Night-to-night variability in obstructive sleep apnea severity: relationship to overnight rostral fluid shift.

STUDY OBJECTIVES Overnight rostral fluid shift from the legs to the neck may narrow the pharynx and contribute to obstructive sleep apnea (OSA) pathogenesis. We hypothesized that night-to-night changes in the apnea-hypopnea index (AHI) would be associated with changes in overnight rostral fluid shift. METHODS Twenty-six patients with OSA (AHI ≥10) underwent two polysomnograms 14 days apart with measurement of neck and leg fluid volumes (LFV), neck circumference and upper-airway cross-sectional area before and after sleep. RESULTS Although mean AHI did not differ between polysomnograms, 35% of patients had a difference in AHI >10, indicating significant intra-individual variability. There were direct correlations between change in non-rapid-eye movement (NREM), but not REM AHI and change in evening LFV between polysomnograms (r = 0.440, p = 0.036 and r = 0.005, p = 0.982, respectively) and between change in supine, but not non-supine AHI and change in evening LFV (r = 0.483, p = 0.020 and r = 0.269, p = 0.280, respectively). An increase in evening LFV between polysomnograms was associated with a greater overnight decrease in LFV (r = 0.560, p = 0.005) and a greater overnight increase in neck fluid volume (r = 0.498, p = 0.016). Additionally, a greater overnight increase in neck circumference was associated with a greater overnight increase in neck fluid volume between polysomnograms (r = 0.453, p = 0.020) and a greater overnight decrease in upper-airway cross-sectional area (r = -0.587, p = 0.005). CONCLUSION Intra-individual variability in OSA severity may be partly explained by day-to-day changes in evening leg fluid volume and overnight rostral fluid shift, which may be most important in the pathogenesis of OSA during NREM and supine sleep.