Crystalloids or colloids?

It is up to you to choose which intravenous fluid is best in each of your surgical candidates. Here is vital information to help you decide.
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Feb 01, 2013


PHOTO BY GREGORY KINDRED
Exactly how to make the decision to use a crystalloid or colloid fluid for the perioperative prevention or treatment of hypotension and hypovolemia is still undefined. What has become evident is that the negative effects of fluid overload increase postoperative morbidity and mortality regardless of fluid choice. Knowing the fundamental differences between and the potential benefits of the various types of crystalloids and colloids is essential to their selection as replacement therapy in surgical candidates.

This article reviews salient aspects for the selection of fluids for intravenous perioperative fluid therapy. Intravenous fluid solutions are broadly classified as crystalloids (salts in water) or colloids (noncrystalline substances consisting of large molecules diluted in a crystalloid). The capillary endothelium is freely permeable to water and dissolved salts (e.g. sodium, chloride) but is relatively impermeable to larger molecules (> 30,000 Da).

CRYSTALLOIDS

Sodium chloride

Normal saline solution (0.9% sodium chloride), often referred to as physiological saline solution, has a tonicity (osmotic pressure) similar to that of plasma (308 mOsm/L) but is actually a nonphysiological, chloride-rich, unbalanced salt solution that, when given intravenously in large volumes (> 30 ml/kg), produces metabolic acidosis, has the potential to impair renal blood flow, and predisposes the animal to postoperative vomiting.1-3

The perioperative administration of 0.9% sodium chloride produces a higher risk of morbidity and mortality and is more likely to produce adverse events in surgical patients than balanced crystalloid solutions.4

Balanced solutions

Physiological and balanced salt solutions possess electrolyte concentrations that have a tonicity similar to that of plasma (290 to 310 mOsm/L: isotonic) and help maintain physiological pH, respectively. Conventional crystalloid replacement solutions are designed to mimic plasma and contain molecules (lactate, acetate, gluconate; ≥ 24 to 28 mEq/L) that can be converted to bicarbonate5,6 in order to minimize changes in plasma pH (approximately 7.4).6

Balanced crystalloid solutions help maintain normal hydration, electrolyte concentrations, and acid-base balance during surgery; however, their ability to improve hemodynamics (cardiac filling pressures, arterial blood pressure, cardiac output) and tissue perfusion (microcirculatory blood flow) is highly variable and generally transient.7,8 Keep in mind that all isotonic crystalloids produce increases in blood flow (cardiac output) and small decreases in arterial blood pressure because of a decrease in plasma viscosity.9,10

Hypotonic and hypertonic crystalloids add water to or extract water from the extravascular fluid compartment, respectively.


Table 1: Crystalloid solutions*
Hypotonic solutions. Examples of minimally hypotonic and isotonic replacement solutions administered to treat hypovolemia and hypotension include lactated Ringer's solution (Abbott Animal Health), Plasma-Lyte A (Baxter), and Normosol R (Hospira, Inc.) (Table 1).

Hypotonic and isotonic crystalloid solutions are poor plasma (volume) expanders. Studies in anesthetized dogs administered an intravenous crystalloid at rates equal to or exceeding 1 ml/kg/min (> 60 ml/kg/hr) suggest that 40% to 75% of the infused volume is retained in the vascular compartment immediately after infusion but that this value decreases to less than 25% within 20 to 30 minutes.8,11

Hypertonic solutions. Hypertonic crystalloid solutions are considered to be plasma expanders because their tonicity causes water to move from interstitial and intracellular sites into the intravascular compartment, thereby increasing plasma volume.12 Hypertonic saline solution (5% to 7.5% sodium chloride) is a unique crystalloid demonstrated to rapidly expand plasma volume. It produces acute beneficial hemodynamic, immunologic, and microcirculatory effects when administered in comparatively low total volumes (3 to 4 ml/kg), thereby decreasing the potential for adverse effects (see "Sodium chloride" above) and volume overload.12,13

Hypertonic saline solution has also been advocated as therapy for traumatic brain injury.4 However, questions about the effects of hypertonic saline solutions on mortality in people remain unresolved and require further investigation.12,13