Acid-base balance in peritoneal dialysis patients: a Stewart-Fencl analysis.

Abstract:
BACKGROUND: Evaluation of acid-base disorders using the Stewart-Fencl principle is based on assessment of independent factors: strong ion difference (SID) and the total concentration of non-volatile weak acids (Atot). This approach allows for a more detailed evaluation of the cause of acid-base imbalance than the conventional bicarbonate-centered approach based on the Henderson-Hasselbalch principle, which is a necessary yet insufficient condition to describe the state of the system. The aim of our study was to assess acid-base disorders in peritoneal dialysis (PD) patients using both of these principles. METHODS: A total of 17 patients with chronic renal failure (10 men), aged 60.7 (22–84) years, treated by PD for 25.7 (1–147) months were examined. A control group included 17 healthy volunteers (HV) (8 males), with a mean age of 42.7 (22–77) years and normal renal function. Patients were treated with a solution containing bicarbonate (25 mmol/L) and lactate (15 mmol/L) as buffers; eleven of them used, during the nighttime dwell, a solution with icodextrin buffered by lactate at a concentration of 40 mmol/L. The following equations were employed for calculations of acid-base parameters according to the Stewart-Fencl principle. The first is SID = [Na⁺] + [K⁺] + 2[Ca²⁺] + 2[Mg²⁺] – [Cl^–] – [UA^–], where SID is the strong ion difference and [UA^–] is the concentration of undetermined anions. For practical calculation of SID, the second equation, SID = [HCO₃^–] + [Alb^–] + [Pi^–], was used, where [Alb^–] and [Pi^–] are the charges carried by albumin and phosphates. The third is Atot, the total concentration of weak non-volatile acids, albumin [Alb] and phosphates [Pi]. RESULTS: The capillary blood pH in PD group was 7.41 (7.27–7.48), [HCO₃^–] levels 23.7 (17.6–29.5) mmol/L, SID 36.3 (29.5–41.3) mmol/L, sodium-chloride difference 39.0 (31.0–44.0) mmol/L, [Pi] 1.60 (0.83–2.54) mmol/L, and [Alb] 39.7 (28.8–43.4) g/L (median, min–max). Bicarbonate in blood correlated positively with SID (Rho = 0.823; p < 0.001), with the sodium-chloride difference (Rho = 0.649; p < 0.01) and pH (Rho = 0.754; p < 0.001), and negatively with residual renal function (Rho = –0.517; p < 0.05). Moreover, the sodium-chloride difference was also found to correlate with SID (Rho = 0.653; p < 0.01). While the groups of PD and HV patients did not differ in median bicarbonate levels, significantly lower median value of SID were observed in PD patients, 36.3 vs. 39.3 mmol/L (p < 0.01); additionally, PD patients were shown to have significantly lower mean value of serum sodium levels, 138 vs. 141 mmol/L (p < 0.01), and serum chlorides levels, 100 vs. 104 mmol/L (p < 0.001). Despite the higher [UA^–] levels in PD patients, 9.1 vs. 5.4 mmol/L (p < 0.001), this parameter was not found to correlate with bicarbonate levels. CONCLUSIONS: The results suggest that the decreased bicarbonate in PD patients results from a combination of decreased sodium-chloride difference and mildly increased unmeasured anions.