In the first paper on shorter dialysis duration, Schupak and Merrill  reported a markedly higher rate of hypertension problems than in the early reports with longer dialysis [5, 6]. The French Dialysis Registry reported a gradual decrease in hemodialysis duration during the 1970s and a higher rate of hypotensive episodes . In 1983, the European Dialysis and Transplant Association reported 'the proportion of deaths in the Federal Republic of Germany was twice as high in short dialysis' .
An early warning that a short duration of dialysis was associated with multiple problems related to water and sodium retention came in the report by Sellars et al. . Exchangeable sodium was significantly increased with short dialysis, and more patients required antihyperten-sive drugs. Another warning came from Germany in the report by Wizemann and Kramer  in 1987. They did not observe any significant differences in serum biochemistry between short (2.5 h) and long dialysis (4 h), except for serum phosphate, which was lower during longer dialysis. However, weight gains were higher, blood pressure control was worse, and hypotensive episodes were markedly more frequent with shorter dialysis .
In the US, the relative mortality risk was about 20% higher in patients receiving a dialysis duration of <3.5 h compared to those with treatment for >3.5 h [29, 30]. The annual mortality in US patients has increased from 10 to 25% over the last three decades, but has remained stable at around 10% in Japan . During the period 19821987, hemodialysis mortality in the US was found to be 22% higher than in Europe and 40% higher than in Japan  and the duration of dialysis was 23.5% shorter in the USA than in Europe and 40% shorter than in Japan . The experience in Tassin, France, clearly indicates that longer dialysis (8 h thrice weekly) than is usual in the US improves patient survival . When comparing the survival of US patients to those dialyzed in Tassin, it is in the older age group that the difference is particularly pronounced. While the risk of death is two times higher in the US in the patients younger than 45 years, it is 12 times higher in patients older than 65 years [34, 35]. This finding is thus similar to the Japan-US comparison, where the relative risk of death in the US also markedly increases with the age of the patients .
The results from the Japanese dialysis registry [36, 37] showed that shorter dialysis increases death rates. In Europe, Valderrabano  reported a lower gross mortality rate in patients who were dialyzed for more than 12 h/ week as compared to those dialyzed for <12 h/week; the difference in mortality was particularly considerable in patients over 65 years old.
A recent DOPPS  showed reduced mortality with longer treatment time. Statistical adjustments were made for patient demographics, comorbidities, dose of dialysis (Kt/V), and body size. Every 30 min longer on hemodi-alysis was associated with a 7% reduced relative risk of mortality. The association was present in USA, Europe, and Japan, but was most pronounced in Japan. A syner-gistic interaction occurred between Kt/V and treatment time (p = 0.007) toward a mortality reduction. An ultrafiltration rate of >10 ml/h/kg was associated with 9% increased mortality risk.
Intradialytic hypotension (IDH) occurs in 25-50% of short, thrice weekly hemodialysis treatments in the US. The detrimental effect of IDH is being increasingly recognized as an important factor in the increased relative risk of death due to acute coronary syndrome, and arrhythmias [39-41]. Dialysis hypotension occurs because a large volume of blood water and solutes are removed over a short period, exceeding the plasma-refilling rate and the reduction of venous capacity [42, 43]. Short dialysis is associated with high-speed ultrafiltration and rapid removal of small molecules, thus swiftly depleting plasma volume. In a study by Ronco et al.  ultrafiltration rates of 0.3, 0.4, 0.5, and 0.6 ml/min/kg were associated with approximate rates for IDH of 8, 15, 26, and 60%, respectively. In addition to an increased mortality with rapid ultrafiltration, a recent DOPPS  also showed markedly higher odds of IDH episodes in patients with an ultrafiltration rate of >10 ml/h/kg.
Stratagems to Reduce IDH without Prolonging
Although the K/DOQI Guidelines  and others [39, 42, 43] admit that to avoid IDH the ultrafiltration rate should not exceed the refilling rate, there is no stress on the lengthening of dialysis sessions, the simplest way to avoid the problem. Instead, multiple maneuvers have been applied to increase the plasma-refilling rate and decrease venous capacity such as: isolated ultrafiltration , high dialysate osmolality [24, 47], dialysate bicarbonate instead of acetate [48, 49], lowered dialysate temperature , and higher dialysate ionized calcium . Rapid lowering of serum potassium during dialysis and high dialysate magnesium were also considered as factors augmenting hypotensive episodes . Finally, predialy-sis withdrawal of blood pressure medications and/or use of blood pressure-rising drugs, such as ephedrine, fludro-cortisone, caffeine, and midodrine have been recommended  .
The most popular recent method of preventing IDH was ultrafiltration and sodium profiling. Although a multitude of approaches has been tried [ 53], the most common was application of a high ultrafiltration rate and high sodium concentration at the beginning of dialysis with a gradual or stepwise decrease in dialysate sodium concentrations and ultrafiltration rates throughout the dialysis session . Whereas short-term studies showed improvement in the incidence of hypotensive episodes, a careful study of sodium balance showed that improvement was related to a positive sodium balance, leading to chronic volume overload, hypertension, myocardial hypertrophy, and increased cardiovascular mortality [55, 56] .
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