Postural Orthostatic Tachycardia Syndrome, commonly known as POTS, is characterized by an abnormal increase in heart rate upon standing. This disorder of the autonomic nervous system, which controls involuntary bodily functions, can manifest with symptoms including dizziness, fatigue, and brain fog. A frequent symptom for those with POTS is dehydration. Understanding the physiological mechanisms behind why POTS leads to dehydration is important.
Reduced Blood Volume
Many individuals with POTS experience a lower-than-normal circulating blood volume, a condition referred to as hypovolemia. The total amount of fluid circulating within the blood vessels, comprising red blood cells and plasma, is important for maintaining adequate tissue perfusion. In POTS, this reduced volume means there is less fluid readily available to support the body’s systems, making individuals more susceptible to dehydration. This deficit is not necessarily due to active fluid loss but rather a baseline reduction in blood volume, contributing to the difficulty in maintaining proper hydration. Hypovolemic POTS, a specific subtype, is characterized by a plasma deficit.
Blood Vessel Regulation
The autonomic nervous system (ANS) plays a central role in regulating blood vessel tone, ensuring consistent blood flow throughout the body. In POTS, there is often a dysfunction in the ANS, specifically impacting the ability of blood vessels in the lower body to constrict effectively when a person stands up. This impaired vasoconstriction leads to blood pooling, where a significant amount of blood accumulates in the legs and abdomen. When blood pools in the lower extremities, less blood returns to the heart and, consequently, to the brain.
Although this pooling does not represent an actual loss of fluid, it creates a state that mimics dehydration to the brain. The body’s baroreceptors, which detect changes in blood pressure, signal this reduced blood return, triggering compensatory mechanisms. The heart rate increases excessively in an attempt to pump the pooled blood back to the upper body, and stress hormones like norepinephrine are released. This struggle to maintain adequate circulation can intensify symptoms.
Kidney and Hormone Activity
The dysfunction within the autonomic nervous system in POTS can directly affect the kidneys’ ability to manage fluid and sodium balance. Normally, hormones like aldosterone and antidiuretic hormone (ADH) regulate water and salt retention, ensuring the body conserves fluid when needed. However, in some individuals with POTS, there can be inappropriate hormonal signaling. For instance, POTS patients may have paradoxically low levels of aldosterone despite their reduced blood volume, which would typically trigger higher aldosterone production to promote fluid retention.
This “renin-aldosterone paradox” suggests that the kidneys may excrete too much sodium and water, contributing to the low blood volume. This can lead to increased urination (polyuria) even when the body is in a state of fluid deficit, further exacerbating dehydration. The impaired ability to retain sodium and water means that the body struggles to expand its blood volume. This fluid loss through the kidneys contributes to chronic dehydration in POTS.
Electrolyte Imbalance
Chronic fluid imbalances and increased fluid loss in POTS can lead to disturbances in essential electrolytes, such as sodium, potassium, and magnesium. Electrolytes are minerals that carry an electric charge and are important for bodily functions, including nerve impulses, muscle contraction, and maintaining fluid balance. Sodium, in particular, plays a significant role in helping the body retain water, as water tends to follow sodium.
When these electrolytes are out of balance, it can worsen dehydration and contribute to many symptoms experienced by POTS patients. For instance, low sodium levels can directly impair the body’s ability to hold onto necessary fluids, while imbalances in potassium and magnesium can affect heart rhythm and muscle function, exacerbating fatigue and cramps. This creates a cycle where dehydration intensifies POTS symptoms, which in turn can make rehydration more difficult.