Page 705 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

692 SPECIAL THERAPY

TABLE 29-2 Treatment Intensity Prescription

Initial Treatment
BUN <200 mg/dL URR <0.5 at no greater than 0.1 URR/hr
200-300 mg/dL URR 0.5-0.3 at no greater than 0.1 URR/hr
>300 mg/dL URR 0.3 at no greater than 0.05-0.07 URR/hr
Second Treatment
BUN <200 mg/dL URR 0.6-0.7 at 0.12-0.15 URR/hr
200-300 mg/dL URR 0.6-0.4 at no greater than 0.05-0.1 URR/hr
>300 mg/dL URR 0.4 at no greater than 0.05-0.1 URR/hr
Third and Subsequent Treatments
BUN <150 mg/dL URR >0.8 at >0.15 URR/hr
150-300 mg/dL URR 0.5-0.6 at 0.15-0.1 URR/hr
>300 mg/dL URR 0.5-0.6 at <0.1 URR/hr

recommendations could exceed safe guidelines at the the dialyzer will be essentially complete and the urea
beginning of the treatment in extremely azotemic animals clearance will equal Q b. By the third and subsequent
if the URR goal is too high or the treatment time is short. treatments, the BUN usually is less than 150 mg/dL,
Extended slow dialysis treatments also facilitate and blood flow can be increased cautiously to 5 mL/
removal of large volumes of fluid that risk volume con- kg/min. For intense, high-efficiency treatments during
traction and hypotension during shorter treatments. the maintenance phase of management, blood flow rates
Treatment intensity is indexed to urea transfer, which between 10 and 20 mL/kg/min or the maximal flow
occurs faster than solutes (e.g., potassium, phosphate, achieved by the vascular access can be used.
and creatinine) that are less diffusible or For severely uremic cats or small dogs with BUN
compartmentalized and poorly transferable. Longer concentrations greater than 250 mg/dL, it is preferable
treatments enhance removal of urea in addition to to extend the treatment time to greater than 5 hours
secluded solutes that do not behave like urea. 55,63,113 while providing exceptionally slow blood flow and urea
clearance rates to deliver a sufficiently gradual target of
Extracorporeal Blood Flow <0.1 URR/hr. In some cases, it may not be possible to

Blood flow is a major determinant of treatment intensity adjust the pump speed sufficiently to deliver a blood flow
andbecomesdefinedinsequenceastheURRgoal;required rate slow enough to correct the azotemia safely. For
volume of processed blood and treatment time are decided. example, a 4 kg cat with an initial BUN of 330 mg/dL
For a 20 kg dog with AKI and a BUN of 295 mg/dL, a would require approximately 1.2 L of blood processing
URR of 0.4 (40%) might be prescribed. The requisite treat- to achieve a treatment URR of 0.4 (or 40%)
ment volume for this target would be 0.4 L/kg or 8 L of (Figure 29-4). If the treatment were delivered safely over
total treatment (see Table 29-2; Figure 29-3). Appropriate 360 minutes (0.07 URR/hr), the required Q b would be
combinations of dialysis time and blood flow rate are next 3.3 mL/min. The dilemma is most dialysis machines can-
computedtoachievethe8L goal.Fora240-minute dialysis not deliver accurately a blood flow at this low rate. A faster
session time (0.1 URR/hr), the required Q b would be Q b will intensify the treatment and shorten the time to
33 mL/min (i.e., 8000 mL/240 minutes; 1.7 mL/ treatment goal unacceptably. At a Q b of 10 mL/min
kg/min), whereas for a 360-minute session time (0.06 (which is still too slow for many machines), the treatment
URR/hr), the required Q b would be 22 mL/min time would be only 120 minutes (0.2 URR/hr) and
(1.1 mL/kg/min). A higher first treatment URR target unsafe for the target URR. In these circumstances, it is
could be selected with appropriate extension of the treat- possible to extend the treatment time and lower the effec-
ment time to maintain a safe hourly URR. tive Q b by alternating periods of active dialysis with delib-
Without URR-derived estimates for Q b , blood flow erate intervals of bypass in which blood flow continues
must be determined empirically to provide adequate but dialysate flow and hence dialysis are stopped. There
and safe treatments. When the initial BUN concentration is some continued diffusion into the dialysate contained
is greater than 300 mg/dL, the blood flow rate should be in the dialyzer as the system is placed in bypass, but gen-
limited to 1 to 1.5 mL/kg/min or less to prevent overly erally, alternating 5 to 10 minutes of dialysis with 5 to 20
intense or rapid treatment. If the BUN concentration is minutes of bypass decreases the effective Q b and hourly
between 150 and 300 mg/dL, blood flow for initial URR, and extends the time to treatment goal by twofold
treatments should be limited to 1.5 to 2.0 mL/kg/ to fourfold. Ultrafiltration continues during the bypass,
min. At these slow blood flow rates, urea extraction across facilitating fluid removal during the extended treatment