Acid-base disorders are common indications for CRRT in ICU.If unrecognized they may result in poor outcomes.Stewart approach may be superior for acid-base analysis in the critically ill. AIM: Assessment of Classical and Stewart approaches for the analysis of acid-base disturbances during CRRT and mortality risk at 30 days.
We enrolled 40 consecutive adult patients on CVVH and mechanical ventilation. All patients received a 35 ml/Kg/h infusion of a standard buffer [5 Liters (mmol/L): [HCO-3]35,[Na+]140,[K+]2,[Ca2+]1,75,[Mg+]0.5,pH 7,4]. We calculated[pH] and SBE with the Henderson-Hasselbach and Siggaard-Andersen equations. Physicochemical analysis was performed using the Stewart equations modified by Figge et al.Apparent strong ion difference (SIDa) was calculated as: SIDa, mEq/L=[Na+]+[K+]+[Mg+]+[Ca2+]-[Cl-]-[lactate]. Effective SID(SIDe) was calculated as:SIDe, mEq/L=1000x2.46x10-11xPaCO2/(10-pH)+[albuminemia]x(0.123 x pH-0.631)+[PO4-]x(0.309xpH-0.469). Acidosis was defined by SBE<-5mEq/l, pH<7.35, SIDa<40mEq/l or SIDe<38mEq/l.
The prevalence of acidosis, assessed by pH, SBE, SIDa or SIDe, was significantly different (P<0.001)at each step. At 0, 6, 12 and 24hr from start of CVVH, pH<7.35 was present in 60, 33, 20 and 10% of patients;SBE<-5mEq/l in 35,10,10 and 5%; SIDe<38mEq/l in 100,88,65 and88%; SIDa<40mEq/l in 73,60,48 and 43% respectively [Fig. 1]. 58% of patients (n=23) died within 1 month from ICU admission. The risk of death was significantly higher for reduction of SIDa (P<0.01), SIDe (P<0.05) and SBE (P<0.05) but not pH (P=NS),independently to APACHE II score and gender. 16 of 17 patients (94%) with SIDa<40mEq/l at 24h died within 1 month from ICU admission (the relative risk of death was 3.1).
Stewart approach seems to be more sensitive for detection of acidosis in ICU patients on CRRT and has a greater impact to mortality than Classical approach.
