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Relative and absolute hypovolemia complicate many clinical conditions, and fluid therapy is a cornerstone of acute critical illness management. The clinician is constantly tasked with assessment of volume status, the need for fluid therapy, and selection of the appropriate fluid and dose guided to a suitable endpoint. Timely fluid therapy maintains macrocirculatory and microcirculatory support and reduces morbidity and mortality.13 In contrast, both under-resuscitation and over-resuscitation adversely affect outcome; inadequate resuscitation risks leaving a patient in compensated shock, and overly aggressive fluid administration results in volume overload without improving oxygen delivery and is associated with worse clinical outcomes.4,5 A thorough understanding of the appropriate selection, timing, and goals of fluid therapy is vital to optimize patient care.

Fluid Distribution and Movement

Water is the most abundant constituent of the body, comprising between 50% and 70% of total body weight. Variations in total body water (TBW) depend primarily on lean body mass, since fat and other tissue contain very little water (Table 47-1). Water is distributed within both intracellular fluid (ICF) and extracellular fluid (ECF) compartments. The distribution of water in an average adult male is shown in Table 47-2. The intracellular space contains two thirds of the TBW, with the remainder distributed to the extracellular space, which is further divided into interstitial and intravascular spaces in a 3:1 ratio. These fluid compartments are not contiguous, but may be treated as such due to similar composition and behavior.

Table 47-1. Total Body Water Estimates
Table 47-2. Size and Composition of Body Fluid Compartments (Values Based on 70-kg Male)

Water freely crosses cell membranes. Osmotic forces within fluid compartments determine water distribution within the body. Intracellular and ECF environments are iso-osmolar, but physiochemically distinct due to tight regulation of dissolved solutes and proteins. Membrane-bound sodium–potassium-ATPase pumps compartmentalize sodium and potassium to the extracellular and intracellular spaces, respectively. Active restriction of sodium to the extracellular space is the foundation for isotonic sodium-based resuscitation solutions.

The intravascular fluid, or plasma, differs from all other fluid compartments in that it exists as a single continuous fluid ...

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