Crystalloids colloids compared

Compare and contrast the relative advantages and disadvantages of crystalloids, colloids, and blood products in the treatment of hypovolemic shock. 

The theoretical advantage of colloids is their prolonged intravascular retention time compared to crystalloid solutions. Isotonic crystalloid solutions have substantial interstitial distribution within minutes of IV administration, but colloids remain in the intravascular space for hours or days, depending on factors such as capillary permeability. However, even with intact capillary permeability, the colloid molecules eventually leak through capillary membranes. 

Iso-oncotic colloid solutions (plasma and plasma protein fractions), such as 5% albumin and 6% hetastarch, offer the advantage of more rapid restoration of intravascular volume with less volume infused, but there is no significant clinical outcome differences compared with crystalloids. 

Human plasma has a colloid osmotic pressure of 3.6 kPa, of which 2.8 kPa is contributed by albumin. Volume-for-volume, 4.5% albumin is approximately four times more effective in expanding the plasma volume than crystalloid solutions, and the effect lasts 6-8 hours, compared to only 15-20 min with crystalloids. Although popular in the past as volume expanders, albumin solutions have fallen into disfavour. They are prepared from pooled human plasma, with all the inherent risks of pooled blood products. Albumin can cause adverse reactions, similar to other transfusion reactions, such as chills, urticaria, and vasodilatation. These may be caused by organic or inorganic substances formed during the processing... 

Although hetastarch and dextran 70 have been used clinically in horses (Greatorex 1977, McFarlane 1999), it remains to be seen whether either offers any advantage in terms of clinical outcome when compared with crystalloids. Furthermore, some of the theoretical benefits of colloids over crystalloids have not been observed in some experimental models and clinical trials. For example, increased extravascular lung water was foimd in an endotoxic pig model (Baum et al 1990) but not a septic rat model (Rackow et al 1989) when crystalloid and hetastarch resuscitation were compared. In preterm hypotensive human infants, treatment with 5% albumin resulted in significantly greater fluid retention than treatment with isotonic saline and the two treatments were equivalent in other respects (So et al 1997). 

Recent meta-analyses of randomized trials of colloid administration to human critical care patients demonstrated no advantage of colloids over crystalloids and concluded that colloids increased mortality when used in some clinical syndromes (Choi et al 1999, Schierhout Roberts 1998). When trauma patients were studied as a subpopulation, a 12.3% difference in mortality rate was identified in favor of crystalloid therapy. Conversely, analysis of pooled data from nontrauma patients identified a 7.8% difference in mortality rate in favor of colloid treatment (Velanovich 1989). The majority of clinical trials in these meta-analyses compared albumin with crystalloid solutions and thus these results should not discourage the use of hydroxyethyl starches in horses. 

The colloids, in particular albumin, are expensive solutions. Therefore, it is difficult to justify the additional cost of colloidal products unless the benefit-to-risk ratio is substantially greater than that associated with inexpensive crystalloid solutions. This does not appear to be the case based on randomized, controlled studies and meta-analyses comparing colloid and crystalloid solutions for acute circulatory insufficiency. Because other colloids, such as hetastarch, almost always have been compared with albumin and not with crystalloid solutions in published clinical studies (with no clinically important differences being found), there is no reason to suspect that these other colloids have any unique advantages as volume expanders. Adverse effects associated with colloids appear to be uncommon and generally are extensions of their pharmacologic activity (Table 24—4), but this is also true of crystalloids. The benefit-to-risk ratio appears to be similar for colloids and crystalloids thus, based on cost, crystalloids are preferred for initial treatment of circulatory insufficiency. 

Of the randomized studies comparing albumin with crystalloid solutions in the perioperative period, the majority found no statistically significant differences between groups. The significant differences that have been found have involved isolated hemodynamic or respiratory variables with no obvious clinical correlates (e.g., duration of mechanical ventilation). Therefore, albumin (and other colloids) cannot be recommended for the prevention or initial treatment of circulatory insufficiency, although their use may be appropriate in... 

A meta-analysis of clinical studies comparing crystalloid and colloid resuscitation indicated no clinical outcome differences. " ... 

Emulsoids differ in several ways from suspensoids they are usually more viscous, not easily precipitated by electrolytes, and usually their surface tension is markedly lower than that of the solvent. They also protect suspensoids from precipitation. Graham (V) said the flow of liquid colloids through a capillary tube is always slow compared with the flow of crystalloid solutions, so that a liquid-transpiration tube may be employed as a colloidoscope. The precipitation of emulsoids by electrolytes follows a different course from that... 

Endeavors to determine the electrochemical equivalent of colloids have not been very successful in most cases owing to the confused relations between the size of the particles and the charge. Sometimes it is quite impossible because the substance is partly colloid and partly crystalloid, e.g., Benzopurple, Congo red. Nevertheless, the amount of electricity on a definite amount of colloid has been determined. In all cases, however, where the particles migrate and are discharged at the electrodes, electricity must be transported. Although it is very small in comparison to that of solutions of electrolytes, owing to the comparatively small number of particles involved, it has been often determined in the case of hydrosols. 

Renal replacement therapy (RRT) was more common during periods when patients received synthetic colloids compared to only crystalloids. Risk of RRT was greater after administration of HES and gelatin versus crystalloids. Propensity score stratification confirmed greater use of RRT in the HES and gelatin periods compared to the crystalloid period. Time to vasopressor cessation, normalisation of serum lactate and mean arterial pressure did not differ among groups. 

HES are S5mthetic colloid solutions used as plasma volume expanders in a range of clinical settings, including hypovolaemia. The EU Pharmacovigilance Risk Assessment Committee reviewed the benefits and risks of HES products in different patient groups. The review concluded that there was a clear indication of harm when HES was used for fluid resuscitation and there was no evidence of extended benefit when compared with crystalloid solutions [34 ]. 

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