Page 696 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Hemodialysis and Extracorporeal Blood Purification 683

70
(61.3)
Liver 60 (57.9)
G (3.6 mg/min) spKt/V 1.6
50

Kd 40
(166 mL/min) Urea pool BUN (mg/dl)
V (31 L) 30 TAC 35.8 mg/dl

20
Kr (0.4 mL/min) (14.0)
10
Kidney
Td Ti
0
Single-pool, fixed-volume model 0 24 48 72 96
A B Time (h)
Fig. 29-1 A, Single-pool, fixed-volume kinetic model of the urea metabolism and representative modeled
kinetic parameters determined in a 33-kg dog on intermittent maintenance hemodialysis consuming
approximately 56 g of dietary protein. Urea is generated in the liver as the major end product of protein
metabolism. The urea generation rate, G (mg of urea/min), determines the accumulation of urea in the urea
pool with a volume, V (L). Its removal from the urea pool is determined by the continuous residual renal
clearance, Kr (mL/min), and intermittently by hemodialysis via the urea clearance of the dialyzer,
Kd (mL/min). B, Graphic illustration of a three-point BUN profile (before and after hemodialysis values in
parentheses) that can be fitted to the single-pool model in the right panel. With direct measurement of renal
and dialyzer urea clearances (Kr, Appendix, Equation 6 and Kd, Appendix, Equation 5, respectively), kinetic
modeling allows computation of the urea generation rate (G, Appendix, Equation 9), the urea distribution
volume (V, Appendix, Equation 10), and the time-average concentration of BUN (TAC, Appendix,
Equation 1). The dose of dialysis expressed as the fractional clearance of the urea distribution volume using
single-pool kinetics (spKt/V, Appendix, Equation 11) also can be calculated. T d is the duration of dialysis,
and Ti is the duration of the interdialytic interval. AUC is the area under the BUN versus the time curve
and can be estimated using a trapezoidal method or ideally calculated by fitting the changes in BUN to the
kinetic model.

DIALYSIS ADEQUACY recovery of renal function. Realistic outcomes for these
animals that are treated with hemodialysis vary depending
The optimal outcome for animals with acute renal failure
on age, chronicity of the disease, comorbidities, and
is survival until renal function has recovered, but second-
residual renal function. Appropriate markers for dialysis
ary goals may vary qualitatively depending on the nature
adequacy include length of survival, owner perceived
of the underlying disease. An optimal outcome addition-
quality of life (e.g., activity, social interaction, appetite),
ally should promote physiologic and metabolic stability
elimination of uremic symptomatology (hypertension,
to facilitate recovery and promote an acceptable quality
of life while minimizing secondary injury to the hyperphosphatemia, anemia), nutritional adequacy, and
recovering kidneys or other organs (heart, lungs, gut, elimination of dialysis-associated complications.
brain). As an outcome, survival is multifactorial and For both acute and chronic dialysis, survival is a diffi-
predicated on the diversity of the underlying cause and cult outcome parameter to correlate specifically to dialytic
interventions. Yet, despite these constraints, the kineti-
comorbidities, in addition to the delivered therapy. As
cally modeled dose of dialysis (Kt/V) has been shown
such, outcome assessment by survival alone may be
to correlate independently with survival as an outcome
disassociated from recovery of renal function or adequate
humans
delivery of dialysis. 34,149 Consequently, more sensitive in 73,106,120,124 undergoing maintenance hemodialy-
sis, and it is likely to be linked similarly to
and predictive outcome measures should be considered
the success of dialysis in animals. The empirical use
for assessment of dialysis adequacy, including recovery
of renal function, improvement of the systemic of proven standards of dialysis adequacy and clinical expe-
manifestations of uremia, and reduction of complications rience in human patients are useful first approximations
attending uremia or its therapy. 19 for appropriate veterinary standards of dialysis adequacy
Survival is the optimal outcome for animals with end- until evidence-based standards are determined in
34,59
animals.
stage renal disease because there is no prospect for