Article Overview of Normal Saline in Fluid Therapy: Mechanisms, and ...

Loss of red blood cells diminishes oxygen-carrying capacity. However, the body increases cardiac output to maintain oxygen delivery (DO2) and increases oxygen extraction. These factors provide a safety margin of about 9 times the resting oxygen requirement. Thus, non–oxygen-carrying fluids (eg, crystalloid or colloid solutions) may be used to restore intravascular volume in mild to moderate blood loss. However, in severe hemorrhagic shock, blood products are required.Early administration of plasma and platelets probably helps minimize the dilutional and consumptive coagulopathy that accompanies major hemorrhage. A ratio of 1 unit of plasma for each 1 unit of red blood cells and each 1 unit of platelets is recommended (1). When the patient is stable, if the hemoglobin is < 7 g/dL (70 g/L), in the absence of cardiac or cerebral vascular disease, oxygen-carrying capacity should be restored by infusion of additional blood (or in the future by blood substitutes). Patients with active coronary or cerebral vascular disease or ongoing hemorrhage require blood when hemoglobin is < 10 g/dL (100 g/L).Crystalloid solutions for intravascular volume replenishment are typically isotonic (eg, 0.9% saline or Ringer's lactate). Water freely travels outside the vasculature, so as little as 10% of isotonic fluid remains in the intravascular space. With hypotonic fluid (eg, 0.45% saline), even less remains in the vasculature, and, thus, this fluid is not used for resuscitation. Both 0.9% saline and Ringer's lactate are equally effective; Ringer's lactate may be preferred in hemorrhagic shock because it somewhat minimizes acidosis and will not cause hyperchloremia. For patients with acute brain injury, 0.9% saline is preferred. Hypertonic saline is not recommended for resuscitation because the evidence suggests there is no difference in outcome when compared to isotonic fluids.Colloid solutions (eg, hydroxyethyl starch, albumin, dextrans) are also effective for volume replacement during major hemorrhage. However, colloid solutions offer no major advantage over crystalloid solutions, hydroxyethyl starch increases risk of renal injury, and albumin has been associated with poorer outcomes in patients with traumatic brain injury. Both dextrans and hydroxyethyl starch may adversely affect coagulation when > 1.5 L is given (2).Blood typically is given as packed red blood cells, which should be cross-matched, but in an urgent situation, 1 to 2 units of type O Rh-negative blood are an acceptable alternative. When > 1 to 2 units are transfused (eg, in major trauma), blood is warmed to 37°C. Patients receiving > 6 units may require replacement of clotting factors with infusion of fresh frozen plasma or cryoprecipitate and platelet transfusion (see also Blood Products).Blood substitutes are oxygen-carrying fluids that can be hemoglobin-based or perfluorocarbons. Hemoglobin-based fluids may contain free hemoglobin that is liposome-encapsulated or modified (eg, by surface modification or cross-linking with other molecules) to limit renal excretion and toxicity. Because the antigen-bearing red blood cell membrane is not present, these substances do not require cross-matching. They can also be stored > 1 year, providing a more stable source than banked blood. Perfluorocarbons are IV carbon-fluorine emulsions that carry large amounts of oxygen. However, no blood substitutes have yet proved to increase survival and some have significant adverse effects (eg, hypotension). Currently, no blood substitutes are commercially available for use. There is ongoing research on other blood substitutes.

Nonhemorrhagic hypovolemia Isotonic crystalloid solutions are typically given for intravascular repletion during shock and hypovolemia. Colloid solutions are generally not used. Patients with dehydration and adequate circulatory volume typically have a free water deficit, and hypotonic solutions (eg, 5% dextrose in water, 0.45% saline) are used.