The dialysis is an extracorporeal blood purification method which is used as a substitution method in case of renal failure. Apart from a kidney transplant, dialysis is one of the most efficient and thus most important renal substitution therapies in case of chronic renal failure and one of the treatment options for acute renal failure. The term “dialysis” or, in general, “blood purification” of this kind is to be understood as an exchange of substances through a membrane, with blood/plasma being present on the one membrane side and a dialysis liquid being present on the other side of the membrane or flowing along it.
During a treatment, blood is pumped out of the patient via a patient access (shunt), is conveyed past a dialysis membrane in the dialyzer (filter) and returned to the patient in cleaned condition. Any poisonous substances (metabolic degradation products) and uremic toxins consisting of small and medium-sized molecular substances (which are able to penetrate the membrane) are transported in a hemodialysis or hemofiltration or hemodiafiltration operating mode (treatment mode) of the dialysis apparatus from the blood mainly by diffusion and/or convection via the membrane to the other filter side into the dialysis liquid solution and are removed in this way. With the exception of the hemofiltration, there is a constant flow of fresh dialysis liquid through the dialyzer (preferably approximately 500 to 800 ml/min), with the preferred flow direction of the dialysis liquid in the dialyzer being countercurrent to the blood flow.
Usually, a hemodialysis treatment is carried out for approximately 4 to 5 hours (dialysis overnight up to 8 hours) for every treatment and at least three times a week (depending on the body weight, renal residual function, cardiac output). Patients who perform the hemodialysis at home avoid the problematic, longer treatment interval at the weekend and carry out the dialysis more frequently, as a rule every second day or even daily.
An important factor of influence on the quality of the dialysis is the blood flow. Generally speaking, the higher the blood flow, the better the result of the treatment. However, it may happen that problems occur in the dialysis shunt implanted in the patient for taking blood and returning the cleaned blood (e.g. restricted or insufficient flow due to wrong/imperfect positioning in the patient, damage etc.), in particular if the blood flow is high in the extracorporeal system. The specific level of the blood flow at which these problems will occur depends on the quality of the patient access and on the blood flow through the shunt as well as its integrity (no stenosis or aneurysm).
The problem/phenomenon of a so-called “recirculation” in the shunt may occur both with a low flow of blood through the shunt and in the state in which the shunt flow is still significantly higher than the preset blood flow. This means that already cleaned blood from the venous needle applied to the patient is sucked in by the arterial needle likewise applied to the patient and mixed with the blood still to be cleaned. This reduces the cleaning performance and the treatment results are perhaps not satisfactory. The same may happen if the arterial and venous needles are swapped by mistake when connecting the patient to the dialysis apparatus. In this case, the venous needle is upstream of the arterial needle and a recirculation effect occurs, too. Similarly, with low shunt quality fluidic effects result in recirculation.