To be clinically useful in goal-directed fluid therapy, a fluid challenge (FC) must achieve a large enough blood volume change (ΔBV) to materially change ventricular preload and stroke volume (ΔSV). The same % change in BV for all the subsequent FCs must also be achieved. We explored under what circumstances these prerequisites can be met by referring to data from a clinical trial where 111 patients underwent three successive intravenous FCs using crystalloid (Ringer´s lactate) or colloid (hydroxyethyl starch), involving a total of 9 mL/kg of fluid, just after induction of general anaesthesia.

The three FC using crystalloid fluid increased SV by 2.5% while the colloid increased the SV by 29.2% (medians, P < 0.001). Moreover, crystalloid expanded the BV by 20.8% while the corresponding expansion by colloid was 29.5% ( P < 0.001). The cardiac response to increased BV was normalized by calculating the ΔSV%/ΔBV% ratio, which was much lower at 0.12 for the crystalloid fluid versus 0.98 for colloid ( P < 0.001). The poor SV response can possibly be explained by the underlying changes in the mean circulatory filling pressure, which suggest crystalloid FCs was less effective in increasing the stressed blood volume (preload). On the other hand, the inferior BV expanding effect of crystalloid is likely due to the fast tissue redistribution pharmacokinetic profile of administered boluses. Finally, the cardiac response to the first FC and impact on oxygen delivery became falsely low in both groups due to a reflex redistribution of a substantial volume of interstitial fluid into the circulation after anaesthetic induction.

Differences in effectiveness between crystalloid and colloid fluid when providing repeated FCs increase the risk of false negative indications of fluid non-responsiveness when crystalloid is used.