The ultimate goal of fluid and electrolyte therapy is to maintain a balance between the intracellular and extracellular compartments. This means that often, in caring for the critically ill patient, fluid replacement and electrolyte balance become an afterthought of therapy instead of a goal. However, maintaining this balance is essential for an optimal outcome.1
Many disease states in pregnancy can change the intracellular and extracellular compartments. Preeclampsia is a prime example of how changes in the dynamics of the intracellular and extracellular compartments can affect overall fluid physiology. Common therapies used during pregnancy such as intravenous tocolysis can change electrolyte balance. The purpose of this chapter is to explain basic fluid and electrolyte balance and to provide guidelines for initiating the therapy.
The human body is composed mostly of water. Approximately, 50% of total body weight in an average female is body water. (total body water [TBW] = 1/2 wt. [kg]). Given that, total blood volume increases by 50% during pregnancy with only 20% of that increase attributed to an increase in red cell mass, one can extrapolate that 60% to 65% of the total body weight in pregnancy can be attributed to TBW. The intracellular fluid compartment (ICF) contains 66% of TBW. The extracellular compartment is composed of 34% TBW. The extracellular fluid compartment (ECF) is composed of both intravascular and the interstitial components. Most of the ECF is interstitial (26%), plasma composes 8% (Fig. 25-1). Other examples of ECF include cerebrospinal fluid, synovial fluid, and secretions from the gastrointestinal tract. The fluid compartments are separated by semipermeable membranes. Water and smaller molecules may pass through the membranes; larger colloid substances and proteins are confined to the intravascular space. Hydrostatic pressure assists in maintaining an overall fluid balance as water moves by osmosis, from the area with the lowest concentration of plasma proteins (interstitial compartment) to the area with the highest concentration of plasma proteins (blood).
A breakdown of the human body fluid compartments.
Osmotic activity is the expression of the concentration of solute or the density of solute particles in a fluid. In the ECF, osmotic activity can be defined as the sum of the individual osmotic activities of each solute in the fluid. Plasma osmolality can be calculated using the following formula:
Colloid osmotic pressure (COP) is produced by serum albumin (60%-80%) with fibrinogen and the globulins accounting for the remainder. Normal COP is decreased in pregnancy. While COP can be measured with electronic equipment, the equation in Table 25-1 below can estimate COP by using total protein (TP) in g/dL (Table 25-1).
TABLE 25-1.Colloid Osmotic Pressurea