Page 701 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

688 SPECIAL THERAPY

an hypothetical continuous urea clearance that would The rationale to scale dialysis dose to the nebulous
achieve a constant blood urea concentration identical to index (V) that cannot be readily measured has kinetic jus-
the average predialysis urea concentration for all intermit- tification and historical acceptance. The first order kinet-
tent treatments provided during the week. This theoreti- ics of urea removal by dialysis proceeds with an
cal concept allows comparisons among dialysis schedules elimination constant equal to K d /V and is a measure of
with differing dialysis times and intervals, including the the intensity of the treatment. Even though V is not
extreme case of continuous therapy. measured directly, it is derived mathematically to yield
A dialysis schedule with three 4-hour treatments per the expression, Kt/V, with kinetic modeling. Recently,
week with a sp Kt/V of 2.0 per treatment is equivalent to however, the universality of scaling dialysis dose to the
a std Kt/V of 2.7. Increasing the schedule to six 2-hour urea distribution volume has been questioned in human
treatments per week ( sp Kt/V, 1.0 per treatment) with patients as the relative distribution volume varies inde-
the same total 12 hours of weekly dialysis substantially pendently of body size, between genders, and in patients
increases the amount of dialysis delivered to the equivalent of differing body composition. 43 Consequently, scaling
std Kt/Vof 3.9 (Appendix, Equation 12). Stated differently, dialysis dose to V may promote under treatment in some
a three times a week, 240-minute treatment schedule individuals and relative overtreatment in others. The
( std Kt/V, 2.7) requiring 12 hours of treatment could be comparative significance of this issue has not been
provided with equivalent efficacy by considerably shorter addressed in animals, but it is likely the diversity of size,
treatments of 70 minutes per session if provided six times species, and breed, in addition to gender, in animal
weekly for a total weekly dialysis time of 7 hours. Although patients that could impose even greater variance in the
reduction of the individual treatment time is possible relative urea distribution volume than seen in humans.
according to this analogy and for illustrative purposes, The effect of dose of dialysis on outcome has been
this recommendation would not be clinically demonstrated in humans with end-stage chronic kidney
prudent. 52,55,66,113 Conversely, decreasing the frequency disease in several large-scale clinical studies.* The dose
of dialysis to two treatments per week would require exten- of dialysis that is adequate to manage dogs and cats with
sion of each treatment to almost 24 hours to achieve an either acute or chronic kidney failure needs to be
equivalent std Kt/V. These quantitative predictions illus- established using appropriate tools for treatment quanti-
trate the marked benefits to increased frequency of therapy fication. However, until these parameters are established,
and are in accordance with recent clinical observations, routine application of UKM extends therapeutic insights
suggesting it is difficult to compensate for decreased of dialysis delivery far beyond reliance on routine chem-
frequency of therapy with longer treatment times. 47,50,172 istry tests and clearly benefits the assessment and clinical
As an alternative to sdt Kt/V for comparing the equiva- management of uremic animals. Kinetic parameters and
lency of intermittent and continuous therapies, including quantitation of dialysis delivery are important tools for
residual renal function, the intermittent kinetics of hemo- quality assurance of dialytic therapy in animals; however,
dialysis can be converted to a continuous equivalent clear- they are not therapeutic goals per se. 186 The provision of
ance (EKR). 25,28,50,183,186 This concept is more intuitive a yet-to-be-defined minimal dose of dialysis is only one of
for most clinicians as the relative contribution of dialysis the requirements of therapeutic adequacy, and manage-
can be compared directly with residual renal function and ment of uremia necessitates an individually tailored global
with other intermittent or continuous dialytic therapies approach to the animal.
(Appendix, Equation 7). Total patient clearance (renal
clearance, Kr, and dialyzer clearance, EKR) is expressed USE OF HEMODIALYSIS TO
in the familiar term (milliliters per minute) of clearance, CORRECT UREMIA
similar to the glomerular filtration rate, and the resulting
total clearance can be used to predict the expected uremic The major application of dialytic therapy is the transient
morbidity, similar to patients with earlier stages of chronic elimination of innumerable and unspecified solutes and
kidney disease. fluid retained during renal failure that would otherwise
A prerequisite for the validity of most urea kinetic be cleared by healthy kidneys. The benefits of intermit-
modeling algorithms is the presumption of steady-state tent dialysis are transient, and with cessation of dialysis,
urea metabolism (i.e., constant food intake (quality and the concentrations of urea and all retained uremia solutes
quantity), constant endogenous nitrogen metabolism with continued generation increase immediately until a
and catabolism, stable body weight, and a regular dialysis new steady state is achieved or until the next dialysis ses-
schedule). 48 These conditions rarely exist for most veter- sion (Figures 29-1 and 29-3). It is firmly established that
inary applications of hemodialysis that are prescribed for uremia is associated with retention of a myriad of low-
acute kidney failure; however, classic double-pool, molecular-weight solutes that are effectively predicted
equilibrated, and EKR analyses appear valid under these by the blood urea concentration; dialytic removal of these
conditions in human patients if careful attention is paid to
the accuracy of all input variables. 26,44,87 *References 28, 56, 71, 73, 106, 120, 124, 171.