Page 703 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

690 SPECIAL THERAPY

BOX 29-3 Components of the animals with marked azotemia (BUN >250 mg/dL) will
experience quantitatively greater urea removal per unit of
Hemodialysis time and blood flow than those with lesser degrees of azo-
Prescription temia. For patients with severe azotemia a low-efficiency
dialyzer with a lower urea clearance may be more appro-
1. Selection of the hemodialyzer (surface area, bundle priate and safer than use of a high-efficiency device.
volume, solute and ultrafiltration characteristics, A smaller dialyzer can be selected also for initial
hemocompatibility, and biocompatibility) treatments with reduced blood flow rates to limit the
2. Selection of extracorporeal circuit and priming resident time of blood in the dialyzer to minimize clot-
solution
ting. At a blood flow rate of 20 mL/min, the resident
3. Blood flow rate (Qb)
time of blood in a 28-mL dialyzer is only 1.4 minutes
4. Dialysis time (Td) and scheduled bypass time 2
compared with 9 minutes in a 1.5-m dialyzer with a
5. Dialysate composition and/or modeling
blood volume of 180 mL.
6. Dialysate flow rate and direction (Qd)
7. Treatment schedule Treatment Intensity
8. Access connection (“single needle,” reversed
direction) Initial dialysis treatments are prescribed to be less inten-
9. Anticoagulation (anticoagulant, target ACT, sive (slower blood flow rate, smaller dialyzer surface area,
protocol) and possibly shorter treatment time) than those pre-
10. Ultrafiltration (volume target, rate) scribed for subsequent treatments. At slow blood flow
11. Ancillary medications rates, urea extraction across the dialyzer approaches
12. Monitoring schedule 100%, and urea clearance (K d-urea , in milliliters per min-
13. Rinse back (solution, volume, air) ute) is approximately equal to extracorporeal blood flow
14. Catheter locking solution (Q b , in milliliters per minute). When high-efficiency and
15. Posttreatment (medications, monitoring)
high-flux dialyzers are used, K d-urea increases quantita-
tively with Q b until blood flow exceeds 200 mL/min. 47
At blood flow rates above 200 mL/min, the relationship
dialysis goals for initial treatments in animals with AKI flattens as urea clearance is influenced by membrane
differ considerably from the goals and prescription for characteristics and dialysate flow in addition to Q b . 47
later dialysis treatments. At blood flow rates greater than 300 mL/min, dialyzer
performance is influenced minimally by increased
Hemodialyzers single-pass flow, but total solute removal during the treat-
Selection of the hemodialyzer is based initially on its con- ment will increase as a function of the cumulative flow
tribution to the extracorporeal volume and secondarily through the dialyzer. The total volume of blood passed
on its diffusive, convective, and biocompatibility through the dialyzer during the treatment (Q b T d )
properties according to guidelines in Table 29-1. The has been established as a reasonable predictor of the
smallest neonatal hemodialyzer currently available has a intensity of the treatment as estimated by the URR
2
0.3 m surface area and a 28-mL blood volume compart- (Figures 29-3 and 29-4). 34,59,96 This relationship can be
ment (F3, Fresenius Medical Care, Waltham, Mass.). For used as an operational parameter to guide the prescription
cats and dogs weighing less than 6 kg, a dialyzer with a and delivery of dialysis to the target URR for differing
2
surface area between 0.2 and 0.4 m and a priming vol- severities of uremia and phases of management
ume less than 30 mL generally is tolerated. A synthetic (Table 29-2).
dialyzer (neonatal or pediatric) with a surface area
2
between 0.4 and 0.8 m and a priming volume less than Dialysis Time
45 mL is appropriate for use in dogs weighing between 6 Once the target URR is defined for the treatment, the
and 12 kg of body weight. Dialyzers with surface areas up approximate volume of blood requiring dialytic
2
to 1.5 m and priming volumes up to 80 mL can be used processing to achieve the goal can be determined
on dogs between 12 and 20 kg of body weight. Larger (Figures 29-3 and 29-4). From this volume (Q b t),
dialyzers with surface areas greater than 2.0 m 2 and appropriate combinations of blood flow rate (Q b ) and
priming volumes greater than 100 mL can be used in dialysis time (t) can be derived. For patients with moder-
dogs weighing more than 30 kg. ate to severe azotemia, a long dialysis session time (slow
2
A dialyzer with a smaller surface area (0.3 to 0.5 m ) Q b ) is preferable to a short session time (fast Q b ) that
than recommended may be chosen preferentially in dogs yields the same volume of processed blood and prescribed
of all sizes for initial hemodialysis treatments when the URR. Prescription of a dialysis session time less than 180
BUN concentration is greater than 200 mg/dL to reduce minutes could promote use of inappropriate blood flow
the intensity of the treatment and risk of dialysis disequi- rates that induce rapid changes in BUN and life-threaten-
librium. Solute removal follows first order kinetics, and ing dialysis complications. Short treatments usually cause