Impact of nocturnal hemodialysis on the variability of heart rate and duration of hypoxemia during sleep

BACKGROUND: Nocturnal hemodialysis (NHD) alleviates uremia-related sleep apnea, a condition characterized by increased sympathetic activity and diminished heart rate (HR) variability. We tested the hypothesis that NHD reduces both hypoxemia and sympathetic neural contributions to HR variability during sleep. METHODS: Episodes of apnea and hypopnea and the duration of nocturnal hypoxemia during sleep were determined in 9 end-stage renal disease (ESRD) patients (age: 44 +/- 2) (mean +/- SEM) before and after conversion from conventional hemodialysis (CHD) to NHD, and in 10 control subjects (age: 45 +/- 3) with normal renal function and without sleep apnea. Low frequency (LF) (0.05-0.15 Hz) and high frequency (HF) (0.15-0.5 Hz) HR spectral power during stage 2 sleep was calculated (Fast Fourier transformation). Patients were studied 4 times (1 day before and on the night after their CHD session) and 6-15 months after conversion to NHD, while receiving NHD and on a non-dialysis night. RESULTS: NHD decreased the frequency of apnea and hypopnea (from 29.7 +/- 9.3 to 8.2 +/- 2.0 episodes per hour, P= 0.02), and duration of nocturnal hypoxemia (from 13.9 +/- 5.2 to 2.6 +/- 1.9% of total sleep time, P= 0.02). As CHD recipients, ESRD patients had faster nocturnal heart rates (79 +/- 2 vs. 58 +/- 1 min-1, P= 0.03) and lower HF (vagal) (78 +/- 27 vs. 6726 +/- 4556 ms2, P= 0.001) spectral power than control subjects. After conversion to NHD, HR fell (from 79 +/- 2 to 66 +/- 1 min-1, P= 0.03) and HF power increased (from 78 +/- 27 to 637 +/- 139 ms2, P= 0.001). The HF/HF+LF ratio, an index of vagal HR modulation, was lower during CHD (0.16 +/- 0.03 vs. 0.42 +/- 0.05 in control subjects, P < 0.05) and increased (to 0.45 +/- 0.05, P < 0.001) after conversion to NHD. The LF/HF ratio, a representation of sympathetic HR modulation, which was significantly higher during CHD than in control subjects (2.77 +/- 0.82 vs. 0.71 +/- 0.11, P < 0.05), was also normalized by NHD (0.74 +/- 0.12, P < 0.05, compared with CHD). CONCLUSION: Higher heart rates and impaired vagal and augmented sympathetic HR modulation during sleep in ESRD patients are normalized by NHD. Potential mechanisms for these observations include attenuation of surges in sympathetic outflow elicited by apnea and hypoxia during sleep, normalization of nocturnal breathing patterns that influence HRV, and removal, by increased dialysis, of a sympatho-excitatory stimulus of renal origin.