Fluid Management and Perturbations in Volume Status

Fluid Management and Perturbations in Volume Status is a topic covered in the Washington Manual of Medical Therapeutics.

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  • Total body water (TBW): Water comprises approximately 60% of lean body weight in men and 50% in women and is distributed in two major compartments: two-thirds is intracellular fluid (ICF) and one-third is extracellular fluid (ECF). ECF is further subdivided into intravascular and interstitial spaces in a ratio of 1:4.
    • Example: For a healthy 70-kg man,Descriptive text is not available for this image
      • ICF = 2/3 TBW = 0.66 × 42 = 28 L
      • ECF = 1/3 TBW = 0.33 × 42 =14 L
      • Intravascular compartment = 0.25 × 14 = 3.5 L
      • Interstitial compartment = 0.75 × 14 = 10.5 L
    • The distribution of water between intravascular and interstitial spaces can be affected by changes to the Starling balance of forces. Low oncotic pressure (i.e., low albumin states) and high hydrostatic pressure (i.e., Na+-retentive states) increase the movement of fluid from vascular to interstitial compartments, which is an important step in the development of edema.
    • Because the majority of water is contained in the intracellular space, the loss of free water does not typically result in the hemodynamic changes associated with volume depletion. Instead, disturbances in TBW change serum osmolality and electrolyte concentrations.
    • The intact kidney adapts to changes in TBW by increasing water excretion or reabsorption. This is mediated by the antidiuretic hormone (ADH; vasopressin), which permits water movement across the distal nephron. Although vasopressin release is predominately responsive to osmotic cues, volume contraction can cause a nonosmotic release of ADH resulting in a reduction of renal water excretion.
  • Total body Na+: 85%–90% of total body Na+ is extracellular and constitutes the predominate solute in the ECF. Changes to the body’s total Na+ content typically results from a loss or gain of this Na+-rich fluid, leading to contraction or expansion of the ECF space. Clinically, this manifests as volume depletion (hypotension, tachycardia) and volume expansion (peripheral or pulmonary edema).
    • Na+ concentration is distinct from Na+ content. Na+ concentration reflects the amount of Na+ distributed in a fixed quantity of water. An increase in TBW can decrease the Na+ concentration even if the body’s total Na+ content remains unchanged.
    • The intact kidney can respond to altered Na+ content in the ECF space by increasing or decreasing Na+ reabsorption. This response is mediated by cardiovascular, renal, hepatic, and central nervous system sensors of the effective circulating volume.

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