Page 694 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Hemodialysis and Extracorporeal Blood Purification 681

BOX 29-1 Indications for Dialytic and Extracorporeal Therapies in Animals

Acute Kidney Injury Chronic Kidney Disease
1. Anuria 1. Perioperative support for renal transplantation
2. Failure of fluid administration or diuretic therapy to 2. Indefinite intermittent renal replacement therapy
initiate an adequate diuresis 3. Support for acute decompensation of chronic kidney
3. Failure of conventional therapy to control the azotemia, disease
biochemical, or clinical manifestations of acute uremia 4. Finite renal replacement therapy for client transition to
4. Life-threatening fluid overload irreversible disease status
5. Life-threatening electrolyte (hyperkalemia, Miscellaneous
hypernatremia, hyponatremia) or acid-base disturbances
1. Severe overhydration, pulmonary edema, congestive heart
6. Severe azotemia—BUN >100 mg/dL; serum creatinine
failure
>10 mg/dL
2. Acute poisoning/drug overdose
7. Clinical course refractory to conservative therapy for 12 to
24 hours
8. Delayed graft function following renal transplantation

are proportional to the respective concentration and across the membrane and does not alter diffusive
thermodynamic potential of the solute on each side of gradients or serum concentrations. The transmembrane
the membrane, and net solute transfer is directed from hydrostatic pressure gradient between the blood and
the solution at higher concentration to the solution at dialysate compartments, the hydraulic permeability, and
lower concentration or thermodynamic potential. When the surface area of the membrane determine the rate of
there is no concentration gradient for a solute across the ultrafiltration and solute transfer. During hemodialysis,
membrane, the solute is at a filtration equilibrium. At this a dialysate-directed transmembrane pressure gradient
point, the driving force for diffusion stops, and there is no (dialysate pressure < blood-side pressure) is generated
further net change in concentration of the respective to initiate and control the rate of ultrafiltration. Indepen-
solutions despite ongoing bidirectional and equal dent changes in the dialysate- and blood-side pressures
molecular exchanges between them. can influence the rate of ultrafiltration by attendant
The diffusive potential for every solute varies under changes to the transmembrane pressure. The hydraulic
differing physiologic condition. Molecular weight is the permeability of a dialyzer is determined by physical
main determinant of kinetic motion and contributes features of the membrane (e.g., composition, thickness,
inversely to the rate of diffusion for individual solutes. pore size) and is rated by its ultrafiltration coefficient,
Small solutes such as urea (60 Da) diffuse faster than K uf , defined as milliliters of fluid transferred per hour
larger solutes such as creatinine (113 Da), and generally per milliliters of mercury of transmembrane pressure.
the plasma concentration of small solutes decrease faster Hemodialyzers are qualified as low flux or high flux
than those of larger solutes during the course of dialy- according to their K uf . A minimal transmembrane pres-
sis. 47,142 The intrinsic permeability of a membrane for sure of 25 mm Hg is required for ultrafiltration to offset
each solute also influences directly its diffusive potential. the oncotic pressure of plasma proteins, which favors fluid
Membrane permeability is determined by its thickness, its reabsorption and opposes ultrafiltration. Convective
effective surface area, and the number, size, and shape of transport can contribute to total solute removal, espe-
its pores or diffusion channels. 142 In addition to intrinsic cially for large solutes with limited diffusibility. However,
solute and membrane characteristics, molecular charge, for standard hemodialysis, ultrafiltration primarily is
protein binding, volume of distribution, and cellular targeted at fluid removal, and convective clearance
seclusion influence the bulk transfer of uremia toxins contributes less than 5% to total solute removal. Convec-
and solutes from the body independently from their tive clearance techniques are exploited further in the
predicted diffusion. process of hemofiltration where solute removal occurs
Convective transport of solutes across dialysis entirely by ultrafiltration with replacement of desired
membranes is associated with the process of ultrafiltra- solutes and fluid with a prefilter or postfilter reinfusion
tion, in which water is driven through the membrane solution. Hemodiafiltration and continuous renal
by hydrostatic pressure gradients. Diffusible solutes replacement therapy (CRRT) represent hybrid treatment
dissolved in the water are swept through the membrane modalities combining both diffusive dialysis and large
by solvent drag. 142 Unlike diffusive transport, convective volume ultrafiltration to achieve solute and fluid
transport does not require a concentration gradient removal. 15,81