Disentangling the ultrafiltration rate-mortality association: the respective roles of session length and weight gain

Abstract

Background and objectives: Rapid ultrafiltration rate is associated with increased mortality among hemodialysis patients. Ultrafiltration rates are determined by interdialytic weight gain and session length. Although both interdialytic weight gain and session length have been linked to mortality, the relationship of each to mortality, independent of the other, is not adequately defined. This study was designed to evaluate whether shorter session length independent of weight gain and larger weight gain independent of session length are associated with increased mortality.

Design, setting, participants, & measurements: Data were taken from a national cohort of 14,643 prevalent, thrice-weekly, in-center hemodialysis patients dialyzing from 2005 to 2009 (median survival time, 25 months) at a single dialysis organization. Patients with adequate urea clearance and delivered dialysis session ≥240 and <240 minutes were pair-matched on interdialytic weight gain (n=1794), and patients with weight gain ≤3 and >3 kg were pair-matched on session length (n=2114); mortality associations were estimated separately.

Results: Compared with delivered session length ≥240, session length <240 minutes was associated with increased all-cause mortality (adjusted hazard ratio [95% confidence interval], 1.32 [1.03 to 1.69]). Compared with weight gain ≤3, weight gain >3 kg was associated with increased mortality (1.29 [1.01 to 1.65]). The associations were consistent across strata of age, sex, weight, and weight gain and session length. Secondary analyses demonstrated dose-response relationships between both and mortality.

Conclusions: Among patients with adequate urea clearance, shorter dialysis session length and greater interdialytic weight gain are associated with increased mortality; thus, both are viable targets for directed intervention.

Figures

Figure 1.

Adjusted associations between delivered DSL and mortality and IDWG and mortality based on Cox regression models. In the DSL analysis, patients were matched on sex, access type (fistula, graft, catheter), age (±2.5 years), IDWG (±0.25 kg), and postdialysis weight (±1.5 kg). In the IDWG analysis, patients were matched on sex, access type (fistula, graft, catheter), age (±2.5 years), delivered DSL (rounded to 5 minutes), and postdialysis weight (±1.5 kg). Multivariable models were stratified on matched pair assignment, and include covariate adjustment terms for age, race (black, nonblack), postdialysis weight (kg), vintage (<1, 2, 3, ≥4 years), diabetes, coronary artery disease, congestive heart failure, missed dialysis sessions over 30 days (0, 1, 2, 3, ≥4), creatinine (quartiles, mg/dl), albumin (≤3.0, 3.1–3.5, 3.6–4.0, >4.0 g/dl), phosphorus (≤4.0, 4.1–5.0, 5.1–6.0, >6.0 mg/dl), and predialysis systolic BP (≤130, 131–150, 151–170, >170 mmHg); in the DSL analysis, the model was also adjusted for IDWG (kg). DSL, dialysis session length; IDWG, interdialytic weight gain; DelDSL, delivered dialysis session length; HR, hazard ratio; 95% CI, 95% confidence interval; ref, reference.

Figure 2.

Adjusted associations between delivered DSL <240 minutes (versus delivered DSL ≥240 minutes) and mortality and IDWG >3 kg (versus IDWG ≤3 kg) and mortality among matched pairs in which differences between matched pairs in delivered DSL and IDWG are parsed into tertiles. Results in the top panel indicate the association between DSL <240 minutes (versus DSL ≥240 minutes) among strata of matched pairs in which the difference in DSL (between pair members) was 1–24 minutes (tertile 1), 24–42 minutes (tertile 2), and 42–95 minutes (tertile 3). Results in the bottom panel indicate the associations between IDWG >3 kg (versus IDWG ≤3 kg) among strata of matched pairs in which the difference in IDWG (between pair members) was 0.1–1.1 kg (tertile 1), 1.1–1.9 (tertile 2) and 1.9–5.9 (tertile 3). Patients with DSL <240 versus ≥240 minutes were matched on sex, access type (fistula, graft, catheter), age (±2.5 years), IDWG (±0.25 kg), and postdialysis weight (±1.5 kg). Patients with IDWG >3 versus ≤3 kg were matched on sex, access type (fistula, graft, catheter), age (±2.5 years), delivered DSL (rounded to the nearest 5 minutes), and postdialysis weight (±1.5 kg). Multivariable models were stratified on matched pair assignment and included covariate adjustment terms for age, race (black, nonblack), postdialysis weight (kg), vintage (<1, 2, 3, ≥4 years), diabetes, coronary artery disease, congestive heart failure, missed dialysis sessions over 30 days (0, 1, 2, 3, ≥4), creatinine (quartiles, mg/dl), albumin (≤3.0, 3.1–3.5, 3.6–4.0, >4.0 g/dl), phosphorus (≤4.0, 4.1–5.0, 5.1–6.0, >6.0 mg/dl), and predialysis systolic BP (≤130, 131–150, 151–170, >170 mmHg) in both analyses. In the DSL analysis, the model was also adjusted for IDWG (kg). DSL, dialysis session length; DelDSL, delivered dialysis session length; IDWG, interdialytic weight gain.