title: Expiratory Time Constant and Sleep Apnea Severity in the Overlap Syndrome.
abstract: OBJECTIVE
  Lung mechanics in the overlap of COPD and sleep apnea impact the severity of sleep apnea. Specifically, increased lung compliance with hyperinflation protects against sleep apnea, whereas increased airway resistance worsens sleep apnea. We sought to assess whether the expiratory time constant, which reflects lung mechanics, is associated with sleep apnea severity in such patients.
  
  
  METHODS
  Polysomnographies in 34 subjects with the overlap syndrome were reviewed. Three time constants were measured for each of up to 5 stages (wake, NREM stages, and REM). The time constants were derived by fitting time and pressure coordinates on the expiratory portion of a nasal pressure signal along an exponentially decaying equation, and solving for the time constant. Demographics, morphometrics, wake end-tidal CO2, right diaphragmatic arc on a chest radiograph, and the apnea-hypopnea index (AHI) were recorded.
  
  
  RESULTS
  The time constant was not associated with age, gender, body mass index, right diaphragmatic arc, or wake end-tidal CO2, and was not significantly different between sleep stages. A mean time constant (TC) was therefore obtained. Subjects with a TC > 0.5 seconds had a greater AHI than those with a TC ≤ 0.5 seconds (median AHI 58 vs. 18, respectively, p = 0.003; Odds ratio of severe sleep apnea 10.6, 95% CI 3.9-51.1, p = 0.005).
  
  
  CONCLUSIONS
  A larger time constant in the overlap syndrome is associated with increased odds of severe sleep apnea, suggesting a greater importance of airway resistance relative to lung compliance in sleep apnea causation in these subjects.
authors.affiliation: Division of Respiratory Disease, Department of Medicine, Bhumibol Adulyadej Hospital Royal Thai Air Force, Thailand.
authors.forename: Darunee
authors.initials: D
authors.lastname: Wiriyaporn
authors.affiliation: Department of Quantitative Health Sciences, Cleveland Clinic, Cleveland, OH.
authors.forename: Lu
authors.initials: L
authors.lastname: Wang
authors.affiliation: Sleep Disorders Center, Neurological Institute, and Department of Pulmonary, Allergy, and Critical Care Medicine, Respiratory Institute, Cleveland Clinic, Cleveland, OH.
authors.forename: Loutfi S
authors.initials: LS
authors.lastname: Aboussouan
mesh_terms: D000368:Aged; D000403:Airway Resistance / Q000502:physiology; D045853:Exhalation / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D017286:Polysomnography; D029424:Pulmonary Disease, Chronic Obstructive / Q000150:complications* / Q000503:physiopathology*; D012119:Respiration; D012129:Respiratory Function Tests / Q000706:statistics & numerical data; D012720:Severity of Illness Index; D012891:Sleep Apnea Syndromes / Q000150:complications* / Q000503:physiopathology*; D012894:Sleep Stages / Q000502:physiology; D013995:Time
keywords: chronic obstructive pulmonary disease; expiratory airflow; obstructive sleep apnea severity; respiratory mechanics