Volume expander

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.

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 quiescent patient can safely tolerate very low haemoglobin levels, less than 1/3 that of a healthy person.

The body automatically detects the lower haemoglobin 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 his blood. In extreme cases, patients have survived with a haemoglobin level of 2 g/dl, about 1/7 the norm, 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 oxygen therapeutics (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 aqueous solutions of mineral salts or other water-soluble molecules. Colloids contain larger insoluble molecules, such as gelatin; blood itself is a colloid.

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.^[1] Therefore, they should theoretically preferentially increase the intravascular volume, whereas crystalloids also increase the interstitial volume and intracellular volume. However, there is still controversy as to the actual difference in efficacy due to this difference in action.^[1] Another difference is that crystalloids generally are much cheaper than colloids.^[1]

Hydroxyethyl starch

Main article: Hydroxyethyl starch

Hydroxyethyl starch (HES/HAES, common trade names: Hespan, Voluven) is one of the most frequently used colloids. An intravenous solution of hydroxyethyl starch is used to prevent shock following severe blood loss caused by trauma, surgery, or some other problem. It increases the blood volume, allowing red blood cells to continue to deliver oxygen to the body.

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 (isotonic). Ringer's lactate or Ringer's acetate is another isotonic solution often used for large-volume fluid replacement. A solution of 5% dextrose in water, sometimes called D5W, is often used instead if the patient is at risk for having low blood sugar or high sodium. The choice of fluids may also depend on the chemical properties of the medications being given.

Intravenous fluids must always be sterile.

Ringer's solution

Lactated Ringer's solution contains 28 mmol/L lactate, 4 mmol/L K⁺ and 1.5 mmol/L Ca²⁺. It is very similar - though not identical to - Hartmann's Solution, the ionic concentrations of which differ.

Ringer's acetate consists of 28 mmol/L acetate, 4 mmol/L K⁺ and 1.5 mmol/L Ca²⁺.

Normal saline

Main article: Saline (medicine)

Normal saline (NS) is the commonly-used term for a solution of 0.91% w/v of NaCl, about 300 mOsm/L.^[2] Less commonly, this solution is referred to as physiological saline or isotonic saline, neither of which is technically accurate. NS is used frequently in intravenous drips (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.^[3]

Glucose (dextrose)

Intravenous sugar solutions, such as with 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.

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
2/3D & 1/3S	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
2/3D & 1/3S	556 mL	444 mL
Half-normal saline	667 mL	333 mL
Normal saline	1000 mL	0 mL
Ringer's lactate	900 mL	100 mL

References

1. ^ ^{a b c} An Update on Intravenous Fluids by Gregory S. Martin, MD, MSc
2. ^ 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.
3. ^ Prough, DS; Bidani, A (1999). "Hyperchloremic metabolic acidosis is a predictable consequence of intraoperative infusion of 0.9% saline". Anesthesiology 90 (5): 1247–1249. doi:10.1097/00000542-199905000-00003. PMID 10319767. http://www.anesthesiology.org/pt/re/anes/fulltext.00000542-199905000-00003.htm;jsessionid=L97P825yCKJn2HYBhbyhzynlZF1lTJQyHGR1JNK7nHTvscph2xfr!536197444!181195628!8091!-1. ^{[dead link]}