Model-Based Analysis of Potassium Removal During Hemodialysis

Potassium ion (K+) kinetics in intra- and extracellular compartments during dialysis was studied by means of a double-pool computer model, which included potassium-dependent active transport (Na-K-ATPase pump) in 38 patients undergoing chronic hemodialysis. Each patient was treated for 2 weeks with a constant K+ dialysate concentration (K+CONST therapy) and afterward for 2 weeks with a time-varying (profiled) K+ dialysate concentration (K+PROF therapy). The two therapies induced different levels of K+ plasma concentration (K+CONST: 3.71 ± 0.88 mmol/L vs. K+PROF: 3.97 ± 0.64 mmol/L, time-averaged values, P < 0.01). The computer model was tuned to accurately fit plasmatic K+ measured in the course and 1 h after K+CONST and K+PROF therapies and was then used to simulate the kinetics of intra- and extracellular K+. Model-based analysis showed that almost all the K+ removal in the first 90 min of dialysis was derived from the extracellular compartment. The different K+ time course in the dialysate and the consequently different Na-K pump activity resulted in a different sharing of removed potassium mass at the end of dialysis: 56% ± 17% from the extracellular compartment in K+PROF versus 41% ± 14% in K+CONST. At the end of both therapies, the K+ distribution was largely unbalanced, and, in the next 3 h, K+ continued to flow in the extracellular space (about 24 mmol). After rebalancing, about 80% of the K+ mass that was removed derived from the intracellular compartment. In conclusion, the Na-K pump plays a major role in K+ apportionment between extracellular and intracellular compartments, and potassium dialysate concentration strongly influences pump activity.