A volume expander is a type of intravenous therapy that has the function of providing volume for the circulatory system. It may be used for fluid replacement or during surgery to prevent nausea and vomiting after surgery.
Physiology
When blood is lost, the greatest immediate need is to stop further blood loss. The second greatest need is replacing the lost volume. This way remaining red blood cells can still oxygenate body tissue. Normal human blood has a significant excess oxygen transport capability, only used in cases of great physical exertion. Provided blood volume is maintained by volume expanders, a rested patient can safely tolerate very low
Hematocrit, less than 1/3 that of a healthy person.
The body detects the lower hemoglobin level, and compensatory mechanisms start up. The heart pumps more blood with each beat. Since the lost blood was replaced with a suitable fluid, the now diluted blood flows more easily, even in the small vessels. As a result of chemical changes, more oxygen is released to the tissues. These adaptations are so effective that if only half of the red blood cells remain, oxygen delivery may still be about 75 percent of normal. A patient at rest uses only 25 percent of the oxygen available in their blood. In extreme cases, patients have survived with a hemoglobin level of 2 g/dl, about 1/7 the normal, although levels this low are very dangerous.
With enough blood loss, ultimately red blood cell levels drop too low for adequate tissue oxygenation, even if volume expanders maintain circulatory volume. In these situations, the only alternatives are blood transfusions, packed red blood cells, or Blood substitute (if available). However, in some circumstances, hyperbaric oxygen therapy can maintain adequate tissue oxygenation even if red blood cell levels are below normal life-sustaining levels.
Types
There are two main types of volume expanders: crystalloids and colloids. Crystalloids are
of mineral salts or other water-soluble molecules. Colloids contain larger insoluble molecules, such as
gelatin;
blood itself is a colloid. There is no evidence that colloids are better than crystalloids in those who have had trauma, burns, or surgery. Colloids are more expensive than crystalloids. As such, colloids are not recommended for volume expansion in these settings.
Crystalloids
The most commonly used crystalloid fluid is
normal saline, a solution of
sodium chloride at 0.9% concentration, which is close to the concentration in the blood (
Isotonicity).
or
Ringer’s acetate is another isotonic solution often used for large-volume fluid replacement. The choice of fluids may also depend on the chemical properties of the medications being given.
Normal saline
Normal saline (NS) is the commonly used term for a solution of 0.9% w/v of
sodium chloride, about 300
Osmolality/L.
[Note that in chemistry, a one normal of NaCl (see normality) is 0.5 molar (see molarity) NaCl assuming complete dissociation. Physiological dissociation is approximately 1.7 ions per mole, so one normal of NaCl is 1/1.7 = 0.588 molar. This is roughly 4 times more concentrated than medical "normal saline" of 0.154 molar.] Less commonly, this solution is referred to as
physiological saline or
isotonic saline, neither of which is technically accurate. NS is used frequently in
(IVs) for patients who cannot take fluids orally and have developed or are in danger of developing
dehydration or
hypovolemia. NS is typically the first fluid used when hypovolemia is severe enough to threaten the adequacy of blood circulation, and has long been believed to be the safest fluid to give quickly in large volumes. However, it is now known that rapid infusion of NS can cause metabolic acidosis.
Ringer's solution
Lactated Ringer's solution contains 28 mmol/L lactate, 4 mmol/L K
+ and 1.5 mmol/L Ca
2+. It is very similar – though not identical – to Hartmann's solution, the ionic concentrations of which differ slightly. Ringer's lactate contains calcium, which may combine with citrate (that is used as a preservative in some blood products) and form a precipitate; therefore, it is not to be used as a diluent in blood transfusions.
1/3 NS 2/3D5
2/3 1/3 is no longer recommended as a maintenance intravenous fluid in children as it is hypotonic and isotonic fluids are preferred.
Glucose (dextrose)
Intravenous sugar solutions, such as those containing
glucose (also called dextrose), have the advantage of providing some energy, and may thereby provide the entire or part of the energy component of parenteral nutrition.
Types of glucose/dextrose include:
-
D5W (5% dextrose in water), which consists of 278 mmol/L dextrose
-
D5NS (5% dextrose in normal saline), which, in addition, contains normal saline.
Plasma-lyte
Plasma-lyte A
and
Plasma-lyte are buffered crystalloid solutions
[
]
Comparison table
| + Composition of common crystalloid solutions
!Solution
!Other Name
!Na+(mmol/L)
!Cl−(mmol/L)
!Glucose(mmol/L)
!Glucose(mg/dl) |
| D5W | 5% Dextrose | 0 | 0 | 278 | 5000 |
| 2/3 D5W & 1/3 NS | 3.3% Dextrose / 0.3% saline | 51 | 51 | 185 | 3333 |
| Half-normal saline | 0.45% NaCl | 77 | 77 | 0 | 0 |
| Normal saline | 0.9% NaCl | 154 | 154 | 0 | 0 |
| Ringer's lactate | Lactated Ringer | 130 | 109 | 0 | 0 |
| D5NS | 5% Dextrose, Normal Saline | 154 | 154 | 278 | 5000 |
| + Effect of adding one litre
!Solution
!Change in ECF
!Change in ICF |
| D5W | 333 mL | 667 mL |
| 2/3 D5W & 1/3 NS | 556 mL | 444 mL |
| Half-normal saline | 667 mL | 333 mL |
| Normal saline | 1000 mL | 0 mL |
| Ringer's lactate | 900 mL | 100 mL |
Colloids
Colloids preserve a high colloid osmotic pressure in the blood, while, on the other hand, this parameter is decreased by crystalloids due to hemodilution.[ Another difference is that crystalloids generally are much cheaper than colloids.][ Common colloids used in the medical context include albumin and fresh frozen plasma.
]
Hydroxyethyl starch
Hydroxyethyl starch (HES/HAES, common trade names: Hespan, Voluven) is controversial. Its use in those who are very ill is associated with an increased risk of death and kidney problems. Therefore, HES/HAES is not recommended in people with known inflammatory conditions such as renal impairment.
Gelofusine
Gelofusine is a colloid volume expander which may be used as a blood plasma replacement when a significant amount of blood is lost due to extreme hemorrhaging, trauma, dehydration, or a similar event. It is an intravenous colloid that behaves much like blood filled with albumins. As a result, it causes an increase in blood volume, blood flow, cardiac output, and oxygen transportation.
