Language Guiding Therapy: The Case of Dehydration versus Volume Depletion

Patients presenting with orthostatic hypotension and normal plasma sodium concentrations are frequently admitted to the hospital with a diagnosis of dehydration. If they are fortunate, they receive fluids containing sodium chloride instead of free water to correct obvious extracellular fluid volume depletion. Confusing this diagnosis highlights the growing and pernicious habit of using the terms dehydration and volume depletion interchangeably at the bedside when the two describe clearly different disturbances.

The heuristic value of describing discrete body fluid spaces affected by disorders of salt and water is a well-established bedside strategy [1-5]. It sprang from an early curiosity about the best treatment for fatal diarrhea [6] and seizures [7] and from classic experiments that formulated the volume behavior and osmolarity of cells [8, 9]. Adapting this information from cells to humans in the late 1930s required more conceptual thinking about the special role of vascular volume in the control of body fluids [2, 10]. The wartime assessment of potential fluid losses encountered by shipwrecked aviators and sailors in the early 1940s further enhanced our understanding of salt and water metabolism [11-13], as did the emerging role of cardiac performance [14, 15].

With the advent of radioactive tracers [16, 17], medical language in the latter part of the 20th century began to discriminate more carefully between dehydration associated with hypertonicity, a principal loss of body water from the intracellular and interstitial compartments, and extracellular fluid volume depletion, a fluid deficiency that clinically affects the vascular tree [3, 5, 18]. The proper use of the terms dehydration and volume depletion informs communication and should improve patient care.