One of the most important pathogenetic factors involved in the onset of intradialysis arrhytmias is the alteration in electrolyte concentration, particularly potassium (K+). Main aim of the present study was to identify and rank the factors determining the intradialysis K+ mass balance (K+MB).
Two studies were performed: in Study A 11 stable prevalent Caucasian anuric patients underwent one standard (~ 4h) and one long-hour (~ 8h) slow-flow bicarbonate haemodialysis (HD) session at the midweek interval. The sessions were pair-matched as far as the dialysate and blood volume processed (90 L) and volume of ultrafiltration are concerned. In Study B 63 stable prevalent Caucasian anuric patients underwent one 4h standard midweek bicarbonate HD session. Dialysate K+ concentration was 2.0 mmol/L in both studies. Blood samples were obtained from the inlet blood tubing immediately before the onset of dialysis and at t60, t120, t180 min and at end of the 4h and 8h sessions for the measurement of plasma K+, blood bicarbonates and blood pH. Additional blood samples were obtained at t360 min for the 8h sessions. Direct dialysate quantification was utilized for K+MBs.
Study A: mean K+MBs resulted significantly higher in the 8h sessions (4h: – 88.4 + 23.2 SD mmol vs 8h: – 101.9 + 32.2 mmol; P = 0.02). Bivariate linear regression analyses between K+MBs and many parameters were performed. Only mean intradialysis plasma K+, area under the curve of hourly inlet diffusion concentration gradients of K+ and bicarbonates (respectively, hcgAUCK+ and hcgAUCbicarbonates) and mean blood bicarbonates resulted significantly related to K+MB in both 4h and 8h sessions. A multiple linear regression output with K+MB as dependent variable and duration of treatment as independent variable showed that only mean intradialysis plasma K+, hcgAUCK+ and duration of HD sessions per se remained statistically significant. Figure 1 shows the trends of plasma K+ concentrations, blood pH and blood bicarbonates levels in the 4h and 8h sessions.
Study B: mean K+MBs were – 86.7 + 22.6 mmol. Trends of hourly plasma K+ concentrations, blood pH and blood bicarbonate concentrations are shown in Figures 2a, 2b and 2c, respectively. Several bivariate linear regression analyses were performed, having K+MB as dependent variable. Again, only mean intradialysis plasma K+ (Figure 3a), hcgAUCK+ (Figure 3b) and mean blood bicarbonates resulted significantly related to K+MB. However, only mean intradialysis plasma K+ and hcgAUCK+ predicted K+MB at the multiple regression analysis.
Our studies establish unequivocably the ranking of factors determining intradialysis K+MB: plasma K+ → dialysate K+ gradient is the main determinant; acid-base balance plays a much less important role. The duration of HD session per se is an independent determinant of K+MB.