Principles of Dialysis
Hemodialysis (HD) refers to the process of using an artificial kidney to clear urea, metabolic waste products, toxins, and
excess fluid from the blood. The term dialysis refers to the net movement of solutes and water across a semipermeable membrane
along a concentration gradient. Hemodialysis is the extracorporeal exchange of water and solutes between blood and an artificial
solution termed dialysate, across manufactured semipermeable membranes. This exchange allows for the removal of metabolic
waste products or toxins from the blood and correction of fluid, electrolyte and acid-base derangements. (Figure 1)
Figure 1: Schematic representation of solute transport across the semi-permeable dialysis membrane. The plasma concentration
of urea and creatinine is high resulting in diffusive movement down concentration gradients into the dialysate. The reverse
is true of bicarbonate resulting in net movement from the dialysate into the plasma.
During hemodialysis water and small molecular weight solutes and uremia toxins pass readily through the membrane pores (diffusion
channels) along diffusive and hydrostatic gradients, but the movement of larger solutes, plasma proteins, and the cellular
components of blood are limited by the size of the pores. Excessive body water and additional solute can forced through the
membrane by ultrafiltration produced by hydrostatic or osmotic forces imposed across the dialysis membrane. Net removal of
uremic solutes is influenced by: (1) the concentration gradient for diffusion, (2) the diffusivity of the solute, (3) permeability
characteristics and surface area of the membrane, (4) blood and dialysate flow within the dialyzer, (5) the duration of dialysis,
(6) the distribution volume of the solutes, and (7) amount of ultrafiltration (convective transfer).
The techniques and equipment used to deliver hemodialysis to veterinary patients are the same as those used in human medicine.
The delivery of hemodialysis is technically demanding and requires: (1) access to the patient's vasculature, (2) a hemodialyzer,
(3) an extracorporeal blood circuit, (4) a dialysis delivery system to formulate and deliver the dialysate, control blood
flow in the extracorporeal circuit, deliver anticoagulant, and monitor the integrity and safety of the entire dialysis process,
(5) physiologic monitoring equipment, (6) a source of purified water, and (7) a specifically trained and dedicated nursing
and professional staff.
Indications for Hemodialysis
Acute uremia is the most common indication for hemodialysis in dogs and cats. Animals with severe acute renal failure generally
die within days from complications of uremia. The majority of animals presented for HD are acutely uremic and nonresponsive
to attempted diuresis with intravenous fluids and pharmacologic manipulation. Often these patients are volume overloaded from
attempted diuresis in the face of oliguria, and many have life threatening hyperkalemia. Hemodialyisis rapidly mitigates hyperkalemia
and can restore fluid balance, thus stabilizing the patient and allowing time for recovery of their own renal function. Hemodialysis
should be initiated when the clinical consequences of the uremia cannot be managed effectively with medical therapy alone.
The prognosis for recovery from acute uremia in dogs and cats treated with hemodialyasis depends on the etiology, extent of
renal damage, co-morbid diseases, and presence of multiple organ system involvement. Based on the published studies, global
survival for dogs with severe acute uremia is approximately 50%. Survival from infectious (60%), hemodynamic and metabolic
etiologies (40%) was greater than survival from toxic causes (20%). The outcome for dogs with acute leptospirosis is especially
favorable with 85% survival with either severe (dialysis-dependent) or milder forms (medically manageable) of ARF. Global
survival for cats requiring HD since 1996 is reported to be 56%. The increased survival rate is due largely to the dramatic
increase in the number of cats presented for acute ureteral obstruction. In a report of 50 cats presented for dialytic management
of acute ureteral obstruction, 70% survived to discharge. Overall, hemodialysis substantially increases the global survival
for both dogs and cats with severe acute uremia beyond what would be expected with conventional management.