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.
PHOTO BY GREGORY KINDRED
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).
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
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.
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).
Table 1: Crystalloid solutions*
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