HCO3 increment in arterial line can reveal significant vascular access recirculation in high-flux hemodialysis: a preliminary report.

We report a new and simple way that can reveal the presence of vascular access recirculation (VAR) in patients undergoing hemodialysis (HD). Acid-base and blood gas parameters (pH, pO(2), pCO(2), and HCO(3)) were measured in blood samples drawn from an arterial fistula needle before the initiation of HD and from arterial and venous lines simultaneously 5 min later, in 31 patients (group A). Vascular access recirculation was measured using the glucose infusion test (GIT) immediately after the withdrawal of the 5-min samples. The same study was repeated in 30 patients in whom HD lines were reversed (group B). A comparison with baseline (predialysis) values of an analysis of the arterial line in group A at 5 min revealed that pCO(2) increased by 1.14+/-2.5 mmHg and HCO(3) by 0.6+/-0.6 mM/L (p<0.02 and p<0.00001, respectively). The corresponding pO(2) and pH values did not show significant differences. Glucose infusion test at 5 min (GITa) was -0.058+/-0.03%. A comparison with baseline (predialysis) values of an analysis of the arterial line in group B at 5 min revealed that pCO(2) increased by 7.7+/-3.5 mmHg and HCO(3) by 2.9+/-1.0 mM/L (p<0.000001 in each case). The pH level was significantly lower in comparison with baseline values (p<0.00001), while pO(2) did not show a significant difference. Glucose infusion test at 5 min (GITb) was 12.0+/-6.1% (p<0.000001 in comparison with GITa values). Clinically significant VAR was defined as HCO(3) increment >1.8 mM/L, based on the receiver-operating characteristics curve, which showed a threshold value of HCO(3) increment >1.8 mmol/L as a predictor of GIT recirculation. Five minutes after the initiation of high-flux HD with a 0 ultrafiltration rate, there is a small increment in arterial HCO(3) values relative to predialysis values. Clinically significant VAR is present when this increment is higher than 1.8 mM/L.

pO2 and pCO2 increment in post-dialyzer blood: the role of dialysate.

Blood returning from a dialyzer during hemodialysis has a higher pO2 and pCO2 content than blood entering the dialyzer, and this has been attributed to the dialysate. The present study investigates this phenomenon. Acid-base and blood-gas parameters (pH, pO2, pCO2 and HCO3) were measured in three groups of stable chronic hemodialysis patients (A, B, and C) undergoing high-flux hemodialysis. In group A (n = 15), "arterial" (a) and "venous" (v) samples were withdrawn simultaneously before dialysis (samples A0), 5 min after circulation of the blood with the dialysate in the by-pass mode (samples A5), and 5 min after high-flux hemodialysis at a zero ultrafiltration rate (samples A10). In group B (n = 11) (a) and (v) samples were withdrawn simultaneously before dialysis (samples B0), 5 min after isolated-ultrafiltration with closed dialysate ports ("isolated-closed" ultrafiltration) (samples B5), and 5 min after high-flux hemodialysis at a zero ultrafiltration rate (samples B10). In group C (n = 14), after an initial arterial blood sample withdrawal before hemodialysis (sample C0), high-flux hemodialysis at a zero ultrafiltration rate was initiated. Five minutes later, blood and dialysate samples were withdrawn simultaneously from the hemodialysis lines (samples C5). In all cases blood and dialysate (bicarbonate) flow rates were set at 0.300 and 0.700 L/min, respectively. FLX-18 hemodialyzers (membrane PEPA 1.8 m2) were used in this study. Analysis of variance revealed significant changes only in venous samples. A comparison of arterial and venous samples revealed no differences between groups A and B before the initiation of dialysis (A0a vs. A0v and B0a vs. B0v, P = NS). The pO2 content was higher in A5v samples than in A5a samples (83.5 +/- 11.2 vs. 88.8 +/- 14.0 mm Hg, P < 0.02), while the level of HCO(3) was higher in A5a samples than in A5v samples (20.8 +/- 2.0 vs. 20.4 +/- 1.8 mEq/L, P < 0.05). A10a samples possessed a higher pH and lower levels of pO2, pCO2, and HCO3 in comparison to A10v samples (P < 0.001 for all). Mean pO2 and pCO2 values in A5v and A10v samples increased by 6.3% and 12.1% and by 1.29% and 52% in comparison to corresponding values of A5a and A10a samples, respectively. The pO2 level was the only parameter that differed significantly between B5a and B5v samples (B5a = 84.6 +/- 10.1 vs. B5v = 98.0 +/- 12.6 mm Hg, P < 0.005). B10a samples possessed a higher pH and lower levels of pCO2, pO2, and HCO3 in comparison to B10v samples (P < 0.0005 for all comparisons). Mean pO2 and pCO2 values in B5v and B10v samples increased by 16.2% and 16.3% and by -0.29% and 64.8% in comparison to corresponding values of B5a and B10a samples, respectively. C5a samples possessed a higher pH and lower levels of pCO2, pO2, and HCO3 in comparison to C5v samples (P < 0.001 for all). Mean pO2 and pCO2 values in C5v samples were, respectively, 16.0% and 65.0% higher than corresponding values of C5a samples. These results indicate that blood returning from the dialyzer after 5 min of high-flux hemodialysis has a higher pO2 and pCO2 than blood entering the dialyzer, and that this difference is due to O2 and CO2 transfer from the dialysate space into the blood.