Acid–base implications of the Gibbs-Donnan effect during continuous veno-venous hemofiltration

Background This in-vitro and in-vivo study investigates the Gibbs-Donnan effect across the filter during continuous veno-venous hemofiltration (CVVH). In particular, we assessed its acid–base implications, applying the physical–chemical approach. Methods A prospective, single-center study was conducted using the PrismaMax machine (Baxter). Two sets of in-vitro CVVH experiments (with and without albumin) were performed to quantify the Gibbs-Donnan effect. Electrolytes, glucose, and osmolarity changes were measured across the filter and in the ultrafiltrate. Strong ion difference and sieving coefficients of the main solutes were calculated. Similar measurements were performed in oligo-anuric critically ill patients undergoing CVVH. Results In-vitro experiments without albumin showed a sieving coefficient of 1 for both positive and negative ions. On the contrary, when albumin was added, the sieving coefficient for sodium and chloride changed linearly with albumin concentration (r = −0.94, p < 0.001 for sodium, r = 0.88, p < 0.001 for chloride), resulting in a progressive linear increase in post-filter strong ion difference (β = 1.1, r = 0.77, p = 0.003). In 22 studied patients, calculated albumin concentration increased across the filter (2.2 ± 0.5 g/dL vs. 3.1 ± 0.7 g/dL), leading to sodium retention (138 ± 6 vs. 141 ± 6 mmol/L, p < 0.001) and chloride excretion (100 ± 5 vs. 97 ± 5 mmol/L, p < 0.001), thus resulting in a higher post-filter strong ion difference (46 ± 4 vs. 40 ± 4 mmol/L, p < 0.001). Conclusions These in-vitro and in-vivo studies demonstrate that albumin linearly affects the sieving coefficient of ions, increasing the strong ion difference of plasma water during its passage through the filter and thus having a systemic alkalizing effect.