Components of Fluid Balance and Monitoring

Macrocirculation

The desired goal of fluid resuscitation is to improve organ perfusion. To better understand how fluid resuscitation can improve organ perfusion, we have summarized its most important underlying principles. According to the Frank-Starling principle, fluid administration will increase left ventricular stroke volume (SV) only if the bolus increases the mean circulatory filling pressure (MCFP) more than the central venous pressure (CVP), and if both ventricles are on the “ascending limb” of the Frank-Starling curve. The venous system is divided into the “unstressed” and “stressed” volumes. The “unstressed volume” is the intravascular volume filling the venous system until intravascular pressure starts increasing, whereas the “stressed volume” is the volume that stretches the veins and causes intravascular pressure to increase. The MCFP is the pressure distending the vasculature (normal values, 8–10 mm Hg). The stressed venous system is the major determinant of the MCFP, which will strongly influence the venous return.

Organ blood flow corresponds to the difference between the mean arterial pressure (MAP) and the CVP. If the MAP is within an organ's autoregulatory range, then the CVP becomes the major determinant of capillary blood flow. Because the venous system has a large capacitance, an increase in blood volume only minimally changes the MCFP. However, as the distending volume increases, the diastolic compliance of the heart decreases. Therefore, with large fluid administration, the cardiac filling pressures, more importantly the CVP, increase faster than the MCFP, decreasing the gradient for venous return, and therefore blood flow. The kidney is affected particularly by high venous pressure, which increases renal subcapsular pressure and reduces renal blood flow and glomerular filtration rate. Studies in patients with chronic congestive heart failure have shown that intrarenal venous flow (IRVF) pattern seemed to correlate with renal congestion.

Microcirculation

Total body water (TBW) represents approximately 60% of body weight and traditionally has been divided into the intracellular (40%) and extracellular spaces (20%). In the extracellular fluid, the water is distributed in different parts: 75% in the interstitium, 20% in the plasma, and 5% acting as transcellular fluid. For decades, the so-called “third space” was considered as another extravascular compartment. Therefore fluid administration was optimized to replace this “loss” in critically ill and patients undergoing major surgery, in addition to deficits because of insensible perspiration and fasting. The “third space” most probably accumulates in the interstitium because of the destruction of an integral structure of the vascular wall, the endothelial glycocalyx. The endothelial glycocalyx is a network of membrane-bound glycoproteins and proteoglycans acting as a gel-like fringe covering the intravascular side of every healthy vessel ( Table 134.1 ). The glycocalyx retains plasma proteins from moving across the endothelium, prevents leucocyte and platelet aggregation, and limits tissue edema. The glycocalyx contains many antioxidative molecules, such as superoxide dismutase, and contains many receptors that influence cellular activation with neutrophils and macrophages in response to injury. In sepsis and multiorgan failure, glycocalyx shedding increases vascular permeability and contributes to capillary leak and can cause an activation of immune cascades. The type of fluids (albumin, synthetic colloids, or crystalloids) and amount of fluid resuscitation also may have an effect on the integrity of the glycocalyx barrier. An intact endothelial glycocalyx is therefore required for the vascular barrier to function properly.

Phases of Fluid Resuscitation

As previously mentioned, most hospitalized patients will receive intravenous fluids. To characterize the role and timing of fluid administration or removal during hospital stay, four clinical phases of fluid therapy have been conceptualized ( Fig. 134.1 ), including (1) rescue, (2) optimization, (3) stabilization, and (4) deescalation (ROS-D). Rescue, or resuscitation, implies the “administration of fluid for immediate management of life-threatening conditions associated with impaired tissue perfusion.” Optimization or titration refers to the “adjustment of fluid type, rate and amount based upon context to achieve optimization of tissue perfusion.” Stabilization aims for achieving a neutral or slightly negative fluid balance to favor organ support, whereas de-escalation is defined by the “minimization of fluid administration; mobilization of extra fluid to optimize fluid balance.” The characteristics of each phase have been summarized in Table 134.2 .

Fluid boluses are used during the rescue phase, whereas fluid challenges are administered during the optimization phase. A fluid bolus typically includes the infusion of 500 mL over a maximum of 15 minutes without close monitoring, and a fluid challenge involves the administration of 250 mL or 3 mL/kg over 5 to 10 min with SV reassessment. A fluid challenge allows optimization of tissue perfusion and tests the effects of a more modest volume given more slowly to prevent inadvertent fluid overload. Some studies have defined fluid responders as an increase of 10% to 15% of SV or CO after fluid challenge. The concept of mini-fluid challenge also has been proposed and defined as the administration of 100 mL over 1 minute with assessment of changes in SV or CO. This test had a sensitivity and specificity of 95% and 78%, respectively, with an area under the receiver operating characterizing curve (AUROC) of 0.92 to predict fluid responsiveness.

The ability of crystalloids to expand the intravascular volume is limited. In healthy volunteers, only 15% of a crystalloid bolus remained in the intravascular space after 3 hours. In septic patients, less than 5% of a crystalloid bolus remains in the intravascular space after an hour. In the stabilization phase, fluid therapy is used for ongoing maintenance to replace normal fluid losses (i.e., renal, gastrointestinal, insensible) or ongoing losses because of unresolved pathology. Patients may experience stabilization and de-escalation and then move back to the rescue or optimization phases because of a new clinical event, and many hospitalized patients will not require fluid boluses.