Different interdialytic intervals and cardiorespiratory fitness in patients undergoing hemodialysis.

BACKGROUND: Long interdialytic interval in thrice-weekly hemodialysis is associated with excess cardiovascular and all-cause mortality risk. Impaired cardiorespiratory fitness is a strong predictor of mortality in hemodialysis. This study investigated differences in cardiorespiratory fitness assessed with cardiopulmonary exercise testing (CPET) between the end of the 2-day and the 3-day interdialytic interval.

METHODS: A total of 28 hemodialysis patients, randomized in two different sequences of evaluation underwent CPET and spirometry examination at the end of the 2-day and the 3-day intervals. The primary outcome was the difference in oxygen uptake at peak exercise [VO2peak(ml/kg/min)] assessed with CPET. Volume status was assessed with interdialytic-weight-gain, lung-ultrasound, bioimpedance spectroscopy (BIS), and inferior vena cava measurements. A total of 14 age- and sex-matched controls were also evaluated. Comparisons of changes in parameters of interest were performed with paired or independent t-tests, or relevant non-parametric tests, as appropriate. Bivariate correlation analyses and generalized-linear-mixed-models were employed to examine associations between changes in CPET parameters and volume indices.

RESULTS: Hemodialysis patients at the end of both 2-day and 3-day intervals presented lower values in all major CPET parameters than controls. VO2peak(ml/kg/min) was significantly higher at the end of the 2-day than the 3-day interval (15.2±4.2 vs 13.6±2.8;p<0.001); similar were the results for VO2peak(ml/min) (1188±257 vs 1074±224;p<0.001) and VO2peak(%predicted) (58.9±9.2 vs 52.3±8.6;p<0.001). Numerical but no statistically significant differences were detected in VO2-anaerobic-threshold(ml/kg/min) and VO2-anaerobic-threshold(ml/min) between the two time-points. Maximal work load (90.1±23.2 vs 79.3±25.1;p<0.001), exercise duration, heart rate at peak exercise and oxygen pulse also showed lower values at the end of the 3-day interval. Forced-expiratory-volume in 1 second levels were similar between the two evaluations. Generalized-linear-mixed-model analysis including interdialytic-weight-gain as random covariate attenuated the observed differences in VO2peak(ml/kg/min). Changes in BIS-derived overhydration indexes were moderately correlated with changes of VO2peak(ml/kg/min).

CONCLUSION: The 3-day interval was associated with further impairment of VO2 at peak exercise. This effect was predominantly driven by excess fluid accumulation during the extra interdialytic day.