Crystalloids versus Colloids for the Critical Care/ITU Patient
Introduction: Fluid Balance, Resuscitation, and the Crystalloid-Colloid Debate
Fluid imbalance in the critical care patient may result from several conditions including hypovolemia, normovolemia with maldistribution of fluid and hypervolemia (Kreimeier, 2000, p. 4).
Hypovolemia is a common cause of fluid imbalance and may be induced by blood loss from trauma, or by dehydration due to fluid loss via gastrointestinal illness, fever, complications of diabetes mellitus, or renal dysfunction (Kreimeier, 2000).
The resultant decrease in circulating blood volume may lead to decreased venous return, and in severe cases, to arterial hypotension (Kreimeier, 2000, p. 4).
In addition, catecholamines are released systemically, leading to peripheral vasoconstriction, increased myocardial contractility and tachycardia, thereby attempting to maintain a stable blood pressure in the face of hypovolemia (Kreimeier, 2000, p. 4).
In extreme cases, heart failure may ensue due to the tachycardia coupled with increased oxygen demand by the heart. In later stages, anaerobic metabolism occurs and may lead to acidosis. The combination of myocardial dysfunction and acidosis can, in turn, lead to multi-organ system failure (Kreimeier, 2000, p. 4).
Thus, fluid resuscitation in the hypovolemic critical care patient is an important aspect of care.
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The body has several mechanisms for controlling the distribution of fluids among the intravascular (including intracellular and extracellular) and the interstitial spaces (van Wissen & Breton, 2004, p. 304).
These mechanisms include fluid mechanics, vasodilatation and vasoconstriction, and sympathetic and hormonal responses (van Wissen & Breton, 2004, p. 304).
Fluid mechanics includes oncotic and hydrostatic pressure. It is via oncotic pressure that colloids and hypertonic solutions help maintain vascular volume by attracting fluid into the vascular space by osmosis—attraction of fluid across the capillary membranes toward the area of greater solute concentration (van Wissen & Breton, 2004, p. 305).
The opposite of this is hydrostatic pressure where fluid moves through a capillary by outward force of the fluid on the membrane. Also during hypovolemia, activation of the renin-angiotensin system leads to peripheral vasoconstriction and the reabsorption by the kidney of sodium and water thereby increasing vascular volume (van Wissen & Breton, 2004, pp. 305-306).
Goals of fluid resuscitation. Sufficient oxygenation is the primary focus of fluid therapy. Thus, maintaining adequate pulmonary function and perfusion pressure are important (Kreimeier, 2000, p. 6).
Most important is the maintenance of adequate circulatory volume (Kreimeier, 2000, p. 6).
The main goal of fluid resuscitation is to maintain perfusion to the brain and heart by restoring intravascular volume (Diehl-Oplinger & Kaminski, 2004, p. 52).
Two main factors that affect the choice of fluid for volume replacement are how the fluid was lost and which solutes need to be replaced (Diehl-Oplinger & Kaminski, 2004, p. 52).
The debate. The basic debate among proponents of each type of fluid is based on each side’s view of the potential negative effects of the other’s preferred fluid (Grocott & Hamilton, 2002, p. 5).
Some argue that cost is central in the fluid resuscitation debate. It is reported that albumin is responsible for a substantial portion of some hospitals’ budgets (Vincern, 2000, p. S33).
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Those in the crystalloid camp also cite the hemodynamic derangements, increased potential for intravascular volume overload, and the potential for severe allergic reactions caused by colloids (Grocott & Hamilton, 2002, p. 5).
Those who favor colloids point out the large volumes required to achieve the desired effect and the resultant tissue edema and potential for poor organ perfusion that may result from infusion of necessarily large volumes of crystalloid fluids (Grocott & Hamilton, 2002, p. 5).
Crystalloids are fluids, such as Ringers and 0.9% sodium chloride, that closely mimic the body’s extracellular fluid (ECF; Diehl-Oplinger & Kaminski, 2004, p. 53).
These can be used to expand both intracellular and extracellular volume. Crystalloids can be isotonic, hypertonic, or hypotonic (Diehl-Oplinger & Kaminski, 2004, p. 53).
Isotonic fluids—which have the same tonicity of as plasma—can be used to restore intravascular volume without changing plasma electrolyte concentration or altering fluid shifts between intracellular and extracellular spaces (Diehl-Oplinger & Kaminski, 2004, p. 53).
Hypertonic fluids draw fluid from inside the cells into the extracellular space causing cells to shrink and increasing the volume of ECF (Diehl-Oplinger & Kaminski, 2004, p. 53).
Conversely, hypotonic fluids help move fluid from the extracellular space into the cells. Overly aggressive use of crystalloids for fluid replacement may lead to volume overload, electrolyte imbalances, coagulopathy, and heart failure (Diehl-Oplinger & Kaminski, 2004, p. 54).
Colloids contain solutes that are too large to pass through capillary membranes (Diehl-Oplinger & Kaminski, 2004, p. 53).
Thus, colloids help draw fluid from both the interstitium and intracellular space in order to increase intravascular volume (Diehl-Oplinger & Kaminski, 2004, p. 53).
Colloids usually consist of dextrans, gelatins, starches, or human albumin solutions (Grocott & Hamilton, 2002, p. 2).
Colloids have a similar effect on fluid shifting as hypertonic crystalloids, but can be given in smaller volumes and have a longer duration of action (Diehl-Oplinger & Kaminski, 2004, p. 53).
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It is recommended to use colloids when crystalloids are inadequate, as a plasma replacement in hypovolemic shock, and in patients with third-spacing of fluid into the interstitial spaces (Diehl-Oplinger & Kaminski, 2004, p. 53).
