title: Night-to-night variability in obstructive sleep apnea severity: relationship to overnight rostral fluid shift.
abstract: OBJECTIVE
  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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Sleep Research Laboratory of the University Health Network Toronto Rehabilitation Institute, Toronto, Ontario, Canada: Centre for Sleep Medicine and Circadian Biology of the University of Toronto, Toronto, Ontario, Canada.
authors.forename: Laura H
authors.initials: LH
authors.lastname: White
authors.affiliation: Sleep Research Laboratory of the University Health Network Toronto Rehabilitation Institute, Toronto, Ontario, Canada: Centre for Sleep Medicine and Circadian Biology of the University of Toronto, Toronto, Ontario, Canada.
authors.forename: Owen D
authors.initials: OD
authors.lastname: Lyons
authors.affiliation: Sleep Research Laboratory of the University Health Network Toronto Rehabilitation Institute, Toronto, Ontario, Canada: Institute of Biomaterial and Biomedical Engineering, University of Toronto, Toronto, Ontario, Canada.
authors.forename: Azadeh
authors.initials: A
authors.lastname: Yadollahi
authors.affiliation: Sleep Research Laboratory of the University Health Network Toronto Rehabilitation Institute, Toronto, Ontario, Canada: Centre for Sleep Medicine and Circadian Biology of the University of Toronto, Toronto, Ontario, Canada: Department of Medicine of the University Health Network Toronto General Hospital, Toronto, Ontario, Canada.
authors.forename: Clodagh M
authors.initials: CM
authors.lastname: Ryan
authors.affiliation: Sleep Research Laboratory of the University Health Network Toronto Rehabilitation Institute, Toronto, Ontario, Canada: Centre for Sleep Medicine and Circadian Biology of the University of Toronto, Toronto, Ontario, Canada: Department of Medicine of the University Health Network Toronto General Hospital, Toronto, Ontario, Canada.
authors.forename: T Douglas
authors.initials: TD
authors.lastname: Bradley
mesh_terms: D005260:Female; D018495:Fluid Shifts / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D017286:Polysomnography / Q000706:statistics & numerical data; D012720:Severity of Illness Index; D020181:Sleep Apnea, Obstructive / Q000503:physiopathology*
keywords: Obstructive sleep apnea; rostral fluid shift; upper airway