A simplified quantitative acid–base approach for patients with acute respiratory diseases

The Stewart-Figge acid–base model has been criticized for being mathematically complex. We aimed to develop simpler formalisms, which can be used at the bedside. The following simplifications were used: (1) [Ca2+] and [Mg2+] are replaced by their mid-reference concentrations (2) pH is set to 7.4. In the new model [SIDa] is replaced by its adjusted form, [SIDa, adj] = [Na+] + [K+] − [Cl−] + 6.5 and [SIG] is replaced by “bicarbonate gap”, [BICgap] = [SIDa, adj] − (0.28⋅[Albumin]) − (1.82⋅[Phosphatei])- [HCO3]. The diagnostic performance of the model was tested in 210 patients with acute respiratory diseases and 17 healthy volunteers. [BICgap] was also compared to albumin-corrected anion gap ([AGc]). The concordant correlation coefficient between [SIDa, adj] and [SIDa] and between [BICgap] and [SIG] was 0.98 in both comparisons. The mean bias (limits of agreement) of [SIDa, adj] − [SIDa] and of [BICgap] − [SIG] were 0.53 meq/l (− 0.46 to 1.53) and 0.50 meq/l (− 0.70 to 1.70), respectively. A [SIDa, adj] 52.5 meq/l had an accuracy of 0.997 (p 11.6 meq/l predicted unmeasured ion (UI) acidosis with an accuracy of 0.997 (p 19.88 meq/l predicted UI acidosis with an accuracy of 0.994 (p