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Dehydration and Hypovolemia: Differences Can Be Important

September 29, 2010

We should be precise in our language, especially when discussing something as complex as human physiology.  Fuzzy language can lead to fuzzy logic, which can lead to problems we didn’t want.

One of the most misused terms in medicine is “dehydrated”.  We use that to indicate that someone’s overall fluid volume is low.  The more appropriate term is “hypovolemic”.  However, most people, including health care providers, do not understand the difference or its importance.

The Greek and Latin roots in the terms provide some distinction.  An understanding of physiology provides the rationale.

“Dehydration” means “loss of water”.  Water is the base fluid for all the body’s cells and their surrounding environment.  Over 60 percent of the body is water.  However, because water flows freely between the cells and their environment, when we are low on water – truly dehydrated – we see an increase in the concentration of the molecules dissolved in water.  In the cells, potassium levels increase, because potassium is the primary positively charged molecule (or cation) inside cells.  Outside cells, because sodium is the primary cation outside cells, sodium concentrations climb.  Because the molecules in the bloodstream exchange relatively freely with the fluid outside cells, a blood test showing a high serum sodium level confirms the diagnosis of dehydration.

“Hypovolemia” means “low blood volume”, which is not identical to dehydration because blood is not pure water.  Rather, blood is a solution of sodium and other salts, proteins, and various types of blood cells.  Low blood volume can only be detected by devices that can measure the volumes or pressures within blood vessels or heart chambers, such as central venous pressure, pulmonary capillary wedge pressures, or end-diastolic volume measurements.

Treatment of dehydration employs free water administration.  Treatment of hypovolemia requires salt-based so-called “crystalloid” infusions.  It should not be a major paradigm shift in medicine to know what we are talking about in order to treat the condition properly, yet all too often we’ve forgotten our roots.

11 Comments leave one →
  1. September 29, 2012 7:57 pm

    Shouldn’t that say an “increase” in serum sodium confirms a diagnosis of dehydration?

    • April 9, 2015 2:45 pm

      You are correct. I’ve fixed the post.

      (Sorry to be so late getting back to you, but I’ve been too busy to pay attention to the blog for quite awhile. Now I’m just starting to get back into it.)

  2. April 12, 2013 8:38 pm

    Nice article! I learned a ton, but I think it must be a typo when you say that “low serum sodium levels confirms the diagnosis of dehydration”. Must be high if it freely exchanges with a high sodium ECF.

    • April 9, 2015 2:44 pm

      Thanks. I’ve fixed the error.

      (Sorry to be so late getting back to you, but I’ve been too busy to pay attention to the blog for quite awhile. Now I’m just starting to get back into it.)

  3. Rachel Nicoll permalink
    October 28, 2013 11:11 am

    Last sentence, 4th paragraph. Should that not say “a blood test showing a HIGH (not low) serum sodium level confirms the diagnosis of dehydration”?

    • April 9, 2015 2:43 pm

      You’re so right. I’ve fixed it. Thank you!!

      (Sorry to be so late getting back to you, but I’ve been too busy to pay attention to the blog for quite awhile. Now I’m just starting to get back into it.)

  4. January 14, 2014 12:00 am

    Reblogged this on heidib1978's Blog and commented:
    For my fellow classmate’s…

  5. January 14, 2014 12:00 am

    Thank you!

  6. Visitor permalink
    February 28, 2014 1:42 pm

    Very well explained. Thank you!

  7. girmaa huqqaa permalink
    November 7, 2016 4:03 am

    very nice explaination thank you!!!

  8. Denise Squillante permalink
    December 20, 2016 11:28 am

    So are you
    saying dehydration would result in high serum sodium level where as hypovolemia would result in a low serum sodium level??

    No. Hypovolemia (low circulating blood volume) doesn’t involve sodium concentrations. Overhydration (which translates to “too much water”) produces a low sodium concentration (“hyponatremia”), whereas underhydration (“too little water”) produces a high sodium concentration.

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