Discussion
The results of this study demonstrate the following: (1) the prognostic information of BP measurements obtained in
outside the dialysis unit either by the patient or by an automatic monitor is greater than that obtained in the dialysis
unit; (2) the prognostic information is nearly all contained in the systolic component of BP, rather than the diastolic component; (3) the relationship of BP to mortality is independent of conventional and unconventional cardiovascular
risk factors (Table 2); and (4) the relationship of BP recordings and mortality followed a W-shaped curve for out-ofdialysis unit recordings (Figure 4).
Patients who were recently hospitalized or sick were excluded. Thus, patients who may have been more hypotensive were not studied. Thus, this study differed in its recruitment criteria compared to epidemiological studies
which have analyzed all patients in the dialysis unit regardless of their level of illness. These large cohort studies report
a consistently higher mortality for lower blood pressures and do not find increase in mortality for increasing the level of
BP. Perhaps this may be because of a stronger signal of low BP for mortality reflecting the poor health of these patients. Similar to what has been reported by large cohort studies using dialysis unit measurements, we found a higher mortality
among patients in the lowest quartile of home and ambulatory BP. However, in sharp contrast to the cohort studies, out-of-dialysis unit BP recordings demonstrated a clear trend of increasing all-cause mortality among patients in
increasing home or ambulatory BP quartiles. These findings suggest that BP recorded outside the dialysis unit may contain
greater prognostic information compared to BP measured in the dialysis unit.
Three studies using ambulatory BP monitoring in hemodialysis patients support the notion that ambulatory BP and mortality are strongly related. Among 57 treated French hypertensive hemodialysis patients, Amar et al10 reported that at follow-up of 34_20 months, patients died of cardiovascular causes. Nocturnal systolic BP was associated with
increased risk of cardiovascular death (risk ratio: 1.41; 95% CI: 1.08 to 1.84). The largest study to date among hemodialysis patients reporting the relationship between 24-hour ambulatory BP and cardiovascular outcomes comprised of 168 patients. Among these nondiabetic patients without preexisting cardiovascular events, Tripepi et al16 reported the
ratio of the average systolic BP during the night and day (night/day systolic ratio) used to indicate the nocturnal fall in
BP or the dipping phenomenon was associated with all-cause and cardiovascular mortality on both univariate and multivariate analyses. In contrast to Tripepi et al, this study included blacks (who had a lower mortality compared to whites) and those with cardiovascular disease (who had a higher mortality as expected). A previous report by Alborzi et
al11 reported that ambulatory BP was of greater prognostic value compared to dialysis unit BP recordings, but these analyses were unadjusted for cardiovascular risk factors. An important aspect of the current report is that the effect of ambulatory and home BP on survival persisted even after adjustments for cardiovascular disease, as well as conventional and nonconventional cardiovascular risk factors for mortality. Furthermore, the current study extends the above reports to a cohort nearly twice as large as the largest study reported to date and with a longer follow-up.
The present study found that home systolic BP threshold for optimal survival was 10 mm Hg higher than ambulatory systolic BP. When compared to ambulatory systolic BP among hemodialysis patients, home systolic BP is on average 12.2 mm Hg higher.5 Thus, it is not surprising to find a higher threshold for optimal outcome with home systolic BP. A shorter follow-up from a subgroup of this cohort has also found a link between increased home BP to mortality among long-term hemodialysis patients.11 The results of this study are also consistent with previously published cohort studies examining the influence of home BP with clinic BP among patients without kidney disease.17,18 Similarly, compared to clinic BP among patients with chronic kidney disease not on dialysis, the risk for end-stage renal disease is increased to a greater extent when home BP recordings are considered.19 The W-shaped relationship for home BP and mortality was unexpected. A similar though less pronounced W pattern was discernable for ambulatory BP. It is possible that when home BP (or ambulatory BP) was found to be high, patients were treated leading to a subsequent improvement in survival. Thus, treatment with antihypertensive medications (that may have a cardioprotective effect) or dry-weight reduction may modify the relationship of the initial measurement of BP and the final outcome. Given that the analysis is limited to a single occasion of BP measurement, the time-dependent relationships cannot be explored.
There are several possibilities why out-of-dialysis unit measurements may have provided better prognostic information.First, multiple blood pressure measurements over the course of the day, as done with home or ambulatory blood pressure monitoring, can average out the troughs and peaks in BP swings, which predialysis and postdialysis BP recordings are unable to provide.20 Second, dialysis unit blood pressures are influenced by the white coat effect— elevated BP only in the dialysis setting—which is less pronounced with home blood pressures and eliminated by ambulatory blood pressure monitoring.21 Third, masked hypertension— elevated BP at home but normal in the dialysis unit—is potentially detected with home BP monitoring and ambulatory blood pressure monitoring and may be of prognostic significance.22 Finally, blood pressures sampled from a broader pool of situations may make them more representative of the person’s typical blood pressure.
There are several strengths and limitations of this report. This study was largely limited to black people, and excluded were certain patients such as those with morbid obesity and atrial fibrillation because of difficulties with accurate blood pressure assessment in this group. Whether the same results would hold in people of other ethnicities and of broader clinical characteristics is not known and will require verification in future cohorts. Although this is the largest study
among dialysis patients reported to date, the sample size of this study was still relatively small. Some strengths of this
study are as follows: (1) by using cubic splines, the threshold of BP that is associated with a better prognosis could be
tested, instead of using arbitrary definitions of normotension and hypertension which are debatable in the hemodialysis
population; (2) the home blood pressure monitor used was a validated device equipped with a memory device and printer, so there was a mechanism in place to confirm the authenticity of the patient reports.