All authors: Division of Allergy, Pulmonary, and Critical Care Medicine, Vanderbilt University School of Medicine, Nashville, TN.
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Fluid resuscitation with crystalloid solutions is among the most common interventions for hospitalized patients. Currently, providers choose between two classes of available crystalloid solutions: 0.9% sodium chloride (saline) and “balanced” crystalloids (such as lactated Ringer solution [ Baxter, Deerfield, IL], Hartmann solution [B. Braun Melsungen AG, Melsungen, Germany], or Plasma-Lyte [Baxter, Old Toongabbie, NSW, Australia]). Although often used interchangeably, saline and balanced crystalloids differ in composition in ways that may impact patients. Given the similar availability and cost of each fluid, along with mounting evidence linking saline to metabolic derangements, acute kidney injury, and mortality, we will argue in this viewpoint that saline should not be the first choice for crystalloid resuscitation fluid.BACKGROUNDCrystalloids are solutions of ions which determine fluid tonicity but are freely permeable through capillary membranes. Early crystalloid solutions comprised of sodium, chloride, and bicarbonate in water were first prepared for treatment of cholera during the 1832 pandemic (1). Addition of calcium and potassium by Sydney Ringer in the 1870s and lactate by Alexis Hartmann in the 1930s laid the foundation for the balanced crystalloids available today. In contrast, the composition of saline derives from Hartog Hamburger’s demonstration in the 1880s that a salt concentration of 0.9% minimized in vitro erythrocyte lysis (1) and how exactly 0.9% sodium chloride entered into clinical use remains unclear.With 154 mmol/L each of sodium and chloride, saline is isotonic to extracellular fluid but contains a chloride concentration 50% higher than plasma and a strong ion difference of zero (Table 1). As a result, rapid administration of large volumes of saline reliably produces a hyperchloremic metabolic acidosis (2). In contrast, the chemical composition of balanced crystalloids is designed to approximate that of extracellular fluid (Table 1). By replacing a portion of the chloride content with bicarbonate, or rapidly metabolized/excreted organic anions, such as L-lactate, acetate, or gluconate, balanced crystalloids provide a more physiologic chloride concentration and strong ion difference. These differences in crystalloid composition have long been known to affect patients’ serum chloride levels and acid-base balance, but mounting data suggest that crystalloid choice may also directly impact organ function and even survival.SALINE CAUSES HYPERCHLOREMIA, ACIDOSIS, AND DECREASED RENAL PERFUSIONAmong healthy volunteers (3, 4), patients undergoing elective surgery (2), and the critically ill (5, 6), rapid IV infusion of saline reliably induces hyperchloremia and metabolic acidosis, each of which is independently associated with increased mortality in at-risk patients (7, 8). The acidosis arising from saline administration may confound interpretation of underlying acid-base disorders, increase interventions by providers to correct pH (9), and potentially prolong length of stay (10). Saline also decreases renal blood flow and glomerular filtration rate (GFR) (3, 11, 12). In animal models, intrarenal infusion of chloride-rich solutions decreases renal blood flow and GFR compared with low-chloride solutions of equal tonicity (11, 12). A recent study of healthy human volunteers showed a significant reduction in renal artery flow velocity and renal cortical tissue perfusion after infusion of 2 L of saline over an hour, but not after Plasma-Lyte infusion (3). A similar trial demonstrated increased renal cortical tissue perfusion with starch solutions prepared in balanced crystalloids rather than saline (13). Saline may diminish renal perfusion by multiple mechanisms. Increased chloride delivery to the distal tubule may stimulate tubuloglomerular feedback, inducing afferent arteriolar vasoconstriction and reducing GFR. Additionally, saline infusions appear to cause more intracapsular renal edema than balanced crystalloids, which may further compromise renal tissue perfusion (3).