Hemofiltration is a therapeutic procedure in internal medicine, particularly nephrology, which allows the removal of urinary substances from the blood and is used to precisely adjust other parameters and thus can contribute to the removal of harmful substances from the blood as a dialysis procedure. Hemofiltration removes fluid from the blood without the need for 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. This hemofilter used is characterized by the fact that it consists of a highly permeable membrane, which leads to the achievement of ultrafiltration rates in the range of 120 to 180 ml/min. The ultrafiltration rate describes the amount of volume that can pass through the membrane per minute, whereby defined molecules can pass through the membrane with varying degrees of efficiency. Of particular importance here is that the ultrafiltrate obtained contains the urinary substances. As a result of this, it is necessary to use a balancing system to replace the ultrafiltrate with a substitution solution after the filter. Thus, the targeted volume withdrawal can be regulated at the balancing system. In order for a sufficient and relevant therapeutic effect to be seen with hemofiltration, the patient must be treated with hemofiltration three times per week. In order to ensure adequate treatment, it is necessary that 40% of the body weight is hemofiltrated and substituted by the affected patient. To achieve the necessary high filtration rates of 120-180 ml/min, a blood flow of 350-450 ml/min must be present. This is only possible if there is very good vascular access, which many patients, especially chronically ill patients with renal impairment, do not have. The use of hemofiltration is not the rule, rather hemofiltration is a reserve procedure that is mostly used only in patients with refractory hypotension during hemodialysis, since clinical studies have shown that hemodynamic stability is considered better when hemofiltration is performed. Because of the reserve status, only one percent of ESRD patients are treated with hemofiltration.
Indications (areas of application)
- Treatment-resistant hypotension during hemodialysis – Hemofiltration is usually only a back-up procedure in patients who require dialysis treatment but suffer from unadjustable hypotension during hemodialysis. If this case is given, the indications hardly differ from hemodialysis.
- Acute renal failure (ANV) – as soon as the body’s own kidney function is no longer sufficient for the clearance (clarification) of the blood, it requires an exogenous (not endogenous) procedure for blood purification. The clearance of urinary substances is determined on the basis of various parameters. If a laboratory test of the patient’s blood reveals a serum urea value above 200 mg/dl, a serum creatinine value above 10 mg/dl, a serum potassium value above 7 mmol/l or a bicarbonate concentration below 15 mmol/l, a dialysis procedure must be performed quickly. However, it should be noted that not only laboratory value may serve as an indication, but also the clinical appearance.
- Overhydration states – is the conservative therapy (exclusively drug therapy) from the therapeutic success to be considered insufficient, so hemofiltration is indicated in these difficult to control overhydration states in therapy.
- Severe hyperphosphatemia (excess phosphate) – an overload of the body with phosphate represents a massive health risk, which is also an indication for the acute use of hemofiltration.
- ARDS (acute respiratory distress syndrome) – in the presence of ARDS, which is associated with occlusion of the pulmonary capillaries and a massive reduction in blood oxygen saturation (SpO2), hemofiltration is a clear indication.
Contraindications
- Exsiccosis – hemofiltration should not be performed in patients with a serious underlying condition associated with significant exsiccosis (dehydration).
The procedure
The basis of hemofiltration is transmembrane pressure applied via a pump, which is the driving force of ultrafiltration. This pressure gradient across the high-permeability membrane causes plasma to be withdrawn from the blood across the membrane. This withdrawal of plasma volume is referred to as ultrafiltration. The consequence of this molecular transport across the membrane is the co-removal of filter-permeable substances. The result of this process is slow detoxification and, if necessary, rapid volume change in the treated patient. However, since such massive fluid removal is not tolerated by the human organism, the removed fluid must be replaced by an electrolyte solution. The following systems of continuous hemofiltration can be distinguished:
- Spontaneous slow ultrafiltration (SCUF)-in this system of hemofiltration, arterial access is essential, because an arteriovenous pressure differential must be established for the required ultrafiltration or hemofiltration, which is generated without the use of pumps. Using a SCUF system, an average of three to five liters of water can be filtered out of the organism during daily therapy, depending on both the choice of filter and the existing blood pressure. This fluid removal can be adequate for a balanced volume balance. However, the “pure” application of SCUF has by no means been designed for the performance of adequate toxin removal. If effective toxin elimination is to be performed, a hemofiltration procedure that has a higher filtration capacity is required. Nevertheless, it should be noted that this improved filtration performance would require a corresponding volume substitution.
