A two-pool kinetic model predicts phosphate concentrations during and shortly following a conventional (three times weekly) hemodialysis session

Comparison of modeled versus reported phosphate removal and modeled versus postdialysis serum phosphate levels in conventional hemodialysis

BACKGROUND

We compared predictions of phosphate removal by a 2-pool kinetic model with measured phosphate removal in spent dialysate as reported by others.

METHODS

Twenty-six studies were identified that reported phosphate removal in 35 groups of patients. In almost all studies, patients were dialyzed for close to 4 h (range 3 to 6 h). For each study, group mean values of predialysis serum phosphate, body size, dialyzer K₀ A urea, blood and dialysate flow rates, and session lengths were input into the kinetic model. Predictions of group mean phosphate removal and postdialysis serum phosphate were compared with reported measured values.

RESULTS

Mean (by patient group) predicted phosphate removal was 931 ± 170 mg/treatment, somewhat higher (p < 0.001) than the reported measured value, 900 mg ± 287. The ratio of predicted/measured removal averaged 1.15 ± 0.427. In 5/35 patient groups (3/26 studies) the predicted/measured phosphate removal was greater than 1.50. If these groups were excluded, the mean measured phosphate removal was 990 mg versus 966 predicted, with a ratio of predicted/measured removal averaging 0.993. Measured group mean postdialysis serum phosphate values (reported in 25/35) were 2.64 ± 0.54, not significantly different from predicted (2.60 ± 0.24 mg/dl, p = NS).

CONCLUSIONS

For conventional 4-h dialysis treatments, phosphate removal and postdialysis serum phosphate values predicted by a 2-pool kinetic model are similar to reported measured values.

Intracellular Phosphate and ATP Depletion Measured by Magnetic Resonance Spectroscopy in Patients Receiving Maintenance Hemodialysis

BACKGROUND

The precise origin of phosphate that is removed during hemodialysis remains unclear; only a minority comes from the extracellular space. One possibility is that the remaining phosphate originates from the intracellular compartment, but there have been no available data from direct assessment of intracellular phosphate in patients undergoing hemodialysis.

METHODS

We used phosphorus magnetic resonance spectroscopy to quantify intracellular inorganic phosphate (Pi), phosphocreatine (PCr), and βATP. In our pilot, single-center, prospective study, 11 patients with ESKD underwent phosphorus (31P) magnetic resonance spectroscopy examination during a 4-hour hemodialysis treatment. Spectra were acquired every 152 seconds during the hemodialysis session. The primary outcome was a change in the PCr-Pi ratio during the session.

RESULTS

During the first hour of hemodialysis, mean phosphatemia decreased significantly (-41%; P<0.001); thereafter, it decreased more slowly until the end of the session. We found a significant increase in the PCr-Pi ratio (+23%; P=0.001) during dialysis, indicating a reduction in intracellular Pi concentration. The PCr-βATP ratio increased significantly (+31%; P=0.001) over a similar time period, indicating a reduction in βATP. The change of the PCr-βATP ratio was significantly correlated to the change of depurated Pi.

CONCLUSIONS

Phosphorus magnetic resonance spectroscopy examination of patients with ESKD during hemodialysis treatment confirmed that depurated Pi originates from the intracellular compartment. This finding raises the possibility that excessive dialytic depuration of phosphate might adversely affect the intracellular availability of high-energy phosphates and ultimately, cellular metabolism. Further studies are needed to investigate the relationship between objective and subjective effects of hemodialysis and decreases of intracellular Pi and βATP content.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

Intracellular Phosphate Concentration Evolution During Hemodialysis by MR Spectroscopy (CIPHEMO), NCT03119818.

Hemodialysis Treatment Time: As Important as it Seems?

Hemodialysis treatment time and Kt/V can both be considered to be primary measures of hemodialysis adequacy, because when either goes to zero, mortality is certain in patients without residual kidney function. Treatment time is important, but it needs to be adjusted based on surface-area-normalized Kt/V, residual kidney function, and expected ultrafiltration rate. Rescaling dose of dialysis measured as Kt/V to body surface area prevents ultrashort dialysis in small patients, women, and children with minimal residual kidney function. Most if not all of the observational studies of associations between outcome and dialysis session length are probably confounded by dose targeting bias. Once adequate Kt/V (taking into account body surface area) has been provided, adequate dialysis time probably is most relevant in terms of limiting the need for a high fluid removal rate. The latter may adversely impact survival by causing recurrent ischemia to cardiovascular and other tissues. There is little high-quality evidence at this time to support a minimum 4-hour treatment time for all patients, regardless of body size, solute removal, or residual kidney function. On the other hand, there is little evidence that prolonging weekly treatment time up to 24 hours per week is harmful. The final decision regarding treatment time is best individualized, based on patient acceptability and experience, residual kidney function, body surface-area-normalized Kt/V, and expected ultrafiltration rate.