Measuring intradialytic hypotension to improve quality of care.

A significant fall in BP during dialysis, so-called intradialytic hypotension (IDH), is an important clinical problem. IDH is common, occurring in 15%–20% of treatments.1 IDH is often associated with distressing symptoms such as lightheadedness, weakness, muscle cramps, and nausea and vomiting. As could be imagined, reducing blood flow to vital organs, even transiently, is associated with a panoply of organ damage, including myocardial stunning,2 ischemic damage to the white matter of the brain, and perhaps disruption of the gastrointestinal barrier against endotoxins with increased inflammation. Severe IDH has been associated with a variety of catastrophes. It is likely linked to intradialytic arrhythmias, and may precipitate these, as well as myocardial infarction. Case reports of intestinal infarction, as well as of IDH-induced blindness due to retinal ischemia, have been reported. Arteriovenous access thrombosis is seen more commonly in patients with IDH.3 Finally, in patients with residual kidney function, repeated ischemic insults to a kidney unable to autoregulate its blood flow may hasten the progression to anuria and loss of the substantial advantages that even small amounts of residual kidney function provide. Some of the cardiovascular event risk associated with IDH may be a reflection of underlying comorbidity. The risk of IDH increases markedly at low values of predialysis BP,1,4 and low predialysis BP has itself been associated with cardiovascular disease and increased short-term mortality risk.4 However, it is very likely that repeated ischemic insults to various body organs caused by IDH are causally linked to poor outcomes including mortality. The root cause of IDH is fluid removal. If one dialyzes a patient and does not remove fluid, the occurrence of IDH is rare tononexistent.Twoaspects offluid removal are important: the rate of removal, and the amount of fluid removal. During the early part of a dialysis session, when most excess fluid is located closer to the central circulation, higher rates of fluid removal are tolerated. By contrast, toward the end of a dialysis session, the same rate of fluid removal might result in hypotension, because the slow rate of fluid transfer from distal edematous body compartments to the circulation results in poor vascular refilling, reduced cardiac output, and thus IDH. One can think of fluid removal on a weekly time scale. Fluid removal depends on the weekly amount of fluid ingestion, and this in turn is driven primarily by sodium intake. Diffusive gain of sodium from the dialysis solution is also a factorwhenhigherdialysis solution sodiumconcentrations are used. Weekly fluid removal also depends on the daily urine volume, which represents ingested fluid that does not need to be removed bydialysis. After residual kidney function has been taken into account, the remaining ingested fluid needs to be removed by dialysis, and the sole determinant of the required ultrafiltration rate will be weekly dialysis time. Dividing up a fixed amount of dialysis time into intervalsmore frequent than three per week is an attractive option theoretically, because thenmorefluid should be closer to the central circulation at the time of attempted removal.However, inpatients in theFrequent HemodialysisNetworkDaily Studywhowere assigned to therapy six times per week, the weekly frequency of IDH was slightly increased relative to the group assigned to conventional thriceweekly treatments. This was due to the greater number of dialysis treatments given in the frequently dialyzed group, as the rate of IDH per treatment was reduced.5 Moving to a more frequent dialysis schedule is associated with a reduction in predialysis BP and a reduced need for antihypertensivemedication.5 Avoidance of the long weekend intradialytic interval is another benefit of adding more treatments per week. With regard to fluid removal needs, one might achieve a considerable reduction in IDH by encouraging patients to eat less sodium, by avoiding high dialysis sodium concentrations, and by increasing weekly dialysis time. An equally important issue with regard to IDH may be the volume of the extracellular fluid (ECF) postdialysis. Using a multifrequency bioimpedance device, one can more rationally target an “optimum” postdialysis weight, based on a comparison of bioimpedance-estimated ECF compared with ECF of healthy patients of similar age, sex, and body size.6 It is not yet known to what extent the ideal ECF suggested by these devices indicates the true “optimum” level in a given dialysis patient, particularly as modulated by cachexia/obesity and general health. In addition, it is not known whether one should decrease the ECF to this optimum level at the end of dialysis or if one should dip slightly below it in order to Published online ahead of print. Publication date available at www.jasn.org.