Background and Aim
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 [Fig.1].
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=1000×2.46×10-11xPaCO2/(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 and 88%; SIDa < 40mEq/l in 73, 60, 48 and 43% respectively [Fig.1-2].
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.