metabolic acidosis : what I do understand

Fluid resuscitation with large amounts of crystalloids is usual in critically ill patients and Normal Saline is the most widely used solution for Fluid resuscitation with crystalloids. It has been observed, however, that large amounts of Normal Saline induce metabolic acidosis. According to Stewart’s model metabolic acidosis is due to a reduced Strong Ion Difference [SID]. However all three independent variables of Stewart’s model must be considered together, i.e. pCO2, [SID] and the total amount of weak acids [ATOT] as normal pH is assured by an adequate proportion of these components. It is important to remind that whatever crystalloid solution, before the infusion, has pCO2, [ATOT] and [SID] equal to zero and that crystalloid solutions containing organic ions, as Lactated Ringer’s, develop a positive [SID] only after the metabolism of the associated organic ions (as Lactate and Acetate). Therefore, when a crystalloid solution is added to normal plasma in a closed system, the independent variables are equally diluted and pH remains constant. When the system is open to gases and, as an example, pCO2 raises from 20 to 40 mmHg, pH decreases, being [ATOT] and [SID] unchanged. Of course, by manipulating also the actual [SID] of the infused solution, pH will change accordingly, so we may expect that a proper combination of Stewart’s independent variables could manipulate the acid-base equilibrium towards acidosis, alkalosis or unchanged pH. It was shown that no acidosis developed in normal animals when the diluting solution had a [SID] of nearly 24 mEq/l. We found in vitro that plasma pH after crystalloid dilution at constant pCO2, depends on the relationship between infused SID (SIDinf) and baseline plasma bicarbonate concentration [HCO3-]. When infused SID is equal to baseline bicarbonate no change in pH occurs, whatever the degree of dilution. HoweverWhen pH increases when SID infused is higher than baseline bicarbonate, while it decreases when infused SID is lower. We verified that the same rules are confirmed even in vivo in experimental animals ventilated at constant pCO2. Moreover the kidney response is specific for each solution, finalized to restore the baseline conditions