Untitled

Abstract

Stewart's quantitative physical chemical approach enables us to understand the acid-base properties of intravenous fluids. In Stewart's analysis, the three independent acid-base variables are partial CO 2 tension, the total concentration of nonvolatile weak acid (A TOT ), and the strong ion difference (SID). Raising and lowering A TOT while holding SID constant cause metabolic acidosis and alkalosis, respectively. Lowering and raising plasma SID while clamping A TOT cause metabolic acidosis and alkalosis, respectively. Fluid infusion causes acid-base effects by forcing extracellular SID and A TOT toward the SID and A TOT of the administered fluid. Thus, fluids with vastly differing pH can have the same acid-base effects. The stimulus is strongest when large volumes are administered, as in correction of hypovolaemia, acute normovolaemic haemodilution, and cardiopulmonary bypass. Zero SID crystalloids such as saline cause a 'dilutional' acidosis by lowering extracellular SID enough to overwhelm the metabolic alkalosis of A TOT dilution. A balanced crystalloid must reduce extracellular SID at a rate that precisely counteracts the A TOT dilutional alkalosis. Experimentally, the crystalloid SID required is 24 mEq/l. When organic anions such as L-lactate are added to fluids they can be regarded as weak ions that do not contribute to fluid SID, provided they are metabolized on infusion. With colloids the presence of A TOT is an additional consideration. Albumin and gelatin preparations contain A TOT , whereas starch preparations do not. Hextend is a hetastarch preparation balanced with L-lactate. It reduces or eliminates infusion related metabolic acidosis, may improve gastric mucosal blood flow, and increases survival in experimental endotoxaemia. Stored whole blood has a very high effective SID because of the added preservative. Large volume transfusion thus causes metabolic alkalosis after metabolism of contained citrate, a tendency that is reduced but not eliminated with packed red cells. Thus, Stewart's approach not only explains fluid induced acid-base phenomena but also provides a framework for the design of fluids for specific acid-base effects.

Role of acidic pH of intravenous fluids in subsequent development of metabolic acidosis- may not be what it seems

“…[4] But metabolic acidosis accompanies infusion of these solutions. [12467] This acidosis was explained by dilution of bicarbonate content in plasma. [267] Subsequently Stewart's physiochemical approach was used to describe the in vivo effect on acid base status by IVFs.…”

“…[12467] This acidosis was explained by dilution of bicarbonate content in plasma. [267] Subsequently Stewart's physiochemical approach was used to describe the in vivo effect on acid base status by IVFs. [67] According to the later concept, the acid-base effect of an IVF is determined by the strong ion deference (SID) and the total weak acid concentration (A TOT ) of the fluid.…”

“…[267] Subsequently Stewart's physiochemical approach was used to describe the in vivo effect on acid base status by IVFs. [67] According to the later concept, the acid-base effect of an IVF is determined by the strong ion deference (SID) and the total weak acid concentration (A TOT ) of the fluid. [67] pH falls with decreasing SID, increasing A TOT or CO 2TOT (total CO 2 content) / PCO 2 .…”

“…[67] According to the later concept, the acid-base effect of an IVF is determined by the strong ion deference (SID) and the total weak acid concentration (A TOT ) of the fluid. [67] pH falls with decreasing SID, increasing A TOT or CO 2TOT (total CO 2 content) / PCO 2 . [67] As the IVFs equilibrate with ECF, they alter SID of ECF, as SID of these IVFs defers appreciably from that of plasma.…”

“…[67] pH falls with decreasing SID, increasing A TOT or CO 2TOT (total CO 2 content) / PCO 2 . [67] As the IVFs equilibrate with ECF, they alter SID of ECF, as SID of these IVFs defers appreciably from that of plasma. [67] SID of D5 and NS is 0, they decrease the SID of plasma and exert an acidifying effect.…”