Hemodialysis and hemofiltration

Fig. 14. Mass transfer across hemodialysis and hemofiltration hoUow-fiber membranes.

A 45-year-old man with mediastinitis and renal and hepatic dysfunction was treated with mediastinal irrigation with povidone iodine (70). He developed toxic plasma iodine concentrations and clinical deterioration hemodialysis and hemofiltration were effective in reducing plasma iodine concentrations. 

Brunet P, Jaber K, Berland Y, Baz M. Anaphylactoid reactions during hemodialysis and hemofiltration role of associating AN69 membrane and angiotensin I-converting enzyme inhibitors. Am J Kidney Dis 1992 19 444-7. 

A 55-year-old woman and a 34-year-old man ingested, with suicidal intent, an unknown amount of what was reported to have been formalin (Koppel et al. 1990). The female patient was found in a coma and admitted to the hospital with shock (systolic blood pressure 50 mm Hg), respiratory insufficiency, and metabolic acidosis. The male patient, who had a history of alcohol abuse, was also hospitalized with shock (systolic blood pressure 60 mm Hg), respiratory insufficiency, and metabolic acidosis. Both patients underwent hemodialysis and hemofiltration treatment. Analysis of the formaldehyde samples ingested by both patients showed no evidence that these products contained methanol, although it was expected to have been detected. A chemical-toxicological screening indicated that no drugs other than fonnaldehyde had been ingested neither methanol or ethanol were detected in blood samples. Three... 

Two membrane-based therapies, hemodialysis and hemofiltration, are used as substitutes for renal function. On comparing these substitution therapies with the natural kidney, it should be remembered that each therapy functions for four to six hours, three times per week, in contrast to the continuous performance of their natural counterpart. Hemodialysis and hemofiltration may also be used to augment normal renal function in the cases of drug detoxification and fluid removal, respectively. 

Blends of isotactic and atactic polymethylmethacrylate (PMMA) in solution have been used to form semipermeable membranes, for both hemodialysis and hemofiltration. Details of the polymer properties are not available (18). 

Mass Transfer Properties. The particle size distribution of Interest In medical devices Is extremely wide, ranging from urea and electrolytes (<1 nm diameter) In hemodialysis to platelets and red cells (3 to 8 ym diameter) In plasmapheresis. Between these extremes, molecular size ranges can be defined that roughly delineate filters suitable for hemodialysis and hemofiltration (that Is, Impermeable to albumin), protein separation (that Is, albumin permeable, IgM Impermeable) and microfiltration (that Is, retentive to platelets and red cells). The lower end of these size distributions Is Indicated In Figure 1. 

As outlined earlier, hemodialysis and hemofiltration require the removal of solutes smaller than albumin from blood. Solute mass transfer rates across hemodialysis membranes cannot exceed the diffusivity of the solute In water. Solute diffusivity decreases with Increasing molecular diameter (Stokes-Einstein relationship) consequently, solute mass transfer rates for hemodlalyzers intrinsically decrease with increasing molecular size. In addition to limitations Imposed by diffusion In solution, mass transfer is further limited by diffusion resistance in the membrane as well as boundary layer effects resulting from laminar flow both of these effects are also functions of molecular size. The quantitation of mass transfer In hemodlalyzers has been reviewed extensively (22). 

Two physical processes, diffusion and convection, are in widespread use as methods to mimic the excretion functions of the native kidney. Therapies utilizing predominantly convective transport are accurately termed hemofiltration, while diffusion-dependent methods are grouped under hemodialysis. Both hemodialysis and hemofiltration are used in the inpatient and outpatient setting to treat renal and nonrenal diseases. In general, the purpmse of these therapies is to either remove toxins circulating in the blood or reduce the blood volume by removal of water. 

Although hemodialysis and hemofiltration are often considered separate therapies, some clinical treatments rely on a combination of the two and therefore can be classified as hemodiafiltration procedures. Treatment techniques can be further stratified as to whether they are intermittent or continuous in nature, and whefiier the vessels accessed are both venous, or arterial and venous. Due to a lack of definitive prospective randomized trials, the relative advantage of continuous versus intermittent treatment is unsown, although continuous administration is felt to be more gentle in some circles, allowing greater time for toxin equilibration and removal (Meyer, 2000). 

As of 1995, more than 30 different polymer blends were being used in the manufacture of membranes for hemodialysis and hemofiltration (Klinkmann and Vienken, 1995). The various membrane types used for renal replacement therapy can be divided into membranes derived from cellulose (83 percent of 1991 worldwide total) and from synthetic materials (the remaining 17 percent) (Klinkmann and Vienken, 1995). Synthetic membranes have been constructed from such materials as polyacrylonitrile (PAN), polysulfone, polyamide, polymethylmethacrylate, polycarbonate, and ethyl-vinylalchohol copolymer (Klinkmann and Vienken, 1995). In the United States, use of cellulosic materials for membrane construction predominates at around 95 percent of the total number of membranes used (Klinkmann and Vienken, 1995). 

Complications that occur during hemodialysis and hemofiltration can be divided into problems related to vascular access and those due to exposure of the blood to the exchange circuit Depending upon the method used, vascular access problems associated with renal replacement therapy are similar to those experienced in patients with vascular grafts or catheters and are covered in those respective sections. 

Artificial kidney designs will likely continue to experience incremental improvements in the materials and hemodynamic areas. New developments in biocompatible materials, superior transport methods for toxin removal, and improved patient management techniques will allow further maturation of hemodialysis and hemofiltration therapy. For example, considerable benefits could be realized from selective toxin removal without concomitant elimination of beneficial proteins. It has been suggested that future devices might utilize the absorption removal pathway with affinity methods as a primary technique to eliminate specific uremic toxins (Klinkmann and Vienken, 1995). 

Catheters are placed when there is a clinical need for repeated sampling, injection, or vascular access, usually on a temporary basis. In kidney failure, catheters allow emergent blood access for hemodialysis and hemofiltration (Canaud, 2000), and provide temporary access as more permanent sites such as arteriovenous fistulas or grafts mature rerotola, 2000). Placement of a catheter or access port is routine for the administration of chemotherapeutic agents and intravenous nutritional supplements. Catheters are often placed when frequent, repeated doses of medication are to be injected. 

F. Villarroel, E. Klein, and F. Holland. Solute Flux in Hemodialysis and Hemofiltration Mem-... 

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