Dehydration and Hypovolemia: Differences Can Be Important

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.