Filtration & fluid control
volume will be limited by the length of the session (usually four hours) and the blood flow from the patient. “In four hours,” Canaud adds, “it’s difficult to get more than 30L.”
Dialysis has been the main therapy used for kidney failure since the early 20th century.
usually you would get 50-70ml per minute of beta two microglobulin clearance, which is already substantial. If you move to high-volume haemodiafiltration, you get 120-150ml.”
But for the patient to maintain their fluid balance, you have to replace the water that’s removed before the blood is re-infused: if you take 23L out, you need to put 23L back in. This fluid you add in is called substitution fluid. Today’s dialysis machines enable the use of such high convective volumes because they can produce substitution fluid live, by sterilising and filtering fresh dialysate.
23%
The percentage by which high-
volume HDF reduced the risk of all-cause mortality compared to high-flux HD.
The New England Journal of Medicine
88
“This is the way to make sure that we have no limitation in the substitution volume that we deliver to the patient,” Canaud explains. In the past, substitution fluid was a bagged sterile solution that was hooked up to the machine. Here, you’d be limited to using around 8-10L per session. Substitution fluid can be added into the blood either before (pre-dilution) or after (post-dilution) it passes through the dialyser. Both methods will do the job, though the pre-dilution method requires about twice as much fluid to get the same level of clearance given it dilutes the blood before it’s filtered. “Post- dilution,” Locatelli stresses, “is the more efficient way for improving the removal of toxins.” The recommended 23L figure is in relation to the post-dilution mode, though this doesn’t include the weight gained by the patient between dialysis sessions due to fluid retention. To put it differently, Locatelli says, you’d need to remove this amount from the blood on top of the 23L. “All together, theoretically, we’d have to remove 25, 26, or even 27 litres.” What, however, if you increase the convective dose beyond 23L? As Canaud says, the data we have so far shows that clearance improves as convective volume goes up. But we don’t know whether there’s a maximum volume where the beneficial effect tapers off. Practically speaking, however, the convective
The future of high-volume HDF These questions notwithstanding, you get the sense that HDF is here to stay. To quote Pedreros-Rosales: “HDF has enough evidence showing benefits in multiple areas, particularly in primary outcomes such as survival, that it should soon be considered a ‘standard.’” A case in point is the so-called ‘CONVINCE’ trial. Published last June in the New England Medical Journal – and boasting Canaud as an author – it found that high-volume HDF reduced the risk of all-cause mortality by 23% compared to high-flux HD. A 2024 meta-analysis of five trials, including a pooled total of 4,143 patients with end-stage kidney disease, found that HDF reduced the risk of death from any cause by nearly 20% compared to the HD group. So what’s stopping more clinics from switching to HDF? For one, regulatory approval may be lacking. This is certainly the case in the US – at the time of writing, only one HDF system has received the green light from the FDA. Another barrier, suggests Pedreros-Rosales, is the cost of therapy. That’s shadowed, Canaud adds, by the slow pace of change across medicine generally. “I think it will take ten years to get 80% or more [of clinics to use high-volume HDF], depending on the country.” In the meantime, there’s plenty of scope to fine-tune the technique. For instance, innovations in membrane design can help prevent albumin from being dragged from the blood along with the toxins. Renal care giant Fresenius reports that their optimised FX CorAL filters can reduce the loss of albumin to under 1.4g in one four-hour HDF session.
Feedback systems can also be used to automatically adjust infusion rates based on the pressure inside the dialyser. Because the amount of water removed from the blood depends on this pressure, it can be used to calculate how much substitution fluid is needed. The Max-Sub feature on Nipro’s Surdial X dialysis machine uses this principle to determine the highest possible substitution volume for a given patient. On the research front, trials and studies are underway to further investigate the benefits of high- volume HDF. The H4RT trial at the University of Bristol is currently examining the clinical and cost- effectiveness of the treatment compared to high-flux HD in people with end-stage kidney disease. There’s research interest in other impacts of the procedure, such as quality of life and cognitive function. That’s echoed by what remains a primary focus: achieving the best possible clearance of waste products. “Step by step,” Locatelli says, “we are moving to try and equal the native kidneys.” ●
Medical Device Developments /
www.nsmedicaldevices.com
ali.can0707/
Shutterstock.com
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