Why Is Saline So Acidic (and Does It Really Matter?)

Abstract

Commercial 0.9% saline solution for infusion has a pH around 5.5. There are many reasons for this acidity, some of them still obscure. It is also true that infusion of normal saline can lead to metabolic acidaemia, yet the link between the acidity of saline solution and the acidaemia it can engender is not straightforward. This commentary draws together the known and putative sources of acidity in saline solutions: it turns out that the acidity of saline solution is essentially unrelated to the acidaemia complicating saline infusion.

Drug-induced metabolic acidosis

“…Acidosis may be due to an unbalanced effect on the CO2/HCO3 -buffer system: HCO3 -is diluted while PCO2 is independently regulated by respiration. 21 An alternative explanation is through its effect on the strong ion difference (SID). 22 The SID is the difference between positively-and negatively-charged fullydissociated ions [(Na + + K + +Mg + +Ca 2+ )-(Cl -+lactate)]), and is argued to be one of three independent variables that determine serum pH (the others being PCO2 and total weak acid concentration).…”

“…Whether this mathematical explanation reflects the biochemical mechanisms is debated. 21,22 'Balanced' crystalloid solutions, which have an electrolyte composition closer to that of serum (such as…”

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

“…[1] But unlike mentioned by the authors, current literature discusses the known and putative reasons for the pH of 5.5 of NS. [2] Dissolved CO 2 from atmosphere, the ionic composition of the solution and inherent properties of the packaging material in which the solutions are supplied, are thought to determine it. [2] We believe that the same set of reasons may also explain the acidic pH (4.2) of D5.…”

“…[23] Thus solutions with high titratable acid have reservoir of hydrogen ion (H + ) irrespective of pH. [3] Citing the work of Gaudry et al .…”

“…[35] Though the in vitro pH of D5 and NS is acidic, their effect on H+ balance is probably negligible. [24] It may be because of their low titratable acid content. [4] But metabolic acidosis accompanies infusion of these solutions.…”