Page 720 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Hemodialysis and Extracorporeal Blood Purification 707

severely azotemic animals to prevent manifestations of COMPLICATIONS OF
dialysis disequilibrium syndrome. HEMODIALYSIS
Application of extracorporeal therapies should not be
limited to single modalities but should be sequenced and The clinical and procedural complications associated with
combined to best match the clinical course and kinetics of hemodialysis in humans and animals have been
the toxicant. Continuous versus intermittent therapies reviewed. 34,59,78,147 The most serious complications
should not be considered mutually exclusive but rather include those associated with the interaction of the
complimentary. There is little justification not to include patient with the dialysis machinery, vascular access, hemo-
a dialytic device with a hemoperfusion cartridge when dynamic stability, and solute disequilibrium. Hemodialy-
contemplating hemoperfusion. For many toxins, hemo- sis is a technically complex therapy applied to patients
dialysis has the potential to improve toxin clearance in with profound physiologic and metabolic derangements.
concert with hemoperfusion despite theoretical Therapeutic complications can be anticipated from both
predictions to the contrary (Figure 29-12). Dose, blood the technical aspects of the process, the dynamic
concentration, changes in protein binding of the toxin, oscillations of solute and fluid homeostasis, exposure to
concurrent drugs/toxins, acid-base status, membrane nonbiologic materials, and sources of contamination
type, and other variables may influence the diffusive and toxicities associated with procedural and medical
potential of a toxin under different clinical conditions. therapies. Often it is difficult to distinguish whether
The presence of the hemodialyzer in the extracorporeal adverse events are caused by the severity of the uremia,
circuit provides the potential for better thermal regula- the intensity of its treatment, or the consequences of
tion and opportunity to correct coexisting fluid volume, the intervention. The frequency and severity of
electrolyte, acid-base, or uremic complications. Place- complications related to homeostatic excursions early
ment of the hemodialyzer after the hemoperfusion in dialysis diminish as the patient adapts to the
cartridge also helps to prevent depletion of calcium and procedures and the uremia is controlled, but they often
glucose by charcoal sorbents and isolates the are replaced by more subtle homeostatic imbalances
hemoperfusion cartridge from the ultrafiltration control imposed chronically.
system that may promote excessive fluid removal and The number and relative frequency but not the types
hemoconcentration in the dialyzer should pressure of complications encountered in veterinary dialysis have
increase in the sorbent bed. changed over the past 25 years. In early years, dialysis dis-
Hemoperfusion with activated charcoal is generally equilibrium and hemorrhage related to anticoagulation
safe but poses potential disadvantages or complications were the most common causes of fatal complications.
not generally experienced with hemodialysis. One of Now, fatal dialysis-related complications are rarely
the principal concerns is the innate hemocompatibility encountered. Even in severely uremic patients, dialysis
of the adsorbent. Hemoperfusion with activated charcoal disequilibrium is averted by tailoring dialysis
(as well as other sorbent materials) can cause thrombocy- prescriptions to each patient and by use of precautionary
topenia and leukopenia because platelets and leukocytes measures including slow, less intensive prescriptions, pro-
become adhered to the sorbent or entrapped in fibrin phylactic mannitol administration, and sodium profiling
films or clots formed on the charcoal. Platelets are to minimize osmotic shifts in high-risk patients.
destroyed also by surface irregularities of the charcoal Symptomatic hypotension remains a persistent threat
bed. These effects are not unique to activated charcoal because of the increasing willingness to dialyze smaller
and are potential complications of polymer-based patients and those with critical comorbidities. Blood pres-
sorbents. Thrombocytopenia can be especially problem- sure should be monitored at 15- to 30-minute intervals
atic if daily treatments are required that preclude ade- throughout the dialysis session to remain proactive and
quate regeneration of platelets between treatments. If attentive to this concern. The susceptibility to hypoten-
hemoperfusion is not combined with hemodialysis, the sive events is influenced by body size, hydration status,
patient may experience significant cooling because of the severity of the uremia, the presence of concurrent car-
the duration the extracorporeal blood is exposed to room diac disease or co-morbid conditions (e.g., hemorrhage,
temperature. The sorbent bed also may become saturated anemia, sepsis, pancreatitis), and current medications
at unpredictable times during the treatment resulting in (e.g., antihypertensives, diuretics). For cats and small
incomplete removal of the toxin. Saturation of the sor- dogs, the volume of the extracorporeal circuit may exceed
bent is easily demonstrated by measuring the extraction 25% to 30% of the intravascular volume and cause
ratio [(A tox –V tox )/A tox ] across the device or its whole hypovolemia as the circuit is filled. The rapid removal
blood clearance [Q b extraction ratio], where A tox and of plasma solutes in the early stages of a dialysis treatment
V tox are the concentrations of the solute or toxin at the decreases intravascular volume and opposes refilling of
inlet and outlet of the hemoperfusion cartridge, respec- fluid from the extravascular space. Excessive or rapid
tively, and Q b is the blood flow rate. ultrafiltration that exceeds vascular refilling is the most