Colloid and Crystalloid Resuscitation
This chapter presents the variety of crystalloid and colloid fluids available for use, and describes the salient features of these fluids, both individually and as a group.
I. Crystalloid Fluids
Crystalloid fluids are electrolyte solutions that move freely from the plasma to the interstitial fluid. The principal ingredient in crystalloid fluids is the inorganic salt, sodium chloride.
A. Volume Distribution
Crystalloid fluids distribute uniformly in extracellular fluid: i.e., plasma and interstitial fluid. Since plasma volume is 25% of the interstitial fluid volume (see Table 7.1), then 25% of an infused crystalloid fluid will expand the plasma volume, and 75% of the infused volume will expand the interstitial fluid (1). Thus, the principal effect of crystalloid fluids is to expand the interstitial fluid volume, not the plasma volume.
B. Isotonic Saline
The most widely used crystalloid fluid is 0.9% sodium chloride (0.9% NaCL), better known as normal saline (a misnomer, as will be demonstrated).
1. Features
The notable features of 0.9% NaCL are shown in Table 10.1 (2). When compared to plasma (also included in the table), 0.9% NaCL has a higher sodium concentration (154 vs. 141 mEq/L), a much higher chloride concentration (154 vs. 103 mEq/L), and a lower pH (5.7 vs. 7.4). The only feature of 0.9% NaCL that matches plasma is the measured osmolality. These comparisons show that normal saline is not normal chemically, but it is isotonic with plasma. Therefore, the appropriate name for this fluid is isotonic saline, not normal saline.
Note: The measured osmolalities in Table 10.1 (measured by freezing point depression) provide a more accurate reflection of in vivo osmotic activity than the calculated osmolarities (which are the summed concentrations of all osmotically active species in a fluid). Note that the measured osmotic activities are lower than the calculated (predicted) activities. This discrepancy is caused by electrostatic interactions between ions in the fluid, which reduces the number of osmotically active particles. This deserves mention because the manufacturers of crystalloid fluids use the calculated osmotic activity to describe the in vivo behavior of the fluid.
2. Volume Effects
The volume effects of 0.9% NaCL in plasma and interstitial fluid are illustrated in Figure 10.1.
Infusion of one liter of 0.9% NaCL adds 275 mL to the plasma volume and 825 mL to the interstitial fluid volume (1). This is the volume distribution expected from a crystalloid fluid.
Note that the total increase in extracellular volume in Figure 10.1 (1,100 mL) is slightly greater than the infused volume. The additional 100 mL of extracellular
fluid is the result of a fluid shift from intracellular to extracellular fluid compartments, prompted by the excess sodium in 0.9% NaCL.
Table 10.1 Comparison of Crystalloid Fluids and Plasma | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
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 FIGURE 10.1 The effects of selected intravenous fluids on plasma volume and interstitial fluid volume. The infusion volume of each fluid is shown in parentheses. Data from Reference 1. |
3. Adverse Effects
Interstitial edema is a risk with all crystalloid fluids, but the risk is greatest with isotonic saline (3) because the sodium load exceeds that in other crystalloid fluids (and sodium is the principal determinant of ex-tracellular volume).
Rapid or large-volume infusions of isotonic saline are often accompanied by a hyperchloremic metabolic acidosis (4), which is attributed to the excess chloride in isotonic saline. The pathological significance of this condition has been debated, but there is evidence that hyperchloremia is associated with increased mortality in critically ill patients (5).
Infusions of isotonic saline are accompanied by a decrease in renal perfusion, presumably as a result of chloride-mediated renal vasoconstriction (6). This has raised concern about the potential for isotonic saline to promote acute kidney injury (AKI). However, at least 12 clinical trials have shown no evidence of a causal link between isotonic saline and AKI (4,6).
C. Ringer’s Lactate
Ringer’s solution (introduced in 1880 by Sydney Ringer, a British physician) is a 0.9% NaCL solution that contains potassium and calcium (which were added to promote the viability of frog heart preparations, a research interest of Dr. Ringer). Lactate was later added as a buffer (by Alexis Hartmann, an American pediatrician) to create Ringer’s lactate solution (also known as Hartmann’s solution).
1. Features
The chemical features of Ringer’s lactate are included in Table 10.1. The following comparisons with 0.9% NaCL are significant:
The addition of potassium and calcium (in concentrations that approximate the free or ionized levels in plasma) is balanced by a reduction in the sodium concentration (to 130 mEq/L) to maintain electrical neutrality.
Lactate is added (as sodium lactate) as a buffer, and is metabolized to bicarbonate in the liver. The chemical reaction is as follows:
Note that oxygen is required for this reaction, which means that lactate will not act as a buffer source when tissue hypoxia is present (i.e., in circulatory shock) (2).
The addition of lactate requires a reduction in chloride concentration for electrical neutrality. The chloride concentration in Ringer’s lactate is close to that in plasma, which minimizes the risk of hyperchloremic metabolic acidosis.
The osmolality of Ringer’s lactate is significantly lower than plasma, and is the lowest of the crystalloid fluids. This hypotonicity makes Ringer’s lactate the least desirable crystalloid fluid for patients with cerebral edema, or those at risk for cerebral edema (e.g., traumatic head injuries).
2. Adverse Effects
The calcium in Ringer’s lactate can bind to the citrated anticoagulant in blood products. For this reason, Ringer’s solutions are contraindicated as diluent fluids for the transfusion of packed red blood cells (2). However, clot formation does not occur if the volume of Ringer’s solution does not exceed 50% of the volume of packed RBCs, or if the fluid is infused rapidly (7).
The lactate content in Ringer’s lactate (28 mmol/L) creates the risk of hyperlactatemia, especially when lactate metabolism is impeded (i.e., in liver failure or circulatory shock). This risk was evident in a study of burn patients, where hyperlactatemia was common when Ringer’s lactate was used for fluid management, but not when a lactate-free Ringer’s fluid was used (8).
Considering this risk of hyperlactatemia, and the diagnostic and prognostic value of serum lactate levels in critically ill patients (see Figure 6.2), it seems wise to avoid Ringer’s lactate solution in patients with elevated lactate levels, liver failure, or circulatory shock.
Note: Blood samples withdrawn through catheters being used for Ringer’s lactate infusions can yield spuriously high lactate levels (9).
D. Normal pH Fluids
There are two crystalloid fluids with a pH in the normal, physiological range: Normosol and Plasma-Lyte. The composition of these fluids is identical, and is shown in Table 10.1.
1. Features
The chloride concentration in these fluids (98 mEq/L) is within the normal, physiological range, and they contain magnesium (3 mg/dL) instead of calcium.
These fluids contain both acetate (27 mmol/L), and gluconate (23 mmol/L) as buffers. Gluconate is a weak alkalinizing agent that adds little to the buffer capacity (2), but acetate is rapidly metabolized to bicarbonate in skeletal muscle via the following oxidation reaction:Related posts:
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