Effects of fluids on sublingual microcirculation: a point of view review

Fluids are a key component of shock resuscitation. Nevertheless, their microvascular effects are complex. In fluid responsive patients, fluids may increase tissue perfusion because of the increase in cardiac output. Since shock states are characterized by a partial loss of the coherence between systemic hemodynamics and microcirculation, the increase in cardiac output does not guarantee improvements in tissue perfusion. Furthermore, the administration of fluids carries risks of hemodilution and tissue edema that can dampen microcirculation. Regarding this, colloid solutions have some theoretical advantages and experimental support. Despite its relevance, few clinical studies have evaluated the effects of fluids on sublingual microcirculation-the more clinically accessible territory for videomicroscopy. This review analyzes physiological bases and experimental and clinical evidence about the complex microvascular effects of fluids.

We found eight observational and four controlled trials carried out on critically ill and surgical patients addressing the effects of fluids on sublingual microcirculation. Most showed that fluid resuscitation can improve microcirculation, especially in the presence of fluid responsiveness and tissue hypoperfusion. Concerning the controlled trials that compared different solutions, one study failed to show benefits of hypertonic over isotonic hydroxyethyl starch, while another found improved microcirculation after early goal-directed therapy with hydroxyethyl starch than with 0.9% NaCl. Since both studies included a small sample, the results are inconclusive. The third trial, which recruited 100 septic patients, concluded that albumin was superior to balanced solution; however, this conclusion is flawed by methodological problems. Some experimental studies also showed controversial results. Some studies which suggested benefits of albumin, hydroxyethyl starch and high viscosity solutions could not be properly replicated in patients. Superiority of balanced crystalloids over 0.9% NaCl was not consistently demonstrated in basic research. Moreover, some clinical and experimental studies have severe limitations, such as the use of inadequate analysis of microcirculation and compression artifacts in the video acquisition.

Fluid administration probably improves sublingual microcirculation when tissue perfusion is altered and cardiac output increases. The superiority of any solution for this purpose has not been clearly demonstrated. High-quality studies are needed to clarify the effects of different solutions on sublingual microcirculation.