Theoretical principles of fluid management according to the physicochemical Stewart approach

Interpreting acid base disturbances according to the physicochemical Stewart approach allows the cause of such abnormalities to be discovered. This method is based on three independent variables: SID (strong ion difference), mainly sodium and chloride; weak acids concentration — Atot, mainly albumins and phosphate; and carbon dioxide tension — pCO2. These three independent variables are responsible for the change of water dissociation and for the change in H+ concentration and, consequently, the change in serum pH value. The SID value of the fluids administered to a patient is responsible for the change of serum SID value and therefore causes a change in the patient’s acid base status. During the infusion of a given fluid, the SID value of the serum becomes closer to the SID value of that fluid; on the other hand, the infusion causes a decrease in Atot concentration. In order to avoid acid base disturbances connected with fluid administration, the SID value of fluids being administered should be greater than 0 and lower then the serum SID. It has been suggested that fluids should be given of which the SID value is as close as possible to the actual serum HCO3 concentration. Knowing the SID value of the fluid administered, and the serum HCO3 concentration, one can expect a change of serum pH after a fluid infusion. Administering a fluid with a SID greater than the HCO3 concentration causes a pH increase towards alkalosis. Likewise, administering a a fluid with a SID lower than the HCO3 concentration causes a pH decrease towards acidosis. It seems that knowledge of the electrolyte concentration and the SID value of an administered fluid is an important factor regarding acid base disturbances.