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Crystalloids Versus Colloids: Exploring Differences in Fluid Requirements by Systematic Review and Meta-Regression

Orbegozo Cortés, Diego MD; Gamarano Barros, Teresa MD; Njimi, Hassane MSc, PhD; Vincent, Jean-Louis MD, PhD

doi: 10.1213/ANE.0000000000000564
Critical Care, Trauma, and Resuscitation: Research Report

BACKGROUND: Positive fluid balance has been associated with worse outcomes, and knowledge of differences in the amounts of different types of fluid needed to achieve the same end points may have important clinical implications. Large molecules persist longer in the blood vessels than smaller molecules, such that less IV colloid may be needed to achieve similar hemodynamic end points compared with crystalloid. Recent clinical data have, however, challenged this physiological concept, with investigators reporting lower-than-expected crystalloid/colloid ratios in various populations.

METHODS: We performed a systematic search in MEDLINE, EMBASE, and CENTRAL up to December 18, 2013, to retrieve all studies comparing (any) crystalloid with (any) colloid in all types of patients. The crystalloid/colloid ratio was calculated for each study. Descriptive analysis was performed for all studies, and a meta-analysis was performed in those studies reporting full data (in terms of means and standard deviations) of infused fluid volumes. Studies were grouped according to study and population characteristics. A meta-regression analysis was then performed to evaluate some of the possible reasons for differences in crystalloid/colloid ratios across studies.

RESULTS: From 976 studies, 48 were retained for the final analysis; 24 of the studies had sufficient data for meta-analysis. The crystalloid/colloid ratio across all the studies included in the meta-analysis was 1.5 (95% confidence interval, 1.36–1.65) with marked heterogeneity among studies (I2 = 94%). From the meta-regression analysis, decade of publication across all publications (P = 0.001) and concentration (tonicity) in the subgroup of albumin studies (P = 0.001) were associated with the administered crystalloid/colloid ratio. The reduction in heterogeneity among studies for all publications in the meta-regression was minimal, with the maximal decrease obtained when decade of publication was considered (R2 = 12%).

CONCLUSIONS: Greater fluid volumes are required to meet the same targets with crystalloids than with colloids, with an estimated ratio of 1.5 (1.36–1.65), but there is marked heterogeneity among studies. The crystalloid/colloid ratio seems to have decreased over the years, and differences in ratios are correlated with the concentration of albumin solutions; however, the main reasons behind the high heterogeneity among studies remain unclear.

From the Department of Intensive Care, Erasme Hospital, Université Libre de Bruxelles, Brussels, Belgium.

Accepted for publication October 12, 2014.

Funding: No external funding.

The authors declare no conflicts of interest.

Reprints will not be available from the authors.

Address correspondence to Jean-Louis Vincent, MD, PhD, Department of Intensive Care, Erasme Hospital, Université Libre de Bruxelles, Route de Lennik 808, 1070 Brussels. Address e-mail to jlvincen@ulb.ac.be.

Because of their large molecular weight (MW) and difficulty crossing the endothelium, colloid solutions are expected to remain in the intravascular space longer than crystalloids. One would therefore anticipate that less colloid would need to be administered than crystalloid to achieve the same end points. Data from animal studies and healthy volunteers strongly support such a concept.1,2 In a recent animal study, Bansch et al.1 infused Ringer’s acetate solution or 5% albumin in a ratio of 4.5 to 1.0 for resuscitation of rats undergoing hemorrhage or with sepsis: the plasma volume-expanding effect was the same for the 2 solutions after 2 and 4 hours of follow-up. Even when fluids are administered rapidly to avoid redistribution of crystalloids into the interstitium, there are important differences. In healthy volunteers, McIlroy and Kharasch2 created moderate hypovolemia by withdrawing 900 mL of blood and thereafter infused 1 L of Ringer’s solution or 6% hetastarch over 5 to 7 minutes: the hetastarch solution was associated with an intravascular volume expansion effect twice as large as that of crystalloid solution at the end of the bolus.

