Dextrans are large molecules composed of chains of sugar molecules. They are formed as a means of storing food by bacteria and yeast and are commonly found in dental plaque. In purified form, certain dextrans can be used intravenously (IV) to expand a patient’s blood volume in medical emergencies where blood pressure drops to dangerously low levels. They work because the molecules are too large to leak out through blood vessel walls and because they are “osmotically active,” meaning they hold water within blood vessels and help maintain blood pressure.
When given by an IV, dextrans have several pharmacologic (drug-like) effects, including some that inhibit blood clotting (anti-platelet and anti-fibrin effects). They also act to decrease blood viscosity and impede aggregation of red blood cells, which can improve circulation through small vessels and capillaries.
While dextran solutions may be helpful to maintain blood pressure in a patient with acute bleeding, their basic action is to dilute the remaining red blood cells by retaining more fluid (plasma) within the blood vessels of the body. In doing so, they may improve circulation, but they are not capable of providing additional oxygen-carrying capacity.
Saline and Ringer’s solutions are also used to expand blood volume in medical emergencies with very low blood pressures. Saline is basically salt water, in which the concentration of salt (sodium chloride) is adjusted to be the same as that normally found in the blood stream. Ringer’s solution is similar, but also contains potassium and calcium in physiologic concentrations along with sodium and chloride. Both products act in the same manner to expand and dilute the blood volume when given by IV. Neither increases oxygen-carrying capacity, and both pass freely out of the blood vessels into the surrounding tissues, so their effect on maintaining blood volume is transient and not sustained. They are eventually lost in the urine, but may cause tissue swelling (edema), especially when kidney function is compromised.
Hetastarch, also known as hydroxyethyl starch, and pentastarch are solutions of large molecules composed of many smaller sugar molecules. Their action is essentially the same as that described above for dextrans, in that they hold water within the blood vessels and expand the blood volume and help maintain blood pressure. As with other non-blood volume expanders, hetastarch and pentastarch do not contribute to the blood’s oxygen-carrying capacity.
These products can significantly dilute the concentration of red blood cells in the blood and, if given in large volumes, can lead to volume overload and congestive heart failure. Elimination is by excretion in the urine. Therefore, caution must be used in patients with compromised kidney function. They also interfere with clotting mechanisms, which may worsen any pre-existing bleeding problems.
Allergic reactions with hives, wheezing, and rarely, severe drops in blood pressure can also be seen with these products. These reactions are more common in patients who are allergic to corn. Safety for use in pregnancy and in children has not been established.