise program, but for POTS patients they directly address the core problem.
Many patients report that the first few weeks are the hardest, and symptoms may temporarily worsen before improving. Having a specific protocol to follow, rather than generic advice to “exercise more,” makes a meaningful difference in adherence and outcomes.
Brain Fog and Cognitive Effects of Low Blood Volume
One of the most disabling aspects of POTS that often gets less clinical attention than heart rate numbers is cognitive dysfunction, commonly called brain fog. This isn’t just fatigue or poor motivation; it has a physiological basis. A study that subjected POTS patients and healthy controls to a sustained cognitive stress test found that patients showed greater slowing in psychomotor speed (about 6% versus 1.4% in controls) and a much larger increase in symptom scores, including difficulty concentrating, by the end of the test.23PubMed Central. Cerebral Blood Flow and Cognitive Performance in Postural Tachycardia Syndrome: Insights from Sustained Cognitive Stress Test
The connection to blood volume is logical: less circulating blood means less delivery to the brain, especially during upright posture when gravity is already working against cerebral perfusion. For many patients, cognitive symptoms track closely with their hydration status and time spent upright. Some find that their thinking is sharpest in the morning before dehydration sets in, or that lying down briefly clears the mental fog within minutes. If you’re managing hypovolemic POTS and noticing that your brain doesn’t work as well as it used to, that’s not anxiety or laziness. It’s reduced cerebral blood flow, and many of the same strategies that improve heart rate on standing (salt, fluid, compression, exercise) also improve cognitive function by getting more blood to the brain.