Chapter 8 – “Dehydration”
Welcome to the eighth installment of “Inconceivable: medical terms that don’t mean what you think they mean.”
You are evaluating a patient with vomiting and diarrhea. She reports that she’s been unable to keep down solids or liquids for over 72 hours. On exam you note a normotensive but slightly tachycardic patient with dry oral mucosa. Laboratory testing reveals mild hypokalemia, but all other electrolytes are within normal limits. After a dose of ondansetron and a liter of IV fluid, she feels well, tolerates crackers and juice, and is discharged. You diagnose the patient with dehydration, but was she actually dehydrated? She was not, but don’t feel bad, dehydration is a term used in medicine every day, and most of the time it’s used incorrectly.
Dehydration is a specific term for loss of total body free water while hypovolemia is any loss of extracellular volume (fluid AND sodium) leading to intravascular depletion and suboptimal tissue perfusion. Our aforementioned patient was hypovolemic but she was not dehydrated.
Hypovolemia and dehydration are not interchangeable but do have overlap. Dehydrated patients are (almost) always hypovolemic, but all hypovolemic patients are not dehydrated. (Think: all pizza is food but all food is not pizza.) However, dehydrated patients typically have a less profound hypovolemia as only 1/3 of free water resides in the extracellular space. Common causes of hypovolemia without dehydration include diarrhea, vomiting, third-spacing, and diuretics.
Dehydration is a laboratory diagnosis while hypovolemia is a clinical diagnosis (tachycardia, dry mucous membranes, poor skin turgor, flat neck veins, etc.), albeit often aided by laboratory data. Sodium is the predominant extracellular ion, therefore serum sodium can be used as a surrogate for tonicity, and dehydrated patients are always hypernatremic .
In contrast, hypovolemic patients can be hypo-, iso-, or hypernatremic depending on the relative amounts of sodium and water lost. ADH release and they body’s thirst drive are much more sensitive to hypertonicity than hypovolemia. This explains why it is relatively easy to become hypovolemic but true dehydration rarely occurs if someone has access to free water. Common causes of dehydration include altered mental status, immobile patients (e.g. nursing home patients, prolonged down time after a fall), or excessive exercise.
Dehydration and hypovolemia ARE NOT synonymous. Dehydration is loss of only free water, and patients are always hypernatremic. Hypovolemia is any loss (typically both water AND sodium) resulting in extracellular volume depletion.
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