Pre-dialysis fluid status, pre-dialysis systolic blood pressure and outcome in prevalent haemodialysis patients: results of an international cohort study on behalf of the MONDO initiative.

Background: Pre-dialysis fluid overload (FO) associates with mortality and causes elevated pre-dialysis systolic blood pressure (pre-SBP). However, low pre-SBP is associated with increased mortality in haemodialysis patients. The objective of this study was to investigate the interaction between pre-dialysis fluid status (FS) and pre-SBP in association with mortality.
Methods: We included all patients from the international Monitoring Dialysis Outcome Initiative (MONDO) database with a pre-dialysis multifrequency bioimpedance spectroscopy measurement in the year 2011. We used all parameters available during a 90-day baseline period. All-cause mortality was recorded during 1-year follow-up. Associations with outcome were assessed with Cox models and a smoothing spline Cox analysis.
Results: We included 8883 patients. In patients with pre-dialysis FO (>+1.1 to +2.5 L), pre-SBP <110 mmHg was associated with an increased risk of death {hazard ratio (HR) 1.52 [95% confidence interval (CI) 1.06-2.17]}. An increased risk of death was also associated with pre-dialysis fluid depletion (FD; <-1.1 L) combined with a pre-SBP <140 mmHg. In normovolemic (NV) patients, low pre-SBP <110 mmHg was associated with better survival [HR 0.46 (95% CI 0.23-0.91)]. Also, post-dialysis FD associated with a survival benefit. Results were similar when inflammation was present. Only high ultrafiltration rate could not explain the higher mortality rates observed.
Conclusion: The relation between pre-SBP and outcome is dependent on pre-dialysis FS. Low pre-SBP appears to be disadvantageous in patients with FO or FD, but not in NV patients. Post-dialysis FD was found to associate with improved survival. Therefore, we suggest interpreting pre-SBP levels in the context of FS and not as an isolated marker.