Hemodiafiltration

Hemodiafiltration (HDF) is a therapeutic procedure in internal medicine, particularly nephrology, which is an extracorporeal blood purification procedure that is a combination of hemodialysis and hemofiltration. The primary field of application of hemodiafiltration is the use of the system in the permanent therapy of chronic renal insufficiency. Due to this combination of the two blood purification methods, it is possible to perform the removal of both low and medium molecular weight substances. The removal of these substances is only feasible with a controlled replacement of the ultrafiltrate with physiological electrolyte solution. The replacement solution is added directly to the blood either before or after the dialyzer. In order to restore volume balance, it is necessary for the added fluid to be removed again by means of the dialyzer. The result of this process is the generation of a higher transmembrane flux. As a result, the pollutants and toxins present in the blood can be eliminated more effectively.

Indications (areas of application)

  • Chronic terminal renal failure-as described earlier, this extracorporeal blood purification procedure is used exclusively for permanent therapy in chronically ill patients and not for acute therapy. The data from the studies conducted to date demonstrate that on-line hemodiafiltration, in particular, as a procedure can relevantly improve the effectiveness of extracorporeal blood purification with superior tolerability. No conclusive studies have been performed to improve morbidity and mortality rates.

Contraindications

Exsiccosis – hemodiafiltration should not be performed in patients with a serious underlying condition associated with significant exsiccosis (dehydration).

The procedure

The principle of hemodiafiltration is based primarily on taking advantage of both hemodialysis and hemofiltration. Hemofiltration removes fluid from the blood without the need for a dialysate (flushing solution). The decisive difference in the use of hemofiltration compared to conventional hemodialysis is the fact that hemofiltration uses a hemofilter instead of a dialyzer. However, the problem with hemofiltration is that only insufficient removal of low-molecular-weight urinary substances can be achieved. To achieve an increase in the amount of eliminable small-molecule substances, it is necessary to use both classical hemodialysis and hemofiltration together and simultaneously. The total elimination rate of medium-molecular harmful substances also increases with the combination of both methods. The parameter describing the elimination of urinary substances is the so-called sieve coefficient. On the basis of the sieve coefficient, the various substances can be classified. For example, substances that can easily overcome the existing membrane have a sieve coefficient of one. Thus, for all substances that have the same sieve coefficient, the elimination of ultrafiltration rate can be derived. To improve filtration performance, modern high-flux dialyzers are used in hemodiafiltration to improve clearance (removal of a defined substance) across the entire molecular weight spectrum. For optimal hemodiafiltration function to be achieved, dialyzer membranes must be highly permeable (optimally permeable). For example, adequate function is achieved when the surface area of the dialyzer membrane is approximately 15-20% larger than in conventional hemodialysis. Based on this, the ultrafiltration rate is not restricted by blood flow. In order to achieve a therapeutically relevant exchange volume, a strong blood flow must be ensured. Thus, hemodiafiltration aims for a higher blood flow than conventional hemodialysis. Forms of hemodiafiltration

  • Classical hemodiafiltration – in this system of hemodiafiltration, a bagged substitution solution is usually used for volume compensation. However, if there is a need for large volume administration with infusion from a bagged substitution solution, this involves additional expense from a technical and financial point of view.Due to the limitation of the exchange volume in this system is limited to a maximum of eight to eleven liters per treatment unit.
  • High-flux dialysis – the peculiarity of this procedure is the use of a so-called backfiltration (backfiltration), with which the ultrafiltration rate can be significantly increased. Consequently, the mass transfer increases. The improvement in the exchange rate by one to two liters per treatment unit is based exclusively on the use of backfiltration in high-flux dialysis. However, it is problematic that backfiltration is associated with a risk of contamination, since microbiological substances in particular and the endotoxins they produce (a certain group of bacteria – gram-negative – release harmful small-molecule substances when they die) can colonize or contaminate the dialysate. Furthermore, it must be considered that the biofilms formed by bacteria can contaminate the piping systems of high-flux dialysis. According to various guidelines, high-flux dialysis can be assigned to hemodiafiltration.
  • On-line procedure – this procedure represents a consistent further development of classical hemodiafiltration, because it creates the conditions to achieve a reduction in the cost of providing the substitution solution, so that an economic application of the procedure is also given for larger exchange volumes. The basic principle for the on-line process is based on obtaining a larger volume of substitution solution by filtration from the dialysate in the dialysis system. In addition, an endotoxin filter is used in the on-line process to minimize the risk of contamination. The electrolyte solution used must therefore pass through this filter and is then divided into two partial streams. The first partial stream ends in the dialysis fluid, while the second is directed into the substitution solution. A further filter system is used to ensure that the bacterial load in the substitution solution can be kept as low as possible. Thus, this method is suitable for the safe removal of pyogenes even with significant microbial contamination.

Potential complications

  • Risk of infection – despite various filter systems, such as an endotoxin filter or a so-called ultrafilter, the risk of infection cannot be ruled out, especially since the procedure is primarily performed on patients who are immunocompromised (have weakened defenses).
  • Hypothermia – patient heat loss in this case is based on extracorporeal (outside the body) circulation. The tube system used here can also contribute to temperature reduction.
  • Electrolyte Derailment – Electrolyte derailment can result from incorrect administration of electrolyte solutions. Moreover, patients are predisposed to electrolyte derailment who have a catabolic metabolic state.
  • Thrombosis – despite numerous measures for anticoagulation, it is still possible that thrombosis with all its sequelae can develop. The cause may be insufficient heparinization and immobility during therapy. In addition, patients with high blood viscosity are particularly at risk due to excessive water removal during hemofiltration.