The research team, led by investigators at the Pain & Fatigue Study Center in the Department of Neurology, drew participants from an NIH-funded parent study designed to examine the relationship between cardiopulmonary exercise testing and post-exertional malaise, the hallmark symptom of ME/CFS. Between August 2021 and July 2025, forty-nine adults aged 25 to 60 years who met the 1994 Fukuda case definition completed both assessments. To ensure a substantial current symptom burden, participants were required to report moderate-to-severe ratings on post-exertional malaise and on at least three of seven additional symptoms, including sore throat, headache, myalgia, and unrefreshing sleep. Individuals with medical conditions that could explain the fatigue, including anemia identified by routine laboratory testing, were excluded, as were those taking medications that blunt cardiovascular responses and those with certain psychiatric diagnoses.

The orthostatic assessment relied on the 10-minute lean test, a protocol originally developed by NASA to detect orthostatic intolerance in astronauts returning from spaceflight. After at least ten minutes of supine rest, baseline blood pressure, heart rate, respiratory rate, and end-tidal carbon dioxide were recorded in triplicate at one-minute intervals using a portable capnograph and pulse oximeter. Participants then stood with feet together roughly six to eight inches from a wall and leaned against it without moving for ten minutes, while the same parameters were captured every minute. This test captures several distinct physiological abnormalities: postural orthostatic tachycardia syndrome, defined as a heart rate rise of at least 30 beats per minute or an absolute heart rate of at least 120 beats per minute; supine hypocapnia, defined as any baseline end-tidal carbon dioxide below 34 mmHg; and the postural orthostatic syndrome of hypocapnia, in which carbon dioxide falls below that threshold only upon standing.

Blood volume was measured with the semiautomated Daxor BVA-100 system, a technique regarded as a reference standard for quantitative intravascular volume assessment. After a baseline blood sample was drawn, human serum albumin labeled with iodine-131 was injected intravenously and allowed to equilibrate. Five additional samples were collected at 15, 21, 27, 33, and 39 minutes to correct for the gradual leakage of albumin out of the vasculature, and plasma volume at time zero was extrapolated from at least three samples with a coefficient of variation of no more than 3.9 percent. Total blood volume was calculated as the sum of plasma volume and red blood cell volume, and all values were normalized to each participant’s ideal body size. Total blood volume was then categorized as hypovolemic if it deviated more than 8 percent below the predicted value, normovolemic within 8 percent, and hypervolemic above 8 percent, thresholds consistent with prior ME/CFS blood volume research.

The results painted a picture of remarkable physiological heterogeneity. Seventeen participants, or 35 percent, were classified as hypovolemic, twenty-one as normovolemic, and eleven as hypervolemic. Red blood cell volume told a similar story: nearly half of the cohort, twenty-four participants, had a red cell volume deficit more than 10 percent below ideal, a phenotype that, when routine hemoglobin and hematocrit values are normal, can be considered an occult red cell mass deficit rather than conventional anemia. Yet when the researchers cross-tabulated these volume categories against lean-test outcomes, no significant associations emerged. Fifty-five percent of participants exhibited at least one orthostatic abnormality, with hypocapnic responses proving at least as common as tachycardia: twenty participants showed the postural orthostatic syndrome of hypocapnia, six showed supine hypocapnia, six met criteria for POTS, and four had both POSH and POTS. The distribution of these abnormalities, however, was statistically indistinguishable across hypovolemic, normovolemic, and hypervolemic groups, and the same held true for red blood cell volume categories.

These findings directly confront a body of earlier work. Multiple previous studies have reported reduced total or compartmental blood volumes, smaller cardiac chambers, and diminished stroke volume in subsets of ME/CFS patients, and one pilot study using a dual-isotope technique found lower blood volumes among patients whose day-to-day symptoms suggested orthostatic intolerance. That earlier study, however, relied on clinical history rather than contemporaneous physiological testing, leaving open the possibility that subjective symptom reports and objective orthostatic physiology diverge. By combining quantitative blood volume analysis with a standardized lean test incorporating capnography, the Mount Sinai team demonstrated that even objectively confirmed orthostatic abnormalities do not track with measured volume status. In other words, a patient can be quantitatively hypovolemic yet pass the lean test, or have entirely normal blood volume yet exhibit marked hypocapnia or tachycardia upon standing.

