Acid-base balance revisited

The occurrence of metabolic acidosis following large infusion of 0.9% NaCl compared to Lactated Ringer’s has been first described by Scheingraber et al. Several other papers investigated the possible underlying mechanism and/or aimed to find out the composition of a crystalloid solution able to “balance” the possible acidotic effect. The Stewart’s approach, which considers both acid base and electrolytes equilibrium and may be useful to understand these phenomena. Briefly, according to Stewart, three independent variables define the acid base status, i.e. Strong Ion Difference (SID), the dissolved carbon dioxide (CO2) tension (PCO2) or, in a closed system, the CO2 content in the plasma, and the total amount of the weak acids, primarily albumin and phosphates, in the extracellular space (ATOT). The crystalloid solutions used for volume resuscitation include strong ions which are always dissociated and present in solution in their ionic form, as Na+, Cl-, K+, possibly associated, with organic ions as acetate, lactate, gluconate, in order to achieve electroneutrality Therefore, when it is claimed that a given solution of crystalloids including organic ions has a [SID] different from 0, it is implicit that this is true only after the complete metabolism of the associated anion. So, if the metabolism does not occurs, a solution as Lactated Ringer’s (which is told to have a SID = +28 mEq/l) behaves as 0.9% NaCl ([SID] = 0 mEq/l). When a crystalloid solution with a given [SID] is infused, several phenomena occur at the same time. Firstly, after the metabolism of the organic associated anion (if any), extracellular [SID] modifies according to the volume of infusion and its [SID]; secondly, both the [A-] and [ATOT] decreases; thirdly, until the system is closed to gases, the CO2 content, i.e. the sum of dissolved CO2 concentration and bicarbonate (HCO3-), are diluted at the same degree of [ATOT], (unless the associated organic anion in the diluting solution is HCO3-). Accordingly, if the solution infused in a closed system is characterized by zero [SID], all the independent determinants ([SID], [ATOT] and CO2 content) are equally diluted and pH remains constant. When the system equilibrates with alveolar PCO2, the CO2 content increases and the pH decreases. If a diluting agent characterized by a positive [SID] is infused the final [SID] of the extracellular space will be higher compared to the one obtained after infusing a solution with 0 [SID] as 0.9% NaCl and less acidosis will develop. Morgan et al. found that, if the solution [SID] is equal to 24 mEq/l, dilutional acidosis is avoided and, therefore, the solution can be called “balanced”. However, we found that the 24 mEq/l proposed by Morgan only refers to a “normal” acid-base condition in which pH equals 7.4 and PCO2 equals 40 mmHg, i.e. the bicarbonate is nearly 24 mmol/l, while the baseline [HCO3-] dictates the pH response to crystalloid infusion. If crystalloid [SID] is equal to baseline [HCO3-], pH remains unchanged at constant PCO2, while it increases or decreases if the [SID] is greater or lower, respectively.