Fluid balance concepts in medicine: Principles and practice

Abstract

The regulation of body fluid balance is a key concern in health and disease and comprises three concepts. The first concept pertains to the relationship between total body water (TBW) and total effective solute and is expressed in terms of the tonicity of the body fluids. Disturbances in tonicity are the main factor responsible for changes in cell volume, which can critically affect brain cell function and survival. Solutes distributed almost exclusively in the extracellular compartment (mainly sodium salts) and in the intracellular compartment (mainly potassium salts) contribute to tonicity, while solutes distributed in TBW have no effect on tonicity. The second body fluid balance concept relates to the regulation and measurement of abnormalities of sodium salt balance and extracellular volume. Estimation of extracellular volume is more complex and error prone than measurement of TBW. A key function of extracellular volume, which is defined as the effective arterial blood volume (EABV), is to ensure adequate perfusion of cells and organs. Other factors, including cardiac output, total and regional capacity of both arteries and veins, Starling forces in the capillaries, and gravity also affect the EABV. Collectively, these factors interact closely with extracellular volume and some of them undergo substantial changes in certain acute and chronic severe illnesses. Their changes result not only in extracellular volume expansion, but in the need for a larger extracellular volume compared with that of healthy individuals. Assessing extracellular volume in severe illness is challenging because the estimates of this volume by commonly used methods are prone to large errors in many illnesses. In addition, the optimal extracellular volume may vary from illness to illness, is only partially based on volume measurements by traditional methods, and has not been determined for each illness. Further research is needed to determine optimal extracellular volume levels in several illnesses. For these reasons, extracellular volume in severe illness merits a separate third concept of body fluid balance.

Keywords: Body fluids; Body water; Congestive heart failure; Extracellular volume; Hepatic cirrhosis; Hypertonicity; Hypotonicity; Nephrotic syndrome; Sepsis.

Figure 1

Body compartment volume (L) in the three categories of hyponatremia. I: Normal state: ECFV = 16 L, ICFV = 24 L, serum sodium concentration ([Na]_S) = 140 mmol/L. II: Hypovolemic hyponatremia; IIa: Loss of 8 L of isotonic sodium solution: ECFV = 8 L, ICFV = 24 L, [Na]_S = 140 mmol/L; IIb: Gain of 8 L of water; ECFV = 10 L, ICFV = 30 L, [Na]_S = 112 mmol/L. III: Hypervolemic hyponatremia; IIIa: Gain of 8 L of isotonic sodium solution; ECFV = 24 L, ICFV = 24 L, [Na]_S = 140 mmol/L; IIIb: Gain of 8 L of water; ECFV = 28 L, ICFV = 28 L, [Na]_S = 120 mmol/L. IV: Euvolemic hyponatremia manifested in the syndrome of Inappropriate ADH secretion, which combines water gain and sodium loss[38,39]; IVa: Gain of 8 L of water; ECFV = 19.2 L, ICFV = 28.8 L, [Na]_S = 116.7 mmol/L; IVb: Loss of 560 mmol of monovalent sodium salt (e.g., NaCl); ECFV = 16 L, ICFV = 32 L, [Na]_S = 105 mmol/L. ECFV: Extracellular fluid volume; ICFV: Intracellular fluid volume.

Figure 2

Total body water estimates from anthropometric formulas. Estimates of total body water computed by the Hume et al[63], Watson et al[64] and Chumlea et al[65] anthropometric formulas for men and women with the same age (40 years) and varying height and weight. AA: African American; C: Caucasians.

Figure 3

Average extracellular fluid volume estimates expressed as percentages of total body water. I-V: Tracer dilution estimates[46]; I: Sucrose, thiosulfate; II: Mannitol, sulfate; III: Bromide, chloride; IV: Sodium; V: Thiocyanate; VI-VII: Newer techniques; VI: Dual-energy X-ray absorptiometry[130]; VII: Bioelectrical impedance[115]; VIII: Simultaneous determination of total body potassium and total body water[195]; IX, X: Glomerular filtration rate markers; IX: Inulin[139]; X: Iothalamate[147]. ECFV: Extracellular fluid volume; TBW: Total body water.

Figure 4

The fraction extracellular volume over total body water in elderly subjects with relatively compensated congestive heart failure and healthy controls. Mean values ECFV/TBW in the study of Sergi et al[228]. The mean ejection fraction of elderly patients with relatively compensate congestive heart failure (CHF) and absence of pleural effusion was 40%. Total body water (TBW) was measured by ²H₂O dilution and extracellular volume (ECFV) by bromide dilution. The fraction ECFV/TBW was significantly higher in subjects with CHF.

Figure 5

Percent changes from normal of body fluid compartments in hypervolemic states. I: Normal body fluid state; II: Congestive heart failure, hepatic cirrhosis, nephrotic syndrome with underfill mechanism of fluid retention; III: Nephrotic syndrome with overfill mechanism of fluid retention. EABV: Effective arterial blood volume; ECFV: Extracellular fluid volume; TBW: Total body water.