SummaryBackground and objectives The kinetics of plasma phosphorus (inorganic phosphorus or phosphate) during hemodialysis treatments cannot be explained by conventional one-or two-compartment models; previous approaches have been limited by assuming that the distribution of phosphorus is confined to classical intracellular and extracellular fluid compartments. In this study a novel pseudo one-compartment model, including phosphorus mobilization from a large second compartment, was proposed and evaluated.Design, setting, participants, & measurements Clinical data were obtained during a crossover study where 22 chronic hemodialysis patients underwent both short (2-hour) and conventional (4-hour) hemodialysis sessions. The model estimated two patient-specific parameters, phosphorus mobilization clearance and phosphorus central distribution volume, by fitting frequent intradialytic and postdialytic plasma phosphorus concentrations using nonlinear regression.Results Phosphorus mobilization clearances varied among patients (45 to 208 ml/min), but estimates during short (98 Ϯ 44 ml/min, mean Ϯ SD) and conventional (99 Ϯ 47 ml/min) sessions were not different (P ϭ 0.74) and correlated with each other (concordance correlation coefficient c of 0.85). Phosphorus central distribution volumes for each patient (short: 11.0 Ϯ 4.2 L and conventional: 11.9 Ϯ 3.8 L) were also correlated ( c of 0.45). ConclusionsThe reproducibility of patient-specific parameters during short and conventional hemodialysis treatments suggests that a pseudo one-compartment model is robust and can describe plasma phosphorus kinetics under conditions of clinical interest.

The stability of wavy supercritical cylindrical Couette flow has been studied extensively, but few measurements of the velocity field in flow have been made. Particle image velocimetry was used to measure the azimuthal and radial velocities in latitudinal planes perpendicular to the axis of rotation for wavy cylindrical Couette flow in the annulus between a rotating inner cylinder and a fixed outer cylinder. These measurements were matched to previous measurements of the axial and radial velocity measured in several meridional planes resulting in an experimentally measured, time-resolved, three-dimensional, three-component velocity field for wavy cylindrical Couette flow. Using this complete velocity field it is possible to evaluate details of the flow field. The vortical motion transports azimuthal momentum radially while the axial exchange of fluid between vortices in wavy flow transports azimuthal momentum axially. As the Reynolds number increases, these effects strengthen. Streams of net axial flow stretch axially along the length of the annulus and wind around the vortices from the inner cylinder to the outer cylinder and back while also winding azimuthally in the annulus. The azimuthal velocity measured at the center of a vortex is similar to the azimuthal wave speed. Measurements of the azimuthal velocity in cylindrical surfaces concentric with the axis of rotation suggest that the origin of the waviness is related to a jet-like azimuthal velocity profile rather than the radial outflow jet. Near both cylinder walls, the shear stress is quite large, decreasing to near zero at the middle of the annular gap.

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Our recent work proposed a pseudo one-compartment model for describing intradialysis and postdialysis rebound kinetics of phosphorus. In this model, phosphorus is removed directly from a central distribution volume with the rate of phosphorus mobilization from a second, very large compartment proportional to the phosphorus mobilization clearance. Here, we evaluated factors of phosphorus mobilization clearance and postdialysis central distribution volume from 774 patients in the HEMO Study. Phosphorus mobilization clearance and postdialysis central distribution volume were 87 (65, 116) ml/min, median (interquartile range), and 9.4 (7.2, 12.0) liter, respectively. The phosphorus mobilization clearance was significantly higher for male patients than for female patients. Both the phosphorus mobilization clearance and the postdialysis central distribution volume were significantly associated with postdialysis body weight but negatively with the predialysis serum phosphorus concentration. The postdialysis central distribution volume was also significantly associated with age. Overall, the postdialysis central distribution volume was 13.6% of the postdialysis body weight. Thus, the phosphorus mobilization clearance during hemodialysis is higher when predialysis serum phosphorus concentration is low and higher in male patients than in female patients. The central distribution volume of phosphorus is a space approximating the extracellular fluid volume.

For frequent dialysis schedules, the urea generation factor (GFAC) of one commonly used Kt/V prediction equation should be adjusted based on length in days of the PIDI and number of treatments per week.

