Fluid balance basics: Then and now
An understanding of fluid balance is based on the distribution of water into various fluid compartments (intravascular, interstitial,
intracellular) and the concentration of salts (electrolytes including sodium, chloride, and potassium) and protein within
these fluid compartments. The complexities of neuroendocrine, renal, and interstitial fluid dynamics including lymph flow,
although appreciated, are often underemphasized.15
For example, the classic Starling equation (Starling's law of the capillary), which emphasizes hydrostatic and osmotic forces,
is generally used to explain fluid balance in the vascular system, Although instructive, this equation is quantitatively inconsistent
with experimental and clinical observations of transvascular fluid flux and edema formation.16
New knowledge about this basic physiologic concept and the mechanisms responsible for intravenous fluid distribution and elimination
continues to emerge and has helped to clarify and redefine best practices for perioperative fluid therapy.17,18
The outdated Starling's law of the capillary has been replaced by the double barrier concept, wherein fluids exit over the
entire length of the capillary.17-19 An endothelial surface layer (ESL), the glycocalyx, is now known to control vascular permeability, thereby serving as the
principal determinant of fluid flux from the vasculature and the development of interstitial edema (Figure 1).19
1. An endothelial surface layer (glycocalyx) is a key factor in determining vascular permeability. Fluid leaves the capillary
throughout its entire length.
The ESL can be degraded by trauma, inflammation (sepsis), and fluid overload, all of which predispose the animal to interstitial
fluid accumulation and edema.19 Importantly, albumin helps to maintain the ESL and vascular fluid homeostasis. Hypoalbuminemia is strongly associated with
a poor outcome, implying that the serum albumin concentration should be maintained within normal limits (> 1.5 g/dl).20
Unfortunately, and as a result of decades of liberal and inadequately monitored fluid administration, the consequences of
fluid overload are well-known (Figure 2).17,18,21,22 They include tissue edema, coagulopathy, pulmonary and renal failure, ileus, delayed wound healing, hypothermia, nosocomial
infections, and abdominal compartment syndrome.
2. Some of the many consequences of fluid overload.