These “black-outs” where you constantly feel like you are going to pass out—but may not completely lose consciousness may be related to a lack of volume or lack of blood flow to the brain. Medically, we use the terms “dehydration” and “hypovolemia.” These terms are not interchangeable (although you will see that quite frequently” and each has a specific meaning.
Dehydration implies a relative lack of water in the blood volume, meaning the blood is too concentrated.
Hypovolemia implies an actual reduction in the total amount of blood in the system—but the blood that is present will have a normal concentration of water.
See, we lose a certain quantity of water every day. The major sources of water loss (like sweat, urine, stool, etc.) are always less concentrated than blood—meaning that there is relatively more water and less solid particles. This way, the blood is constantly a dehydrating pressure. This must be compensated by drinking some equal volume of water-based fluids or we will rapidly begin to shut down organ function.
Certain medical conditions including diabetes and POTS cause an excess (or rapid) fluid loss, and this additional loss must constantly be made up—or the deficit grows bigger and bigger each day.
However, passing out—or cerebral hypoperfusion is much more complex than solely dehydration. For example, you can be perfectly well hydrated but have no oxygen source and pass out very quickly. Your blood sugar could also suddenly take a nosedive from 140 to 40 and you could pass out—the list goes on and on. For the POTS patient, remember to keep up your fluid and salt intake, and to make sure you are tested for potential blood sugar issues.
Disclaimer: Even though this information is coming from a qualified medical professional, it should not be taken as a personalized diagnosis or treatment plan. POTS patients in particular tend to have multiple medical issues including secondary diagnoses of Mast Cell and Ehlers Danlos Disease. Please discuss this information with your doctor for a more personalized approach. Read our full disclaimer here.