The great fluid debate: when will physiology prevail?

248 August 2013 I N this issue of ANesthesiology, Konrad et al.1 present a study in pigs in which they compare the renal microvascular oxygenation effects by using either a balanced crystalloid solution or a balanced hydroxyethyl starch 6% 130/0.4 solution (hes) for acute normovolemic hemodilution to a fixed hematocrit of 15%. They have chosen this method to assess the impact of crystalloids and starches on the most sensitive physiological compartment, the microcirculation, of the organ most sensitive to the deleterious effects of fluids, the kidney. Their measurement of kidney microcirculatory oxygen further targets one of the unwanted effects of fluids, namely, the hemodilution-induced reduction of the oxygen-carrying capacity of blood.2 They, thereby, examine which of the fluid types exerts harm when used in the context of hemodilution; this issue is a key question that is central to the current debate regarding fluid treatments. They conclude that, in their model, more harm is inflicted on the kidney by the use of crystalloids than by a balanced hes 130/0.4 solution. The advantage of using a physiological model is that one can address several mechanistic aspects of a clinically relevant hypothesis and/ or controversy in a controlled manner. in this respect, this study is very timely because several large multicentral clinical trials have been completed comparing the effects of crystalloid solutions with the effects of starch solutions.3–5* The great fluid debate appears to be as heated as it has ever been, as studies purporting that starches are harmful,3,4 clash with other investigations that reach the opposite conclusion.5* Furthermore, this debate has been clouded not only by a recent case of inappropriate scientific conduct,6 but also by the issuance of, in my opinion, premature recommendations into the relative benefits/drawbacks of starches versus crystalloids.7 A physiological approach is useful for addressing this situation because it can deliver clarity regarding the disputed issues, focus on mechanisms, permit studies to be conducted in a controlled fashion, and most importantly, provide guidance for the design of appropriate clinical trials that can generate evidence for clinical recommendations. in this current study, the authors apply hemodilution to healthy pigs using either a balanced crystalloid solution or a balanced starch solution to reduce the hematocrit to 15% in a stepwise fashion. As they suggest, this procedure is designed to mimic the situation that is encountered in cardiac surgery. This assumption, however, merits critical appraisal because a hematocrit of 15% is rarely encountered during cardiopulmonary bypass. indeed, the authors’ finding that few deleterious functional effects (e.g., deficiencies in creatinine and inulin clearance) from crystalloids occurred at hematocrit values of as low as 20% could The Great Fluid Debate