This long-held theory has, however, been challenged by some experts, especially in view of recent trials, in which there was no large difference between the amounts of colloid and crystalloid solutions administered.3–9 There are several possible reasons for the differences in crystalloid/colloid ratios reported in the various studies. First, the study populations are different; for example, capillary leakage with fluid losses into the interstitium, as in sepsis, may limit the vascular effects of colloids and reduce the normal crystalloid/colloid ratio. Second, fluid administration is now started earlier so that by the time patients are enrolled in a study, they may have already received a significant amount of fluid, complicating the calculation of a crystalloid/colloid ratio. Third, the studied fluids may be different; albumin, starches, gelatins, and dextrans have different pharmacokinetic properties, different effects on the glycocalyx and microcirculation, and different effects on plasma expansion. Fourth, the ratio may vary according to the time at which it is measured; for example, many study protocols allow for additional crystalloid to be administered once maximal doses of colloid have been used so that a calculation at this time point will reflect a crystalloid to mixed crystalloid + colloid ratio rather than a pure crystalloid/colloid ratio. Similarly, the ratio may vary according to the end point chosen to assess the effect of fluid administration.

Several studies have already shown that a positive fluid balance can worsen outcomes in hospitalized patients, especially when congestive heart failure or renal dysfunction is present,10,11 so information regarding differences in the amounts of different types of fluids needed to achieve the same end points is important when considering which fluid(s) to administer in clinical practice. We, therefore, performed a systematic literature review to evaluate the reported crystalloid/colloid ratios in clinical studies comparing these types of fluids. We also performed a meta-regression to attempt to unravel some of the possible reasons for the observed differences.

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METHODS

We systematically searched MEDLINE, EMBASE, and CENTRAL until December 18, 2013, for all studies comparing any crystalloid versus any colloid for fluid replacement. A highly sensitive search was built using indexed terms in each database, filtered to human studies in adult populations, as described in the Appendix. We did not limit our searches by date or language. We also examined the reference lists of included studies and previously published reviews to insure no studies had been missed. Articles were excluded if they included burn patients; involved preoperative volume loading or normovolemic hemodilution, fluid administration during paracentesis, or fluids given by formula (with an already set ratio); evaluated healthy volunteers; or evaluated only priming for cardiac surgery. Papers that had later been retracted also were excluded. When 2 articles presented the same set of data, we used the most detailed report.

Two authors read the titles and abstracts of reports identified by the search to produce a list of possibly relevant articles and checked the list to determine which articles fit the inclusion criteria. After reading the full text of preselected articles, any disagreements were resolved after consultation with a third author. Data were extracted to prespecified tables considering year and decade of publication, number of subjects, characteristics of the administered solutions, and study design. Articles were classified as randomized controlled trials (RCTs) or not.

Studies were grouped into 5 categories according to the studied population: severe sepsis/septic shock; other cause of shock; cardiovascular surgery; other surgery; and mixed populations. Because some studies had resuscitation protocols for periods of instability along with concomitant maintenance administration of other fluids, we classified studies into those that evaluated acute resuscitation with strict protocols and those that did not. An additional classification was made, separating studies according to whether they reported the infused volumes less than or more than 12 hours after randomization. Studies also were grouped into those that incorporated a measure of preload (central venous pressure [CVP], pulmonary artery occlusion pressure, left atrial pressure) or not in their protocols. A final classification was made to evaluate the possible role of observer bias, classifying studies as double-blinded or not. For studies that included albumin, a separate categorization was made considering the concentration of the solutions and, similarly, studies of HES solutions were classified according to the MW of the HES, with low MW considered as MW ≤130 kDa and high MW as >130 kDa. The quality of the studies was evaluated using the Jadad score.

