Filtration & fl uid control 2million+
The number of patients worldwide currently reliant on dialysis to stay alive.
National Kidney Foundation
“Dialysis has saved the lives of many patients with kidney failure,” is how Dr Cristian Pedreros- Rosales, a nephrologist and associate professor at the department of internal medicine at the University of Concepción in Chile, puts it. “But it is an insufficient purification method.”
In recent decades, however, a new technique has emerged that promises to be more effective: high- volume haemodiafiltration (HDF). Studies have shown it to outperform haemodialysis (HD) at clearing larger toxins – and better yet, to have a lower risk of mortality. Today, uptake of high-volume HDF is broadly on the rise in hospitals around the world. And according to many in the field, it’s only a matter of time before it becomes the standard of care everywhere.
More ways to clean the blood The limitations of haemodialysis can plausibly be understood in a single word: diffusion. “In haemodialysis,” explains Bernard Canaud, a nephrologist at the Montpellier School of Medicine and a senior chief scientist at Fresenius Medical Care, “you only have diffusive action. In haemodiafiltration, you have diffusive plus convective clearance. That’s what makes the difference.”
Diffusion and convection are both methods of removing toxins from the blood. With the former,
waste products move from the solution with a higher concentration of particles (blood) into one with a lower concentration (dialysate) – as if they were being pulled from a crowded room into one that’s empty. This, in turn, happens inside the dialyser. Here, blood is separated from dialysate – a solution of purified water, electrolytes and salt – by a semipermeable membrane. For their part, blood and dialysate flow in opposite directions. Small waste molecules can then move from the blood to dialysate, but those that are too large to pass through the membrane’s pores stay behind. You can improve clearance by using membranes with bigger pores, a process known as high-flux HD. But for this to work, you can only make the pores so big, otherwise you risk essential large proteins like albumin accidentally passing through and leaving the patient with a deficiency. High-flux HD allows some medium-sized
molecules, including the inflammatory protein beta two microglobulin, to escape, and is currently considered the standard of care. Yet as Pedreros- Rosales concedes, it still leaves bigger waste products in the blood. “These larger toxins,” he explains, “are more cardiotoxic than smaller ones like urea, and are linked to higher mortality and cardiovascular events – the most significant issues for dialysis patients.” Haemodiafiltration, for its part, uses high-permeability membrane too. But it also cleans the blood via convection, whereby the water in the blood is pulled across the membrane due to differences in pressure between the fluids on either side. Waste products are then dragged through the membrane with it. Modern dialysis machines will automatically calculate and generate the necessary pressure based on the volume of water you want to remove from the blood. In practice, this means that convection offers better clearance of middle-sized toxins, says Francesco Locatelli, a professor of nephrology at the Alessandro Manzoni Hospital near Milan – though how effective it is depends on the total volume of fluid taken from the blood over the session. When the volume is high enough, Locatelli explains, “this is the only way of removing toxins of molecular weight higher than urea – the so-called ‘small’ molecules – and to also remove the middle molecules until the size of approximately 20 [kilodaltons].”
High volume, higher clearance The ‘high volume’ part of ‘high-volume HDF’ refers to the large amount of fluid removed from the patient’s blood during one session – known as the ‘convective volume’. Studies have shown 23L to be the point where patients start to see a beneficial effect from HDF, with Pedreros-Rosales explaining that’s why it’s the recommended minimum convective dose. These bigger volumes, adds Canaud, allow for more clearance of waste. With high-flux HD, “I’d say that
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