The null result does not mean blood volume is clinically irrelevant. Identifying hypovolemia in an individual patient may still matter, because volume expansion remains a logical therapeutic target for that subgroup. But the new data help explain why pharmacological volume expansion has disappointed in controlled trials: randomized studies of fludrocortisone, a mineralocorticoid that promotes sodium and water retention, showed limited benefit in ME/CFS, implying that correcting hypovolemia alone is often insufficient. The authors argue that orthostatic intolerance in this illness likely arises from multiple converging mechanisms that static volume measurements cannot capture. Among the candidates are impaired venous constriction and excessive venous capacitance, which promote gravitational pooling of blood in the legs; autonomic dysregulation, evidenced by reduced vagal modulation and altered heart rate variability at rest, during sleep, and during orthostatic challenge; and abnormalities of neurohumoral control, including the renin-aldosterone paradox described in POTS and small-fiber neuropathy identified in subsets of patients.

Cerebral perfusion adds another critical dimension. Previous controlled work has shown that upright posture can substantially reduce cerebral blood flow in ME/CFS patients even when heart rate and blood pressure remain within conventional limits, and that this cerebral blood flow reduction during tilt testing is independent of deconditioning as defined by exercise testing. Deconditioning itself, which can reduce plasma volume and ventricular mass over weeks of bed rest, is unlikely to account for the high prevalence of orthostatic intolerance in this population, since patients show greater cerebral blood flow reductions than healthy, sedentary controls. Taken together, these observations support a model in which orthostatic pathology in ME/CFS reflects dynamic failures of autonomic, vascular, respiratory, and cerebral hemodynamic regulation rather than a simple deficit in the amount of blood in the circulation.

The study has limitations that the authors acknowledge candidly. The sample of forty-nine participants, recruited from a single tertiary care center and limited to patients well enough to undergo testing, constrains statistical power and may underrepresent the most severely affected patients. The eight-day interval between the lean test and the blood volume assessment, together with the lack of standardization of acute hydration and salt intake, may have introduced temporal variability in both volume and hemodynamic measures. Anxiety-related hyperventilation and other breathing-pattern abnormalities could not be excluded as contributors to the hypocapnic findings, because state anxiety and tidal volume were not assessed. The 10-minute lean test also imposes milder orthostatic stress than head-up tilt, so hypovolemia-driven instability might conceivably emerge only under more prolonged or severe challenge. The absence of a healthy control group further precluded direct comparison of volume patterns between patients and volunteers.

Even with those caveats, the message for the field is clear and consequential. Orthostatic intolerance was highly prevalent in this cohort, yet neither total blood volume nor red blood cell volume category predicted who would exhibit supine hypocapnia, POSH, or POTS on standardized testing. The authors call for multicenter studies that incorporate dynamic measurements of cerebral blood flow, respiratory pattern, and carbon dioxide regulation during graded orthostatic stress, integrated with blood volume status, to build comprehensive pathophysiological models of the illness. Such phenotype-directed approaches, they argue, will be essential for developing rational therapies that address the actual mechanisms driving orthostatic symptoms, rather than assuming that a single measurable deficit, however intuitively appealing, holds the key to one of ME/CFS’s most disabling features.

Subject of Research: The relationship between measured blood volume, red blood cell volume, and orthostatic intolerance in adults with myalgic encephalomyelitis/chronic fatigue syndrome

Article Title: Total blood volume and red blood cell volume are not associated with orthostatic intolerance in adults with myalgic encephalomyelitis/chronic fatigue syndrome

Article References: Total blood volume and red blood cell volume are not associated with orthostatic intolerance in adults with myalgic encephalomyelitis/chronic fatigue syndrome. (n.d.). https://doi.org/10.14814/phy2.71087

Image Credits: AI Generated

DOI: 10.14814/phy2.71087

Keywords: ME/CFS, orthostatic intolerance, blood volume, red blood cell volume, POTS, hypocapnia, lean test, capnography, autonomic dysfunction, cerebral blood flow, hypovolemia, Physiological Reports