Must hypervolaemia be avoided? A critique of the evidence

Anaesthetists are cautioned to avoid hypervolaemia in their patients. The most cited reason is that hypervolaemia elicits the release of atrial natriuretic peptides that damage the endothelial glycocalyx layer. Although shedding of the glycocalyx causes extravasation of protein in inflammatory disorders, it is more uncertain whether hypervolaemia alone is enough to cause clinically important shedding. This review scrutinises the methodology used in two key papers that propose such a link. The most cited one reports that hydroxyethyl starch and 5% albumin, when creating a hypervolaemic state, only expands the plasma by 40% of the infused volume. This result was obtained by comparing measurements of the plasma volume performed with the indocyanine green (ICG) dye method before and after the infusion. However, the transit time of the dye, as well as inequality in the concentration between vascular beds, both act to underestimate the plasma volume, particularly as times were extrapolated backwards to time zero instead of to (the more correct) 1 minute. A re-calculation based on theoretical ICG data, taking account of the transit time, shows the plasma volume expansion was closer to 100% than to 40% of the infused volume. This figure is supported by the dilution of the reported blood haemoglobin and plasma protein concentrations, as well as by other sources. In conclusion, only weak evidence supports a fluid-induced release of atrial peptides of sufficient size to alter the kinetics of colloid fluid by shedding of the endothelial glycocalyx layer.