- Continuous arteriovenous hemofiltration (CAVH) – the key difference of this system compared to SCUF is that in addition to the ultrafiltration performed in both systems, volume substitution is also performed. Thus, CAVH represents a system that has an improved filtration performance compared to SCUF, but can compensate for this by a corresponding volume substitution. Important for the function of the CAVH system is the use of filters that have a small surface area. As a rule, the filter surface does not exceed half a square meter. Furthermore, it should be noted that filters have the advantage of lower resistance and relatively low thrombogenicity (probability of clotting) due to their surface area. From this it can be concluded that filters with a large surface area are only suitable to a very limited extent for the non-pump assisted procedures because of the high resistance. To further reduce resistance, the blood tubing system is kept as short as possible. Since there is no contact between blood and air during continuous arteriovenous hemofiltration, this can further reduce thrombogenicity. The risk of thrombus formation is further reduced by anticoagulation (anticoagulation) directly upstream of the filter. To ensure optimal function of the system, the hemofilter should be placed slightly below the level of the heart. It should also be mentioned that the filtration rate can be variably adjusted to the infusion needs of the patient. The amount of ultrafiltrate produced is directly dependent on the negative pressure in the filtrate compartment. If the amount of ultrafiltrate is now to be regulated, this can be done by the height of the filter relative to the patient. This possibility of regulation is based on the principle that the distance of the drip point from the filter outlet determines the filtration performance. Thus, the closer the drip point is to the filter outlet, the lower the filtration performance. The filtrate volume exceeding the desired ultrafiltration must subsequently be substituted to the same extent to avoid volume depletion (the removal of volume from the body). The volume to be substituted, which contains electrolytes and necessary buffer solution, is added after the filter. If a catabolic metabolism is present in a patient who is uremic due to renal insufficiency, both higher filtration and substitution volumes must be present to achieve adequate filtration. Catabolic metabolism is defined as a massive increase in protein breakdown compared to build-up.This metabolic situation is thus accompanied by a high proportion of harmful protein degradation products.
- Continuous arteriovenous hemofiltration (CAVH) with filtration pump – by using CAVH without a filtration pump, the achieved filtration volume may not be sufficient, so it must be increased by using a pump. By using this pump, an increase in the filtration amount occurs, which is based on the fact that the transmembrane pressure is increased by negating the pressure in the filtrate compartment. On the other hand, however, the higher ultrafiltration greatly increases the risk of thrombus formation, since the higher ultrafiltration leads to a considerable concentration of blood in the filter. Because of this, it seems necessary to add volume substitution directly upstream of the filter to the process. Under these conditions, more favorable flow conditions in the filter.
- Continuous veno-venous hemofiltration (CVVH) – since this system achieves regulated blood flow by using a blood pump, it does not require a filter with a small surface area to function properly. By using filters with a larger surface area, it is possible to increase the filtration rate in a relevant way. Furthermore, it should be emphasized that by using the pumps in the CVVH system compared to the methods with spontaneous filtration, the safer balancing by the double pump method or gravimetric ultrafiltration measurement becomes possible.
Possible complications
- Risk of infection – this risk is primarily based on unhygienic work of the staff. Because hemofiltration is performed particularly in patients who are immunocompromised (immunocompromised), this risk poses a particular danger to the patients involved.
- Bleeding – Bleeding during hemofiltration can occur primarily due to excessive systemic heparinization (drug administration of heparin to reduce blood clotting) or also as a consequence of various coagulation disorders of the patient. Consequences respectively the symptoms are mucosal bleeding, bleeding from puncture sites and laboratory pathological coagulation values.
- Hypothermia – patient heat loss in this case is based on extracorporeal (outside the body) circulation. The tubing system used here can also contribute to temperature reduction.
- Balancing error
- Electrolyte Derailment – Electrolyte derailment can result from the incorrect administration of electrolyte solutions. Moreover, patients are predisposed to electrolyte derailment who have a catabolic metabolic state.
- Air embolism – the presence of air bubbles in the blood can cause air embolism. The risk is relatively variable, as different amounts can cause air embolism.
- Thrombosis – despite numerous measures for anticoagulation, it is still possible that thrombosis with all its sequelae can occur. 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.
