Intravascular volume expanders are administered to a patient to optimize their hemodynamic status in the perioperative setting. They can be used to maintain euvolemia in a patient who is adequately perfusing, or they can be used to counteract pathologic states such as hypovolemic shock. Ultimately, intravascular volume expanders are used to maintain euvolemia so that the circulatory system can deliver oxygen and nutrients to the vital organs of the body. The two main types of intravenous volume expanders are crystalloids and colloids. This chapter will provide information relevant to both forms of volume expanders as well as general fluid management.
Crystalloids are composed of electrolytes and water. For example, normal saline (NS) is a common crystalloid solution composed of sodium and chloride ions. Other balanced salt solutions include Lactated Ringer’s (LR) and Plasma-Lyte (PL) solutions both of which contain electrolytes as well as the buffers lactate and acetate, respectively. Other crystalloid formulations may contain water-soluble molecules such as dextrose or potassium. NS, LR, and PL are preferred for fluid resuscitation. In contrast, crystalloids such as D5W, ½ NS, and hypertonic saline are given only in special circumstances and not typically used as intravascular volume expanders.
When considering crystalloids, it is important to consider the acidity of the solution. For example, large-volume administration of NS is a risk factor for hyperchloremic acidosis. Hyperchloremia may also contribute to a lower glomerular filtration rate and increases the risk of an acute kidney injury. In contrast, since LR and PL are buffered electrolyte solutions with less chloride than NS, they are less likely to contribute to metabolic acidosis.
NS and PL are preferred when diluting packed red blood cells (pRBCs). pRBCs contain citrate; the calcium in LR can chelate the citrate, thus causing precipitation.
A common misconception is that NS is preferred over LR in end-stage renal disease patients. The theory of using NS in ESRD patients is to avoid administering additional potassium. However, studies have shown that since LR contains 4 mEq/L, the patient’s potassium will only trend to 4 mEq/L with administration of LR. On the other hand, NS can contribute to acidosis, causing an increase in extracellular K+.
On the other hand, NS would be preferred over LR in patients with liver dysfunction since lactate is metabolized by the liver, and therefore accumulation of lactate in this population can elevate serum lactate levels.
It is important to note the distribution of crystalloids after being infused. NS, LR, and PL contain molecules with a smaller molecular weight in relation to the colloids discussed in the following section. Therefore, the aforementioned solutions may have increased movement across the capillary membrane and into the extravascular space. This may contribute toward peripheral and/or pulmonary edema. After administration, only about 20% of a crystalloid bolus will remain intravascular.