We used a pragmatic approach, recording the total amount of infused fluid used in each cohort (colloids + crystalloids versus crystalloids) for analysis because the majority of articles did not make a distinction between these groups and because in clinical practice colloids generally are administered along with some crystalloid. When this value was not given directly, we calculated it by adding the mean values for each fluid, but it was then not possible to calculate the standard deviations. The ratio was then calculated as the ratio of total infused fluid in the 2 cohorts. We recorded the earliest values for volume of infused fluids reported in each study, because over longer periods, colloid infusion often is restricted and combined with crystalloids, which tends to underestimate the real effect of the colloids.

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Statistical Analysis

All studies were considered for descriptive purposes and initial exploration of data. Meta-analysis for RCTs that reported complete data for fluid volumes in terms of means and standard deviations was performed. The crystalloid/colloid ratio was used as the summary measure, taking into consideration the variability in the units used to report the infused volume (mL, mL/kg, mL/kg/h). The pooled-ratio and its 95% confidence interval were calculated using the DerSimonian and Laird random effects model, considering the large heterogeneity in study design. Heterogeneity was evaluated with the Q Cochrane statistic and the I2 index, which provides an approximation of the proportion of total variability in point estimates that can be attributed to heterogeneity. To offset the fact that the statistical power of the heterogeneity test may be low, we tested the homogeneity hypothesis at the 10% level.

We conducted a meta-regression analysis to examine the association of treatment effect with some of the reported study characteristics and hence to investigate which study characteristics could explain the heterogeneity found in the main meta-analysis. Each considered predictor variable (type of solution, decade and year of publication, type of population, time of assessment less than or more than 12 hours after resuscitation, presence or not of a clear resuscitation protocol, doubled-blinded or not, use of a preload target for fluid administration) was tested independently in the model. The between-study variability was estimated by the R2, which can be regarded as the amount of (residual) heterogeneity in the reduced model that can be explained by the presence of the predictor in the full model. All reported P values are 2-sided, and a P value of <0.05 was considered statistically significant, unless otherwise indicated. Statistical analyses were performed using R statistical software (version 3.0.1, R Foundation, Vienna, Austria).

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RESULTS

A total of 976 publications were identified, and 48 met our inclusion criteria (Fig. 1),3–9,12–52 presenting data on 71 different comparisons between colloid and crystalloid cohorts. The earliest trial was from 1977,27 and the most recent from November 2013.52 Of the 48 articles, 43 were RCTs.3–5,7–9,12,14–31,33–40,42–47,49–52 Meta-analysis was performed only in the 36 cohorts (24 articles4,7,12,14–22,24,27,31,33,34,36,39,42,45,46,50,51) that presented full mean and standard deviation data in both groups.

Figure 1

Figure 1

The end points used to guide fluid administration differed among the studies and included heart rate, mean arterial blood pressure, systolic arterial pressure, serum lactate levels, base excess, CVP, left atrial pressure, pulmonary artery occlusion pressure, cardiac index, stroke volume, left ventricular stroke work index, central venous oxygen saturation (ScvO2), urine output, and even physician judgment. The characteristics of the studies and the estimated crystalloid/colloid ratios for the different types of colloids are shown in Tables 1 to 3. A timeline showing the reported ratios according to year of study publication is shown in Figure 2. The older studies generally reported greater crystalloid/colloid ratios and more frequently used albumin than the more recent studies. The crystalloid/colloid ratio varied between 0.8941 and 4.7433 in 23 cohorts on albumin, 0.8421 and 3.9936 in 35 cohorts on starches, 1.0025 and 2.8014 in 11 cohorts on gelatins, 2.1214 in the 1 cohort on dextran, and 1.50 in 1 cohort with a mixture of colloids.52 When considering the different study populations, the ratios varied between 1.008 and 2.266 for severe sepsis/septic shock, 0.8941 and 4.5113 for other types of shock, 0.8421 and 2.8014 for cardiac surgery, 0.8930 and 4.7433 for other surgery, and 1.309 and 1.5052 for heterogeneous.

