Postural Orthostatic Tachycardia Syndrome (POTS) is a disorder of the autonomic nervous system, which is the body’s control system for involuntary functions like heart rate, blood pressure, and digestion. It is characterized by orthostatic intolerance, meaning symptoms worsen upon standing. It is formally diagnosed when an adult’s heart rate increases by 30 beats per minute or more within 10 minutes of standing up, without a significant drop in blood pressure. Hypovolemic POTS is a recognized subtype distinguished by a primary underlying problem of low circulating blood volume, known as hypovolemia. This reduced volume triggers the abnormal heart rate response as the body attempts to compensate for insufficient blood return to the heart and brain.
The Underlying Mechanism of Low Blood Volume
Hypovolemia refers to a reduction in total blood volume, specifically a deficit in plasma volume. Studies suggest that patients with this subtype may have a plasma volume approximately 13% lower than expected for their body size. This volume deficit leads to reduced venous return, meaning less blood makes its way back to the heart when a person stands upright. The heart must then beat faster—the defining tachycardia of POTS—to maintain sufficient cardiac output and keep blood flowing to the brain.
The failure to maintain adequate blood volume in Hypovolemic POTS often involves issues with fluid and electrolyte regulation. One theory suggests an impairment in the renin-angiotensin-aldosterone axis, a hormonal system that helps regulate blood volume and pressure. Paradoxically, patients in this low-volume state often have low levels of circulating renin and aldosterone, hormones that should be elevated to promote salt and water retention. This failure results in the kidneys excreting too much sodium and water, actively contributing to the sustained low blood volume.
This mechanism differs from other POTS subtypes. For instance, Neuropathic POTS involves nerve damage leading to excessive blood pooling in the lower extremities. Hypovolemic POTS is defined by the absolute reduction in plasma volume as the dominant driver of the compensatory tachycardia, creating a state of “thoracic hypovolemia” during standing.
Specific Clinical Presentation
The symptoms experienced by people with Hypovolemic POTS are a direct result of the body’s struggle to manage a low circulating volume, particularly when upright. Severe fatigue and generalized weakness are often reported, linked to the overall low blood flow and reduced cardiac output. Exercise intolerance is also a prominent complaint, as the limited blood volume cannot meet the increased circulatory demands of physical activity.
Orthostatic symptoms like dizziness, lightheadedness, and pre-syncope upon standing are intensified because the body has less volume to begin with. Some individuals may also experience excessive thirst, a physiological attempt by the body to correct the volume deficit. Reduced perfusion to the extremities can lead to coldness in the hands and feet, a sign that the body is prioritizing blood flow to core organs.
Confirming the Diagnosis
The initial diagnosis of POTS relies on an Active Stand Test or a Tilt Table Test, which measures the excessive heart rate increase upon standing. To specifically confirm the hypovolemic subtype, diagnostic steps must focus on quantifying the blood volume deficit. Standard blood tests, such as hematocrit, may be misleading because they measure the ratio of red blood cells to plasma, not the absolute volume. A high hematocrit in a hypovolemic patient can wrongly suggest normal volume, as the blood is concentrated due to low plasma.
The definitive method for confirming hypovolemia involves direct measurement of total blood volume, including plasma volume and red blood cell volume. This is typically done using indicator dilution methods, such as the injection of a known tracer like radioactive iodine. These tests quantify the total volume of blood in the body and are considered the gold standard for identifying the absolute volume deficit. Additionally, a 24-hour urinary sodium test is often used as an indirect marker, suggesting that the kidneys are inappropriately losing salt and contributing to the hypovolemia.
Therapeutic Strategies for Volume Restoration
The management of Hypovolemic POTS centers directly on expanding the circulating blood volume to reduce the need for the heart’s compensatory rapid beating. Lifestyle and dietary modifications are the first line of therapy, involving aggressive fluid and salt intake. Patients are advised to consume a high-sodium diet, often targeting 10 to 12 grams of salt per day, coupled with significantly increased fluid consumption, typically around 2 to 3 liters of water or other non-caffeinated fluids daily.
Compression garments are another non-pharmacological strategy. High-waisted compression stockings or abdominal binders are effective, as they counteract the gravitational pooling of blood in the legs and abdomen when upright. By externally squeezing the veins, these garments assist in pushing blood back toward the heart, effectively increasing the central circulating volume.
Pharmacological interventions are used when lifestyle changes are insufficient, focusing on medications that directly promote volume expansion. Fludrocortisone is a synthetic mineralocorticoid that acts on the kidneys to enhance the reabsorption of sodium and water, directly addressing the underlying fluid regulation issue. In some cases, Desmopressin (DDAVP), which reduces the amount of water the kidneys excrete, may be used to further aid in fluid retention and volume expansion.