**2. Background**

The aim of haemodialysis is to replicate normal physiology as much as possible. Although this may sound straightforward initially, there are a vast array of factors to consider. The ideal treatment should give good survival rates, prevent cardiovascular events and hospitalizations, effectively manage fluid and salt balance and address the anaemia and mineral‐bone disorder associated with chronic kidney disease (CKD). Patient well‐being, cognition, sleep, the ability to work and nutritional status are also hugely important factors which need addressing and the list could continue. For a treatment, which for the majority of patients is performed for 12 hours of the week (just 7% of the week in terms of time), this is an incredibly tall order and it is not entirely surprising that outcomes remain poor.

Conventional haemodialysis (CHD) is the most common treatment schedule and lasts for 3–4 hours thrice weekly. This treatment mainly takes place in a hospital or in a dedicated dialysis unit. Other treatment regimes include short daily haemodialysis (SDHD), which is performed for 1.5–3 hours 5–7 times per week, long nocturnal daily haemodialysis (LNDHD) which is performed for 6–8 hours 5–7 times per week and long conventional haemodialysis (LHD), which is typically 8 hours 3 times per week.

Although CHD is the most common treatment regime now, LHD was the most common treatment initially in the 1960s [5]. This treatment came about purely by convention. Home haemodialysis in the United Kingdom was started in London in 1964 by Shaldon and his team [6] and expanded following this. Home haemodialysis was necessary as hospital facilities were sparse, treatment times were lengthy and home dialysis offered both financial and logistical benefit. Prevalence in the United Kingdom peaked in 1982 when 62% of HD patients were at home [3]. As dialysis treatment time shortened and patient numbers increased, haemodialysis practice changed from a predominantly home‐based therapy to a predominantly hospital‐ based therapy.

The National Cooperative Dialysis Study (NCDS) (*n* = 151) [7] was published in 1981. It showed no difference between the shorter and longer duration dialysis groups (2.5–3.5 hours three times per week vs. 4.5–5 hours three times per week). This further paved the way to the adoption of CHD. Data from this study were later used to develop a method for calculating haemodialysis adequacy [8] *K*<sup>t</sup> /*V*urea (*K*<sup>t</sup> /*V*) which is now used worldwide. *K*<sup>t</sup> /*V* exclusively looks at small molecule clearance as a marker of dialysis adequacy. There is an association between dialysis dose and mortality [9]; however, the benefit (from the NCDS data) was seen up until a *K*<sup>t</sup> /*V* of 1.2 with no survival advantage with doses above this. This was later echoed in the much larger HEMO study (involving 1846 patients) and again showed no advantage in increasing the dialysis dose above a *K*<sup>t</sup> /*V* of 1.3. Dialysis treatment time was not investigated in this study.

It would seem therefore that with *K*<sup>t</sup> /*V* we have reached a ceiling where improvements can no longer be made within the restrictions of a thrice weekly schedule, and we must start looking at other ways to improve outcomes in haemodialysis. The most significant causes of death in haemodialysis patients are cardiovascular in nature and this has been well known for some time. UK registry data show that almost a third of deaths in dialysis patients are cardiovascular in nature [4]. It is also clear that the two‐day gap in CHD is harmful with all‐cause and cardiovascular mortality being higher on the day after the long interval [10]. Further data show the highest rate of cardiovascular events in the first month after starting haemodialysis and a high‐risk period extending to 4 months [11].

Based on all of this, a strong argument can be made to further explore more frequent and extended haemodialysis treatments.