Table 1

Table 1

Table 2

Table 2

Table 3

Table 3

Figure 2

Figure 2

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Meta-Analysis of RCTs

The pooled crystalloid/colloid ratio in the RCTs included in the meta-analysis was 1.50 (1.36–1.64) (Fig. 3), with considerable heterogeneity seen among studies (Q = 605, P < 0.001 and I2 = 94%). Meta-regression was performed for all the categories detailed earlier, and the estimated ratios are shown in Table 4. There were statistically significant differences in the crystalloid/colloid ratio according to decade of publication when considering all the studies and according to concentration for the albumin studies. There were no significant differences according to type of solution, indication for fluid, studied population, use of a preload target to guide fluid administration, double blinding, or time of assessment. None of the explored variables substantively decreased the heterogeneity from the general model (Table 4). The estimated decrease in the ratio over time was 1% (95% confidence interval, 0%–3%) per year, but this finding was not statistically significant (P = 0.16) (Fig. 4).

Figure 3

Figure 3

Table 4

Table 4

Figure 4

Figure 4

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DISCUSSION

Our review supports the physiologic concept of reduced fluid requirements when using colloid compared with crystalloid solutions. Experimental data in humans have demonstrated that colloids have a more prolonged effect in the intravascular compartment compared to crystalloids.2,53 Svensén and Hahn53 demonstrated that the intravascular expansion from Ringer’s acetate solution decreased rapidly by >65% at 30 minutes after the end of the fluid infusion, whereas the effect of 6% dextran 70 remained stable for >2 hours. In our review, the ratio of the total amount of crystalloid compared with colloid received to achieve hemodynamic targets was substantially >1 in most studies, and the global ratio calculated from the RCT data was 1.5. This finding must be interpreted from a clinical practice point of view, in which patients who receive colloids usually also receive crystalloids to complete fluid resuscitation. Indeed, in studies in which authors evaluated the infused volumes <12 hours after randomization, when fluid administration was still largely restricted to the study fluid, crystalloid/colloid ratios were greater than those that evaluated the volumes later when crystalloids had generally been added to the colloid regimes (1.59 vs 1.35). The observation that larger quantities of crystalloid than colloid were needed was true for all patient groups, including those with sepsis, suggesting that the permeability alterations associated with sepsis are not associated with complete leak of large molecules outside the vessels.

Some recent studies have reported no major differences in the amounts of colloids and crystalloids required3,8; therefore, doubts have been raised regarding the concept that larger volumes of crystalloids than colloids are needed to achieve the same end points. By the time patients were randomized to one type of fluid versus another in these studies, however, they had already received a significant amount of fluid: In the VISEP (i.e., Volume Substitution and Insulin Therapy in Severe Sepsis) study,3 for example, patients had already received a median of 2000 mL of crystalloid plus 725 mL of colloids in the crystalloid group and 2000 mL of crystalloid plus 979 mL of colloid in the HES group; the CVP was 12 mm Hg, ScvO2 75%, and lactate 2.2 mEqL at baseline, suggesting a degree of fluid resuscitation had already been achieved. In the 6S study,8 CVP 10 (7–13) mm Hg, ScvO2 75%, and lactate 2.0 mEqL were reported at baseline. Our data show that the crystalloid/colloid ratio has tended to decrease over the years (Fig. 2), possibly because fluid administration is being started earlier than in the past, such that patients already had received more fluid at the time of inclusion in the more recent studies compared with older trials.

Experimental data in humans have suggested different intravascular volume expansion effects with different colloid solutions.54 In general in our review, crystalloid/colloid ratios were greater for albumin than for synthetic colloids, and especially gelatins. This finding may not be too surprising for gelatins, with an MW of only 30 to 35,000, that is, about one-half the MW of albumin. Moreover, although there may be some degree of albumin extravasation from the intravascular compartment,55 interventional studies have shown an increase in albumin levels when albumin is administered to critically ill patients, including those with more severe permeability alterations,56 suggesting that albumin may remain more in the intravascular space than other solutions and thus reduce the total amount of fluid needed. Nevertheless, the meta-regression suggests that the type of fluid per se (starch versus albumin versus gelatin) does not influence the crystalloid/colloid ratio, although the concentration of albumin did have an effect.