Colloids enhance blood volume, improve hemodynamics, and may reduce the need for blood transfusions (Diehl-Oplinger & Kaminski, 2004, p. 53).
However, colloids may precipitate allergic reactions including anaphylactic shock (Diehl-Oplinger & Kaminski, 2004, p. 54; Grocott & Hamilton, 2002, p. 4).
Risks and Benefits Associated with Each Fluid Type
A major concern with fluid resuscitation in the hypervolemic patient is the possibility of fluid overload resulting in pulmonary edema (Kreimeier, 2000, p. 6).
Different replacement fluids will produce varying effects on volume expansion. To provide significant volume expansion using crystalloids, it is necessary to infuse large volumes of fluids (Grocott & Hamilton, 2002, p. 1).
This may lead to significant tissue edema caused by accumulation of fluid into the interstitial compartment. This could, in turn, lead to organ failure because of reduced perfusion due to the compression of capillaries and increased diffusion distances within the tissues; (Grocott & Hamilton, 2002, pp. 1-2).
Some essential characteristics of colloids are that they persist intravascularly, they cause hemodilution of clotting factors, and they decrease blood viscosity (Grocott & Hamilton, 2002, pp. 3-4).
The choice of fluid may affect some pathophysiologic processes. For example, in shock-specific microcirculatory disorder, synthetic colloids (as opposed to crystalloids) may decrease post-ischemic microvascular leukocyte adherence (Kreimeier, 2000, p. 5).
Research Comparing Colloids and Crystalloids
An early systematic review of RCTs of crystalloids vs. colloids was presented by Schierhout and Roberts (1998).
They aimed to determine the effect of these fluids on mortality in critically ill patients who were randomly assigned to receive either colloids or crystalloids for resuscitation (p. 961).
They included studies of burn and trauma patients, surgical patients, and patients with other conditions such as sepsis (p. 961).
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They excluded trials where fluids were given for reasons other than fluid resuscitation (p. 962).
All-cause mortality at the end of each trial’s defined follow-up period was the principal outcome of the systematic review. The analysis was based on data for 1315 patients in 19 trials (p.962).
The results suggested an increased risk between 4% and 7% depending on whether studies without “adequate allocation concealment” were included in the analysis (p. 962).
The authors concluded that the increase in deaths after colloid resuscitation, the non-association of colloids with improved survival, and the increased cost of colloids over crystalloids suggests that their use outside of RCTs is not warranted (p. 964).
Further, they did not find evidence of differences of effect among different types of patients needing fluid resuscitation (p. 962).
A similar systematic review was published in 1999 examining the effects of isotonic crystalloids versus colloids in fluid resuscitation on pulmonary edema, mortality and length of stay (Choi, Yip, Quinonez & Cook, 1999, p. 200).
This review found no differences between the two fluid types with respect to pulmonary edema, length of stay, or mortality (p. 206).
The results did suggest an apparent lowered mortality when using crystalloids in trauma patients (Choi et al., 1999, p. 207).
However, the authors do concede that data from studies to date had not provided sufficient power to draw any clinically significant conclusions about pulmonary edema or mortality based on fluid choice (p. 206).
They also propose confounding factors such as comorbidity, severity of the acute illness, and ITU-acquired complications that may have a stronger effect on mortality than the choice of colloid or crystalloid fluid therapy (p. 208).
In an update to the 1998 review, Roberts and colleagues performed a similar analysis, except that they stratified by fluid type rather than the patient’s injury type (Roberts, Bunn, Chinnock, Kre & Schierhout, 2005, no pagination).
They found that the pooled relative risks did not show a benefit on mortality of colloids over crystalloids. Further, they suggest that the danger, if any, from colloids may have been overstated in the previous meta-analysis (new relative risk of 1.02 vs. 1.29 in the analysis published in 1998).
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However, the authors still assert that the increased costs of colloids do not justify their use outside of RCTs in specific subsets of patients Roberts et al., 2005).
A double-blind randomized controlled trial of the use of albumin versus saline in 6997 critically ill patients was recently reported by the SAFE Study Group (2004).
Subjects receiving albumin had a 0.99 relative risk of death when compared with those receiving saline (p. 2252).
Basically, use of either fluid resulted in a similar number of deaths from all causes in a large, heterogeneous sample of ICU patients (p. 2253).
These results do not support the results of the early Cochrane Injuries Group meta-analysis that albumin fluid resuscitation is associated with increased mortality in critically ill patients (Schierhout & Roberts, 1998).
Evaluation of the Findings
Considering the above evidence, it does not appear that a consensus is at hand. Grocott and Hamilton (2002, p. 5) assert that none of the randomized clinical trials (RCTs) designed to answer this debate were sufficiently powered to answer whether either fluid choice leads to decreased mortality. These authors also point out that the recent meta-analyses of the RCTs combined results from studies using many different replacement fluids for varying indications, on too large a variety of patients to be meaningful (Grocott & Hamilton, 2002, p. 5).
One author presented a review of three systematic reviews including the two published in the late 1990’s as reported above (Webb, 2000).
This author points out that the relative risk reported by Schierhout and Roberts (1998) was not statistically significant (Webb, p. S27).
Also, heterogeneity of the included RCTs was a problem (p. S27).
This author reported that although the results of the Schierhout and Roberts analysis and that of Choi et al. (1999) were similar, the latter group was more conservative in their interpretation of the data (Webb, p. S27).
Shared limitations of the reviews included heterogeneous patient samples, use of disparate fluids, varying endpoints, failure to consider between-group differences, differing fluid management approaches, and failure to consider concomitant therapies (Webb, p. S28).
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Also, none of the included trials incorporated blinding into their designs. Webb concludes that the importance should be on how fluids are used, not on which fluid to use (p. 30).