Volume depletion versus dehydration: how understanding the difference can guide therapy

Abstract

Although often used interchangeably, dehydration and volume depletion are not synonyms. Dehydration refers to loss of total-body water, producing hypertonicity, which now is the preferred term in lieu of dehydration, whereas volume depletion refers to a deficit in extracellular fluid volume. In particular, hypertonicity implies intracellular volume contraction, whereas volume depletion implies blood volume contraction. Using a case of hyperglycemic hypertonic nonketosis as an example, we examine the changing composition of body fluid spaces to explore the distinction between dehydration and hypertonicity from volume depletion.

Figure 1. Schematic Representation of Body Water Compartments

Figure 2. Body Fluid Derangements and Fluid Therapy Strategy in HHNK with Volume Depletion & Hypertonicity

Circulating volume depletion is the overriding initial therapeutic goal followed by interstitial fluid depletion which account for 1/4^th and 3/4^th of the ECFV deficit respectively. Correction of tonicity with free water or hypotonic solutions is conducted slowly ensuring serum Na⁺_G falls at a rate < 10 mEq/day.