Language Guiding Therapy: The Case of Dehydration versus Volume Depletion

Abstract

Indiscriminate use of the terms dehydration and volume depletion, so carefully crafted by our predecessors, risks confusion and therapeutic errors. These two conditions should be distinguished at the bedside and in how we speak to one another. Dehydration largely refers to intracellular water deficits stemming from hypertonicity and a disturbance in water metabolism. The diagnosis of dehydration cannot be established without laboratory analysis of p[Na +] or calculation of serum tonicity. In contrast, volume depletion describes the net loss of total body sodium and a reduction in intravascular volume and is best termed extracellular fluid volume depletion. The diagnosis of this condition relies principally on history, careful physical examination, and adjunctive data from laboratory studies. The pathophysiology of both dehydration and extracellular fluid volume depletion must be understood if these conditions are to be recognized and appropriately treated when they occur separately or together. There is no inclusive therapy for all situations. For example, indiscriminate treatment with 0.45% saline cannot be recommended when these conditions coexist because extracellular fluid volume depletion is often treated rapidly with 0.9% saline and dehydration is often treated more slowly with 5% dextrose.

The hyponatremic patient: A systematic approach to laboratory diagnosis

“…Thus, in a patient with hy-ponatremia, normal or elevated effective serum osmolality (measured as serum osmolality less serum urea level in millimoles per litre) suggests the presence of either pseudohyponatremia (due to hyperparaproteinemia or hypertriglyceridemia) or increased concentrations of other osmoles, such as glucose and mannitol. 1,6,7 In the context of marked hyperlipidemia (hypertriglyceridemia) and lactescent serum, lipids occupy space in the volume of serum, leading to lower readings in the concentrations of sodium and free water per litre of serum. However, the physiologically significant serum water and sodium and serum osmolality remain unaffected.…”

An Investigation into Equations for Estimating Water Requirements and the Development of New Equations for Predicting Total Water Intake

“…Some investigators consider any loss of total body water as dehydration (Feinsod et al, 2004;Gross et al, 1992;Kavouras, 2002;Robinson & Weber, 2004;Shireffs, 2003;Thomas et al, 2008). Others consider dehydration as a loss of body water from the intracellular and interstitial compartments (Mange et al, 1997;Thomas, Tariq, Makhdomm, Haddad, & Moinuddin, 2004). Armstrong (2007) described dehydration more specifically as uncompensated water loss via urine, sweat, feces, and respiratory vapor.…”

“…This is a serious condition which can occur acutely and therefore must be identified and treated quickly and appropriately for a good outcome (Guyton & Hall, 2006). It is accepted throughout the literature that ECF volume is maintained at the expense of a hyperosmolar ICF because without adequate blood flow, organ failure and tissue death can occur (Mange et al, 1997;Thomas et al, 2004;Whitmire, 2008). Thus, although the intravascular portion of the ECF comprises only 25% of the ECF, or 8% of TBW, there are numerous physiological mechanisms in place to closely guard its volume (Kavouras, 2002).…”

“…Hypotonic intravascular volume depletion occurs when more sodium is lost relative to water loss (Mange et al, 1997). This will promote shifting of water into the intracellular space.…”

“…The electrolyte concentration in the cell will be altered and cell swelling will occur. If isotonic fluids are lost and replaced with water alone, the hypotonic vasculature will try to equilibrate with ICF by shifting fluids into cells thereby improving the homeostatic equalization between body compartments while decreasing vascular volume (Mange et al, 1997).…”