Page 699 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

686 SPECIAL THERAPY

the kinetic prediction of dialysis dose can be derived very kinetic analysis by using more mathematically complex
simply from the measured predialysis and postdialysis double-pool 142 or noncompartmental kinetic modeling
BUN concentrations. It must be emphasized, that this methods (Figure 29-5). The double-pool variable
expression represents a gross oversimplification of the volume kinetic model accounts for intercompartmental
events and kinetic variables during therapeutic hemodial- solute diffusion during and after completion of hemodi-
ysis and should be used only to provide a rough estimate alysis, and dp Kt/V is regarded as the standard for dialysis
of the dialysis dose. dose. Optionally, correction algorithms that account for
During a therapeutic dialysis session, the relationships these compartmental deviations have been applied to sin-
between G, V, and K (illustrated in Figure 29-1) are more gle-pool assessments using additional blood sampling
complex, highly interdependent, and cannot be described and appropriate software in human patients. 41,48,69 These
mathematically by a single simple relationship. Mathe- correction formulas minimize many of the limitations of
matical description of each variable, however, can be single-pool estimates but have not been validated in
defined in terms of the other two with formal urea kinetic animals. More accurate predictions of dialysis dose also
modeling (Appendix, Equations 8 through 10). When can be obtained using single-pool kinetic calculations
one of the variables (G, V, or K) is known, the others by incorporating an equilibrated BUN obtained 45 to
can be resolved by simultaneous iterative solution of 60 minutes after cessation of the treatment as the end-
the equations to yield a unique solution for the unknowns dialysis value. Use of the equilibrated BUN in the sin-
when residual renal clearance (K r ), instantaneous dialyzer gle-pool calculations yields e Kt/V as a measured dialysis
clearance (K d ), ultrafiltration volume, and the measured dose that closely approximates the dp Kt/V and better
changes in BUN during and after the treatment are reflects whole patient clearance. Both the e Kt/V and
47,48,141–143
known. These computations are performed the dp Kt/Vassessments of dialysis dose will be lower than
easily with commercially available software or can be dose predicted as the sp Kt/V.
programmed into routine spreadsheet applications. Online measurement of these kinetic determinants of
The simplified single-pool, fixed-volume model dialyzer performance and dialysis dose can be computed
presumes conditions not generally valid in therapeutic in real-time with ionic dialysance techniques that advance
dialysis sessions and loses accuracy if total body water
(TBW) changes during or between treatments. The
model also loses accuracy during high-efficiency Liver
treatments of short duration, when the urea distribution G
does not behave as a single homogenous compartment.
Delayed diffusion from the intracellular compartment Central Peripheral
or variations in diffusion among discrete fluid Kd
compartment Kc compartment
compartments (e.g., skin, muscle, gut) with different per- V1 V2
fusion and transference characteristics creates a solute dis-
equilibrium between compartments that promotes a
Kr
postdialysis rebound of urea that is not predicted by
immediate postdialysis blood sampling. 47,55,126,144 Kidney
Deviations in the assumptions for single-pool, fixed-vol-
Double-pool variable-volume model
ume kinetics can be minimized by measurement of the Figure 29-5 Graphic illustration of the double-pool variable-
postdialysis urea at 45 to 60 minutes after the end of volume kinetic model of the urea metabolism during high efficiency
the dialysis treatment rather than immediately hemodialysis. In this model, the urea generation rate (G), the renal
postdialysis. By this time, intercompartmental shifts (or clearance (Kr), and the dialyzer clearance (Kd) are the major
rebound) have reestablished solute equilibrium, and the determinants of urea content in the central compartment (volume
plasma concentration reflects the equilibrated concentra- V1). An additional peripheral compartment (volume V2)
tion of urea across all body compartments. 47,151,163 As continuously exchanges solutes and water with the central pool.
stated previously, therapeutic hemodialysis deviates con- The bidirectional rate constant for urea transference between the
siderably from the single-compartment model illustrated two pools is indicated by Kc. When Kc ¼ 1, urea diffuses freely
between the compartments and the system reverts to a single-pool
in Figure 29-1. Retained solutes, including urea, can be model. A lower Kc implies a slower diffusional component into and
distributed in multiple compartments, which are partially out of the peripheral compartment. If the peripheral compartment
secluded from the dialyzer by delayed transfer or remains unaccounted for, single-pool kinetic modeling results in a
differences in regional perfusion. Most dialysis treatments lower apparent V, a more rapid decrease of the urea concentration
also require ultrafiltration, and urea generation proceeds in the central pool, a greater postdialysis rebound, and
throughout the session which further deviate the serum overestimation of the dose of dialysis, Kt/V. Anatomically, the two
urea concentration from single-pool predictions. These compartments can represent the extracellular and intracellular
deviations from single-pool, fixed volume assumptions spaces, respectively, or body areas with different perfusion
can be improved to provide greater accuracy to urea characteristics.