When many people hear the term "volume depleted" in reference to a person's condition, they mistakenly think it means the same thing as saying a person is dehydrated. But the difference between the two is an important thing to understand.
Total body water makes up 45 to 60% of body weight depending on age, gender and race. Total body water is then broken up into intracellular and extracellular body fluid. Each of these compartments represents 45 and 55% of the total body water. The extracellular compartment is further divided into plasma volume, interstitial volume, and bone and connective tissue water. Finally, blood volume includes the sum of extracellular plasma volume and red blood cell volume. (1) What does all of that mean? Simply put, the amount of water in your body is not the same as the amount of fluid in your tissues or the amount of water in your blood (plasma).
The term "hypovolemia" refers to decreased volume in the vascular system. This can be with or without the loss of total body water (dehydration). In fact, severe dehydration can cause hypovolemia, as can vasodilation. It is vasodilation that is at the root cause of one recognized POTS subtype - Hypovolemic POTS.
Vasodilation is the widening of blood vessels causing an increase of blood flow and a decrease in blood pressure. In a healthy body this happens automatically as a part of the autonomic nervous system in response to increases in temperature, low oxygen levels, medications, exercise, and other things. (2) When vasodilation does not occur the way it should it can result in hypovolemia, meaning the vascular system in the body does not have enough fluid volume. In the case of hypovolemia, drinking water alone can not correct the issue because filling up the tank (the stomach) with water can not be guaranteed to increase the volume within the vascular system if the flow is restricted by a dysfunctional pathway, in this case vasodilation in a dysfunctional Autonomic Nervous System.
Hypovolemic POTS describes a person with POTS symptoms as a result of low blood volume (hypovolemia). For a person with Hypovolemic POTS, treatment is centered around balancing an adequate intravascular volume by increasing fluid intake to 2 to 3 liters per day and salt intake to 10 to 12 grams per day, in many cases medications are used to assist the body in correct vasodilation. IV hydration is also sometimes used to help the body maintain the body's fluid balance.
For more information about the subtypes of POTS, including hypovolemic POTS, please visit https://youtu.be/U0s26KdDDdU?feature=shared
For a better understanding of how the Autonomic Nervous System functions, please visit https://www.dinet.org/info/dysautonomia-disorders-diagnostics-info/understanding-the-autonomic-nervous-system-r208/
References:
1. Guatam Bhave MD, PhD, Eric G Neilson, MD, "Volume Depletion versus Dehydration: How understanding the difference can guide therapy", published by National Library of Medicine, June 2011 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4096820/
(2) Cleveland Clinic website, "Vasodilation" https://my.clevelandclinic.org/health/diseases/23352-vasodilation#symptoms-and-causes
Image: Cleveland Clinic website "vasodilation"
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