Page 709 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

696 SPECIAL THERAPY

Animal patients inadvertently may be protected from anticoagulation difficulties and makes monitoring essen-
moderate or overt hemodynamic events by the imposed tial throughout the dialysis session. Automated activated
lower temperature limits of human dialysis delivery clotting time (ACT) is used most commonly to prescribe
systems. Recent studies in human patients demonstrated and monitor safe heparin requirements, but other coagu-
hemodynamic tolerance is better preserved in dialysis lation measures can be used with equal reliability.
treatments when the patient maintains isothermic balance Automated ACT has proven reliable and predictive with
or is slightly cooled. 30,109,110,150,176 To obviate tempera- point-of-care convenience and cost-effectiveness.
ture-mediated hemodynamic events, core body tempera- Low-molecular-weight heparins are used with increased
ture should be monitored in patients throughout the frequency in human dialysis, but there appears to be
dialysis session—especially in patients undergoing rapid little difference in their respective efficacy during the dial-
ultrafiltration or those predisposed to hypotension. If ysis session. The transition to low-molecular-weight
core temperature increases above normal, dialysate heparins is directed to minimize heparin-induced
temperature should be adjusted to maintain an complications, especially heparin-induced thrombocyto-
isothermic core temperature throughout the treat- penia. 170 To date, there is little experience with the use
ment. 127 For animals predisposed or symptomatic for of low-molecular-weight heparins in veterinary dialysis,
hypotension during dialysis, decreasing the dialysate tem- but the relative safety of unfractionated heparin and cost

perature by 0.5 C to 1.5 C could induce peripheral have obviated a need for change.
vasoconstriction and central redistribution of blood, The predisposition for clotting the dialysis circuit
increase vascular resistance, and improve oxygenation varies with individual characteristics of the animal in addi-
during the treatment. 176 Integrated biofeedback systems tion to its underlying disease, the choice of hemodialyzer
with blood temperature sensors on the arterial blood line membrane, predialysis hematocrit, extracorporeal blood
are available to monitor and prevent temperature-related flow rate, volume of the extracorporeal circuit, predialysis
hypotensive or vasodilatory events. An effector system ACT, and rate of ultrafiltration. A standard protocol for
dissipates increased heat through programmed anticoagulation during hemodialysis includes a loading
alterations in dialysate temperature, which decrease the dose of heparin from 10 to 25 units/kg IV (cats) and
temperature of the returning blood to maintain an from 25 to 50 units/kg IV (dogs). The loading dose is
isothermal core body temperature throughout the administered 5 to 10 minutes before starting dialysis to
dialysis session. 102,109,127,145 establish an ACT in the target range of 1.5 to 1.8 times
the reference ACT or approximately 150 to 180 seconds.
Anticoagulation After starting dialysis, a continuous infusion of heparin at
The interaction of blood with the materials and 20 to 50 U/hr (cats) or 50 to 100 U/kg/hr (dogs) is
irregularities of the dialysis membrane and extracorporeal provided to maintain the ACT in the target range. The
circuit activate the coagulation cascade, promote throm- hourly heparin dose is adjusted or intermittent boluses
bosis in the extracorporeal circuit, and necessitate routine of heparin are administered based on sequential ACT
anticoagulation of patients during the dialysis session. 170 measurements performed every 30 to 60 minutes to main-
In fact, it was the discovery of the anticoagulant, hirudin, tain the ACT target. The target ACT can be increased to
1
that enabled the initial development of hemodialysis. All 200 to 250 seconds if the animal demonstrates a propen-
triggers and components of the coagulation cascade, and sity to clot the extracorporeal circuit. The loading and
activation and aggregation of platelets participate variably hourly dose is set to an ACT target of 125 to 150 seconds
to induce clotting during dialysis. Active strategies to if there is moderate risk of bleeding.
balance anticoagulation and coagulation must be Under some clinical circumstances, the risks of bleed-
employed to prevent these events for dialysis to succeed ing from heparin administration are too great despite the
yet remain safe. necessity to provide dialysis. These equally compelling
Inadequate anticoagulation promotes thrombosis of circumstances mandate alternative anticoagulation
the dialyzer, causing inefficient treatments, blood loss strategies that preclude the use of heparin or systemic
in the extracorporeal circuit, and potential for an abrupt anticoagulation of the patient. The decision to avoid
cessation of the treatment. Excessive anticoagulation can systemic anticoagulation or perform a “no heparin” treat-
cause serious bleeding, although this is infrequent. ment is determined by the animal’s relative risks for
Unfractionated heparin has been used as the standard consequential bleeding. Active bleeding, recent or
anticoagulant for intermittent hemodialysis for 40 impending major surgery, percutaneous biopsy (within
years. 36 Despite this experience, coagulation remains 24 to 48 hours), severe trauma, hyphema, gastric
variable from animal-to-animal and treatment-to-treat- ulceration, uremic lung, and a predisposition for CNS
ment and remains problematic to control. The large hemorrhage represent contraindications for systemic
extracorporeal circuit and slow blood flow rate required heparinization and candidate conditions for “no heparin”
in severely uremic animals contributes to the hemodialysis.