Why Isotonic Fluids?
This figure shows the two major water compartments in the human body and demonstrates where isotonic and hypotonic intravenous crystalloid fluids distribute within these compartments. For clinical purposes, total body water (TBW) is distributed into two compartments: the intracellular fluid (ICF) compartment (2/3 of TBW) and the extracellular fluid (ECF) compartment (1/3 of TBW). Within the ECF there are two compartments, the interstitial fluid (ISF) compartment or fluid surrounding cells (75% of ECF or 25% of TBW) and the intravascular fluid compartment or plasma (25% of ECF or 8% of TBW). In dengue patients with increased vascular permeability, plasma leaks from the intravascular compartment to the ISF. As a result, instead of a 75%-25% ratio of ISF to intravascular compartment, respectively, the ratio can be 80%-90% to 20%-10%. Physicians need to give dengue patients fluids that contain the same amount of electrolytes as the plasma, such as isotonic crystalloids or solutions that are more concentrated than plasma, for example, a colloid to restore the plasma volume while minimizing fluid overload.
The goal is to get as much IVF to stay in vascular space as possible to minimize risk of fluid overload. One liter of 0.9NS results in 250 cc in the vascular space whereas one liter of half-normal saline only 83 cc will go to vascular space.