Background: During standard hemodialysis, amino acid losses are substantial, amounting to 6 to 9 g per treatment. When these nutritional supplements are infused during dialysis, losses are increased, but a net positive balance can be achieved if the infusion rate is high enough. High-flux dialyzers, used with increasing frequency in modern dialysis centers because of their more permeable synthetic membranes, should cause further amino acid losses; however, the increase has not been measured, and the effect on plasma levels has not been examined. Assessment of net balance requires measurement of blood concentrations as well as of clearance.
Methods: To quantitate the effect of high-flux dialysis on amino acid balance, we measured clearances, plasma levels, and losses of individual amino acids during hemodialysis in patients with acute renal failure who required daily parenteral nutrition.
Results: Nearly all predialysis amino acid levels in plasma were within the normal range, probably because of control of uremia with prior dialyses and from continuous infusion. In paired studies, clearances were higher (150 +/- 15 mL/min vs 107 +/- 11 mL/min, p < .01), and levels fell more at mid-dialysis with high-flux membranes (28% +/- 5%) than with conventional cellulosic membranes (4 +/- 5%, p < .05). Mean losses of amino acid were 5.2 +/- 0.6 g per conventional dialysis, representing 60% of the total infused, and 7.3 +/- 1.8 g per high-flux dialysis, or 80% of the simultaneous infusion. Fractional losses decreased at higher infusion rates, but losses of individual amino acids varied from one fourth to more than 10 times the amount infused. Compared with other small solutes, plasma levels were relatively well maintained even during high-flux dialysis, a factor that enhanced removal by the dialyzer. Total balance depended more on the infusion rate than on the dialysis membrane.
Conclusions: These studies show that positive balance can be achieved with concurrent infusion during dialysis, especially at higher amino acid delivery rates. High-flux dialysis causes a greater disturbance of amino acid equilibrium than conventional dialysis does, but 24-hour gains far exceeded losses in the dialysate for most of the amino acids.