Hyperchloremic Metabolic Acidosis and fluid Resuscitation

Adequate fluid resuscitation is still a major treatment to optimize hemodynamics and to restore organ perfusion in the case of volume depletion. To achieve this goal, a wide variety of fluids are available to the clinician. Due to their different composition, the usual classification opposites crystalloids and colloids. Despite numerous studies, controversy still exists on the ideal fluid resuscitation. The trend to choose preferentially one or another during the last century has moved just like a pendulum. Acidosis as a consequence of fluid replacement is well known since more than 80 years ago. Initially, according to the classical Henderson-Hasselbalch approach, this disturbance was simply explained by a phenomenon of plasma bicarbonate dilution which was responsible for a proportional decreased pH. Consequently, acidosis has been called “dilutional acidosis”. But, in the 1970s, Stewart described a new concept for the interpretation of acid-base equilibrium. In this approach, pH variations result from changes in 3 independent variables which are the strong ion difference (SID), the total charge in weak acids and the PaCO2. This concept emphasizes the implication of chloride and weak acids in acid-base equilibrium. Considering this approach, it is clear that the infusion of fluids containing high concentration of chloride leads to hyperchloremic metabolic acidosis. In this way, acidosis is not related to a simple dilution, but to the decreased SID which results totally from hyperchloremia. According this concept, crystalloids and colloids are now sub-classified into balanced or unbalanced categories. Balanced solutions are those that contain a concentration of chloride close to that of the plasma, whereas the unbalanced fluids are those characterized by a proportional high chloride concentration. Since about 10 years ago, normal saline, an unbalanced solution, remains the most popular choice of IV fluid. Due to its preferential administration, hyperchloremic metabolic acidosis is more and more frequently observed during the perioperative period and in critically ill patients. Numerous experimental and clinical trial have confirmed this phenomenon. Finally, whatever the exact mechanism, iatrogenic hyperchloremic metabolic acidosis produced by unbalanced expanders is now demonstrated. But, the real question is about the clinical relevant of potentially harmful effect of these changes, especially of hyperchloremia. Actually, the answer is not univocal. Only short-term infusion of unbalanced fluids has been studied, so that, only slight and transient hyperchloremic metabolic acidosis are described. Nevertheless, some recent data support transient postoperative cerebral, renal or digestive dysfunction in patients with hyperchloremia. A worsen outcome and a shorter survival time have been also found in experimental septic rats, long-term resuscitated with unbalanced solutions. However, these results need to be confirmed by further prospective randomized clinical trial of clinical outcome. In other words, present data which are still essentially biological modifications, cannot permit objectively to avoid totally or partially volume expansion with unbalanced solutions.