Hemofilter ready for use.
In medicine, hemofiltration, also haemofiltration, is a renal replacement therapy which is used in the intensive care setting. It is usually used to treat acute kidney injury (AKI), but may be of benefit in multiple organ dysfunction syndrome or sepsis. During hemofiltration, a patient's blood is passed through a set of tubing (a filtration circuit) via a machine to a semipermeable membrane (the filter) where waste products and water (collectively called ultrafiltrate) are removed by convection. Replacement fluid is added and the blood is returned to the patient.
- 1 The principle of hemofiltration
- 2 Replacement fluid composition
- 3 Hemodiafiltration
- 4 Intermittent vs. continuous modes of therapy
- 5 Is on-line intermittent hemodiafiltration (IHDF) better than regular hemodialysis?
- 6 See also
- 7 References
- 8 External links
The principle of hemofiltration
As in dialysis, in hemofiltration one achieves movement of solutes across a semi-permeable membrane. However, solute movement with hemofiltration is governed by convection rather than by diffusion. With haemofiltrate, dialysate is not used. Instead, a positive hydrostatic pressure drives water and solutes across the filter membrane from the blood compartment to the filtrate compartment, from which it is drained. Solutes, both small and large, get dragged through the membrane at a similar rate by the flow of water that has been engendered by the hydrostatic pressure. Thus convection overcomes the reduced removal rate of larger solutes (due to their slow speed of diffusion) seen in hemodialysis.
Replacement fluid composition
An isotonic replacement fluid is added to the blood to replace fluid volume and electrolytes. The replacement fluid must be of high purity, because it is infused directly into the blood line of the extracorporeal circuit. The replacement hemofiltration fluid usually contains lactate or acetate as a bicarbonate-generating base, or bicarbonate itself. Use of lactate can occasionally be troublesome in patients with lactic acidosis or with severe liver disease, because in such cases the conversion of lactate to bicarbonate can be impaired. In such patients use of bicarbonate as a base is preferred.
Hemofiltration is sometimes used in combination with hemodialysis, when it is termed hemodiafiltration. Blood is pumped through the blood compartment of a high flux dialyzer, and a high rate of ultrafiltration is used, so there is a high rate of movement of water and solutes from blood to dialysate that must be replaced by substitution fluid that is infused directly into the blood line. However, dialysis solution is also run through the dialysate compartment of the dialyzer. The combination is theoretically useful because it results in good removal of both large and small molecular weight solutes.
Intermittent vs. continuous modes of therapy
These treatments can be given intermittently, or continuously. The latter is usually done in an intensive care unit setting. There may be little difference in outcome between the two in the context of acute renal failure.
On-line intermittent hemofiltration (IHF) or hemodiafiltration (IHDF)
Either of these treatments can be given in outpatient dialysis units, three or more times a week, usually 3–5 hours per treatment. IHDF is used almost exclusively, with only a few centers using IHF. With both IHF or IHDF, the substitution fluid is prepared on-line from dialysis solution by running dialysis solution through a set of two membranes to purify it before infusing it directly into the blood line. In the United States, regulatory agencies have not yet approved on-line creation of substitution fluid because of concerns about its purity. For this reason, hemodiafiltration, had historically never been used in an outpatient setting in the United States.
Continuous hemofiltration (CHF) or hemodiafiltration (CHDF)
Continuous hemofiltration was first described in a 1977 paper by Kramer et al as a treatment for fluid overload. Hemofiltration is most commonly used in an intensive care unit setting, where it is either given as 8–12 hour treatments, so called SLEF (slow extended hemofiltration), or as CHF (continuous hemofiltration), also sometimes called continuous veno-venous hemofiltration (CVVH) or continuous renal replacement therapy (CRRT). Hemodiafiltration (SLED-F or CHDF or CVVHDF) also is widely used in this fashion. In the United States, the substitution fluid used in CHF or CHDF is commercially prepared, prepackaged, and sterile (or sometimes is prepared in the local hospital pharmacy), avoiding regulatory issues of on-line creation of replacement fluid from dialysis solution.
With slow continuous therapies, the blood flow rates are usually in the range of 100-200 ml/min, and access is usually achieved through a central venous catheter placed in one of the large central veins. In such cases a blood pump is used to drive blood flow through the filter. Native access for hemodialysis (e.g. AV fistulas or grafts) are unsuitable for CHF because the prolonged residence of the access needles required might damage such accesses.
The length of time before the circuit clots and becomes unusable, often referred to as circuit life, can vary depending on the medication used to keep blood from clotting. Heparin and regional citrate are often used, though heparin carries a higher risk of bleeding.
Is on-line intermittent hemodiafiltration (IHDF) better than regular hemodialysis?
There is controversy about whether intermittent on-line hemodiafiltration (IHDF) gives better results than hemodialysis in an outpatient setting. In Europe, several observational studies have compared outcomes in patients getting dialysis with those getting IHDF.
A recent study assesses the better outcome of patients treated with OL-HDF rather than standard HD.
- Cruz D, Bellomo R, Kellum J, de Cal M, Ronco C. The future of extracorporeal support. Critical Care Medicine. 2008;36(Suppl):S243–S252. doi:10.1097/CCM.0b013e318168e4f6.
- Hoffmann JN, Faist E. Removal of mediators by continuous hemofiltration in septic patients. World Journal of Surgery. 2001;25(5):651–659. doi:10.1007/s002680020027.
- Pannu N, Klarenbach S, Wiebe N, Manns B, Tonelli M. Renal replacement therapy in patients with acute renal failure. JAMA. 2008;299(7):793. doi:10.1001/jama.299.7.793.
- Kramer P, Wigger W, Rieger J, Matthaei D, Scheler F. Arteriovenous haemofiltration: a new and simple method for treatment of over-hydrated patients resistant to diuretics. Klinische Wochenschrift. 1977;55(22):1121–1122. doi:10.1007/BF01477940.
- Bellomo R, Ronco C. Continuous renal replacement therapy in the intensive care unit. Intensive Care Medicine. 1999;25(8):781–789. doi:10.1007/s001340050953.
- Abdeen O, Mehta RL. Dialysis modalities in the intensive care unit. Critical Care Clinics. 2002 Apr;18(2):223–247. doi:10.1016/s0749-0704(01)00007-0.
- Wu M, Hsu Y, Bai, C, Lin Y, Wu C, Tam K (2012). "Regional citrate versus heparin anticoagulation for continuous renal replacement therapy: a meta-analysis of randomized controlled trials". American Journal of Kidney Diseases 59 (6): 810–818. doi:10.1053/j.ajkd.2011.11.030.
- Maduell F, Moreso F, Pons M, Ramos R, Mora-Macia J, Carreras J et al. High-efficiency postdilution online hemodiafiltration reduces all-cause mortality in hemodialysis patients. J Am Soc Nephrol. 2013 [Retrieved 2 November 2014];24(3):487–497. doi:10.1681/ASN.2012080875.
- http://vam.anest.ufl.edu/dialysis/ A free interactive simulation of a CVVH machine