Regional blood volume and peripheral blood flow in POTS

A common framing of POTS focuses on blood volume: patients are told they are hypovolemic, prescribed salt and fluid loading, and the condition is managed as a quantity problem. A 2004 study by Stewart and colleagues in the American Journal of Physiology challenges that framing with direct measurement. The problem in many POTS patients is not how much blood exists. It is where the blood goes when the patient stands up.

What the Study Measured and How

Stewart and colleagues recruited POTS patients and healthy controls and measured blood distribution regionally during orthostatic challenge — not just global volume but where the blood was actually sitting. They combined impedance cardiography, which tracks thoracic fluid content and cardiac output, with venous plethysmography, which measures blood accumulation in the limbs. This gave them a regional picture that standard heart rate and blood pressure monitoring cannot provide.

The orthostatic challenge revealed a clear pattern in POTS patients: upright posture was associated with reduced thoracic blood volume. The chest — and by extension, the heart — was receiving inadequate venous return despite the patient having circulating blood. Total blood volume was not dramatically reduced in all cases. The problem was distribution: blood was accumulating in peripheral compartments, particularly the legs and splanchnic (abdominal) circulation, instead of returning to the thorax.

This peripheral pooling was associated with reduced cardiac preload — less blood entering the heart with each beat — which reduced stroke volume and triggered the compensatory tachycardia that defines POTS. The heart rate increase is the system's attempt to maintain cardiac output when each beat is delivering less. This is the core POTS mechanism: not a primary cardiac problem, not a simple volume deficit, but a failure of blood distribution under gravitational stress.

Why Vasoconstriction Failure Drives the Distribution Problem

In healthy individuals, standing triggers a rapid peripheral vasoconstriction response. The vessels in the legs and splanchnic circulation constrict, limiting how much blood can pool in dependent regions, maintaining venous return to the thorax and sustaining cardiac filling. This is a reflex arc the autonomic system runs continuously while upright.

In the POTS patients Stewart studied, this vasoconstriction was insufficient. The peripheral vessels were not constricting adequately or quickly enough to prevent pooling. The failure was not in blood volume per se but in the dynamic redistribution mechanism that keeps volume flowing upward against gravity. The total amount of blood in the system could look perfectly normal on a blood test. The distribution of that blood under gravitational challenge was not.

This finding has a direct corollary in why salt and water loading often produces incomplete results in POTS. If the mechanism is a failure of peripheral vasoconstriction allowing blood to pool, adding more blood volume to the system shifts the problem but does not resolve it. The peripheral pooling continues; there is simply marginally more volume available to pool. Treatments that physically limit peripheral pooling — compression stockings, abdominal binders — address the distribution failure more directly by reducing the space available for blood to accumulate in dependent regions.

Cardiac Output, Stroke Volume, and What Standard Testing Misses

The study also measured cardiac output directly during orthostatic stress. In POTS patients, upright cardiac output was reduced compared to controls despite elevated heart rates. This is the mechanical consequence of reduced preload: the heart beats faster but each beat pumps less. Stroke volume was falling as heart rate was rising — the tachycardia was compensating for, but not fully restoring, the output loss caused by insufficient venous return.

Standard POTS evaluation does not measure cardiac output during tilt. It measures heart rate and blood pressure. This means the stroke volume reduction that Stewart documented goes undetected in routine evaluation. A patient with significantly reduced upright stroke volume and elevated heart rate will receive a POTS diagnosis — correctly — but the evaluation will have told the clinician nothing about the magnitude of the cardiac underfilling or the degree to which tachycardia is failing to compensate for it.

Impedance cardiography during tilt testing makes this visible. It is not standard. But this paper establishes that the information it provides is mechanistically important — not just for confirming the diagnosis but for understanding which treatment targets are actually relevant for a given patient.

The Splanchnic Component

The paper also identified pooling in the splanchnic circulation — the extensive network of vessels supplying the abdominal organs. This region has a large capacitance: it can accommodate a substantial fraction of total blood volume. Under normal conditions, splanchnic vasoconstriction during standing limits how much blood pools here. When this vasoconstriction fails, the splanchnic bed becomes a major reservoir for blood that should be returning to the heart.

This splanchnic component helps explain a clinical pattern that patients often describe: symptoms that are worse after eating, when splanchnic blood flow increases further to support digestion, reducing the volume available for systemic circulation. It also suggests that abdominal compression — not just leg compression — may be mechanistically important for patients with significant splanchnic pooling.

What This Means for Evaluation and Treatment Strategy

If your treating clinician has told you that your blood work looks normal and your blood pressure holds up on tilt, this paper explains what that evaluation did not measure: whether your blood is distributing appropriately when you stand. Normal labs and a preserved blood pressure do not tell you where your blood volume is sitting. The heart rate elevation — which did get measured — is the downstream consequence of a distribution failure the evaluation was not designed to detect.

The practical questions this research raises: Does your evaluation include cardiac output or stroke volume measurement during orthostatic stress? Has anyone assessed peripheral and splanchnic venous pooling directly? The answers will not change your POTS diagnosis, but they may substantially change which interventions are most likely to address the mechanism driving your symptoms rather than only the symptoms themselves.