Inverse ratios, in which the amount of colloid used was larger than that of crystalloids, were seen largely in older studies,21,27,30,37 but also in the CRYstalloids or COlloids (CRYCO) study published in 2008.41 Nevertheless, although the CRYCO study included a large number of patients, it was a cohort, noninterventional study so that the groups were not randomized, which limits the interpretation of the ratio in this study.

This review has several limitations, including the heterogeneity of the study types and patient populations (including preoperative and postoperative patients), numbers of patients enrolled, objectives, designs, different protocols, and end points for fluid administration, fluid challenge, and shock (fluid resuscitation), which made it difficult to compare the studies. This is confirmed by the fact that in the meta-regression none of the investigated covariates reduced the heterogeneity between studies to any extent so that explanatory factors could not be identified, and are likely related to differences in study design that cannot be quantified from the available published data. Moreover, the concomitant administration of crystalloids in the colloid cohorts leads to an underestimation of the real volume expansion power of colloids but reflects the situation in clinical practice where crystalloids generally are administered with colloids. Finally, most of the studies did not present data for total infused volumes in each cohort as means and standard deviations, or the data were not complete so we were unable to include many studies, including some of the larger ones, in the meta-regression analysis. Thus, extrapolation of our results to other groups of critically ill patients is limited.

In conclusion, however, our literature review confirms, in a clinical context, the basic physiological concept that less colloid than crystalloid needs to be administered to achieve the same hemodynamic end points. Reported crystalloid/colloid ratios are quite variable, largely related to the heterogeneity of study populations and the amounts of crystalloid administered concomitantly with colloid.

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APPENDIX

SEARCH SYNTAX

Date: December 18, 2013:

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PUBMED

(“Colloids”[Mesh] OR “Starch”[Mesh] OR “Gelatin”[Mesh] OR “Albumins”[Mesh] OR “Dextrans”[Mesh]) AND (“crystalloid solutions”[Supplementary Concept] OR “Isotonic Solutions”[Mesh]) AND (“humans”[MeSH Terms] AND “adult”[MeSH Terms])

336 articles

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EMBASE

‘colloid’/exp OR ‘starch’/exp OR ‘hetastarch’/exp OR ‘gelatin’/exp OR ‘dextran’/exp OR ‘albumin’/exp AND (‘crystalloid’/exp OR ‘isotonic solution’/exp) AND ([adult]/lim OR [aged]/lim) AND [humans]/lim AND [embase]/lim

435 articles

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CENTRAL

Crystalloid AND Colloid as Key-terms filtered by Trials

205 articles

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DISCLOSURES

Name: Diego Orbegozo Cortés, MD.

Contribution: This author helped design the study, perform the literature search and synthesis, and prepare the manuscript.

Attestation: Diego Orbegozo Cortés attests to having approved the final manuscript and to the integrity of the original data and the analysis reported in this manuscript.

Name: Teresa Gamarano Barros, MD.

Contribution: This author helped design the study, perform the literature search and synthesis, and prepare the manuscript.

Attestation: Teresa Gamarano Barros attests to having approved the final manuscript, attests to the integrity of the original data and the analysis reported in this manuscript, and is the archival author.

Name: Hassane Njimi, MSc, PhD.

Contribution: This author performed all statistical analyses and helped interpret the data.

Attestation: Hassane Njimi attests to having approved the final manuscript.

Name: Jean-Louis Vincent, MD, PhD.

Contribution: This author helped design the study and critically review the manuscript.

Attestation: Jean-Louis Vincent attests to having approved the final manuscript.

This manuscript was handled by: Steven L. Shafer, MD.

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