♦ Background: Contrary to ultrafiltration, the three-pore model predictions of icodextrin absorption from the peritoneal cavity have not yet been reported likely, in part, due to difficulties in estimating the degradation of glucosepolymer chains by α-amylase activity in dialysate. We incorporated this degradation process in a modified three-pore model of peritoneal transport to predict ultrafiltration and icodextrin absorption simultaneously in rats and humans. ♦ Methods: Separate three-pore models were constructed for humans and rats. The model for humans was adapted from PD Adequest 2.0 including a clearance term out of the peritoneal cavity to account for the absorption of large molecules to the peritoneal tissues, and considering patients who routinely used icodextrin by establishing steady-state plasma concentrations. The model for rats employed a standard three-pore model in which human kinetic parameters were scaled for a rat based on differences in body weight. Both models described the icodextrin molecular weight (MW) distribution as five distinct MW fractions. First order kinetics was applied using degradation rate constants obtained from previous in-vitro measurements using gel permeation chromatography. Ultrafiltration and absorption were predicted during a 4-hour exchange in rats, and 9 and 14-hour exchanges in humans with slow to fast transport characteristics with and without the effect of amylase activity. ♦ Results: In rats, the icodextrin MW profile shifted towards the low MW fractions due to complete disappearance of the MW fractions greater than 27.5 kDa. Including the effect of amylase activity (60 U/L) resulted in 21.1% increase in ultrafiltration (UF) (7.6 mL vs 6.0 mL) and 7.1% increase in icodextrin absorption (CHO) (62.5% with vs 58.1%). In humans, the shift in MW profile was less pronounced. The fast transport (H) patient absorbed more icodextrin than the slow transport (L) patient during both 14-hour (H: 47.9% vs L: 40.2%) and 9-hour (H: 37.4% vs L: 31.7%) exchanges. While the UF was higher during the longer exchange, it varied modestly among the patient types (14-hour range: 460 -509 mL vs 9-hour range: 382 -389 mL). When averaged over all patients, the increases in UF and CHO during the 14-hour exchange due to amylase activity (7 U/L) were 15% and 1.5%, respectively. ♦ Conclusion: The icodextrin absorption values predicted by the model agreed with those measured in rats and humans to accurately show the increased absorption in rats. Also, the model confirmed the previous suggestions by predicting an increase in UF specific to amylase activity in dialysate, likely due to the added osmolality by the small molecules generated as a result of the degradation process. As expected, this increase was more pronounced in rats than in humans because of higher dialysate concentrations of amylase in rats.Perit Dial Int 2015; 35(3):288-296 www.PDIConnect.com epub ahead of print:

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♦ Background: Intermittent peritoneal dialysis (IPD) is an old strategy that has generally been eclipsed, in the home setting, by daily peritoneal therapies. However, for a select group of patients with exhausted vascular access or inability to receive PD at home, in-center IPD may remain an option or may serve as an incremental strategy before initiation of full-dose PD. We investigated the residual kidney clearance requirements necessary to allow thrice-weekly IPD regimens to meet current adequacy targets. ♦ Methods: The 3-pore model of peritoneal transport was used to examine 2 thrice-weekly IPD dialysis modalities: 5 -6 dwells with 10 -12 L total volume (low-dose IPD), and 50% tidal with 20 -24 L total volume (high-dose IPD). We assumed an 8-hour dialysis duration and 1.5% dextrose solution, with a 2-L fill volume, except in tidal mode. The PD Adequest application (version 2.0: Baxter Healthcare Corporation, Deerfield, IL, USA) and typical patient kinetic parameters derived from a large dataset [data on file from Treatment Adequacy Review for Gaining Enhanced Therapy (Baxter Healthcare Corporation)] were used to model urea clearances. The minimum glomerular filtration rate (GFR) required to achieve a total weekly urea Kt/V of 1.7 was calculated. ♦ Results: In the absence of any dialysis, the minimum residual GFR necessary to achieve a weekly urea Kt/V of 1.7 was 9.7 mL/min/1.73 m 2 . Depending on membrane transport type, the low-dose IPD modality met urea clearance targets for patients with a GFR between 6.0 mL/ min/1.73 m 2 and 7.6 mL/min/1.73 m 2 . Similarly, the high-dose IPD modality met the urea clearance target for patients with a GFR between 4.7 mL/min/1.73 m 2 and 6.5 mL/min/1.73 m 2 . ♦ Conclusions: In patients with residual GFR of at least 7.6 mL/min/1.73 m 2 , thrice-weekly low-dose IPD (10 L) achieved a Kt/V urea of 1.7 across all transport types. Increasing the IPD volume resulted in a decreased residual GFR requirement of 4.7 mL/min/1.73 m 2 (24 L, 50% tidal). In patients with residual kidney function and dietary compliance, IPD may be a viable strategy in certain clinical situations.