In contrast to crystalloids, colloids do not dissolve into a true solution, and therefore do not pass readily across semipermeable membranes. As such, colloids effectively remain in the intravascular space and increase the oncotic pressure of the plasma. This effectively shifts fluid from the interstitial compartment to the intravascular compartment. In clinical practice, these theoretical benefits are generally short-lived (given metabolism of colloidal proteins/sugars) and for most patients there is little therapeutic advantage of colloids over crystalloids or vice versa. Examples of colloids include 5% albumin, 25% albumin, the dextrans, hetastarch, and fresh-frozen plasma (FFP). Because each of these agents contains a substance (proteins and complex sugars) that will ultimately be metabolized, the oncotic agent will be ultimately lost and only the remaining hypotonic fluid delivery agent will remain. As such, use of large volumes of colloidal agents is more likely to induce fluid overload compared to crystalloids. Although smaller volumes of colloids have equal efficacy as larger volumes of crystalloids, they generally must be infused more slowly. Often the net result is that the time to clinical benefit is the same regardless of which class of fluid is utilized. For example, 500 mL of normal saline is required to increase the systolic BP to the same degree as seen with approximately 250 mL of 5% albumin; however, the normal saline can be administered twice as fast.

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