title: Fractal analysis of tracheal sounds during maximal forced exhalation.
abstract: BACKGROUND
  Fractal structures or processes, many of which are found in nature, are marked by the presence of a formal pattern that repeats itself even as the scale changes, a property termed 'self-similarity'. The bronchial tree is structurally a fractal system and inspiration is a fractal process. Our objective was to show that the dynamic process of emptying air from the bronchi as measured by tracheal sounds should also behave fractally over the period of time during which exhalation takes place.
  
  
  METHODS
  Twenty non-smoking patients (9 males and 11 females) aged 53.6+/-16.3 years with persistent but clinically stable moderate to severe asthma and a control group of 11 healthy non-smoking subjects (6 males and 5 females) aged 46.0+/-12.5 years were enrolled. Respiratory sounds were registered using a microphone held by an elastic band directly against the patient's skin at the level of the cricoid cartilage. Analysis was done on the average of three forced expirations made after a deep inspiration. The Hurst's coefficient was computed by Hurst's rescaled range analysis (H) and by analysis of the variance-time relationship of the aggregated process (H_v).
  
  
  RESULTS
  Values of H and H_v were around 0.75, indicating that the sound signals were fractal over time. The responses of controls and patients to bronchodilator administration of 1 mg terbutaline were similar (p>0.05). Mean intra-subject variation coefficients were less than 15%.
  
  
  CONCLUSIONS
  This analysis of tracheal sound during forced expiratory maneuvers confirms the dynamic fractal behavior of the respiratory system.
authors.affiliation: Department of Respiratory Medicine, Hospital Universitario Germans Trias i Pujol, Badalona, Spain. jafiz@ns.hugtip.scs.es
authors.forename: José A
authors.initials: JA
authors.lastname: Fiz Fernández
authors.forename: Jose
authors.initials: J
authors.lastname: Izquierdo Ramirez
authors.forename: Mari A
authors.initials: MA
authors.lastname: García Fernandez
authors.forename: Juan
authors.initials: J
authors.lastname: Ruiz Manzano
authors.forename: José
authors.initials: J
authors.lastname: Morera Prat
mesh_terms: D000328:Adult; D000368:Aged; D000465:Algorithms; D001249:Asthma / Q000503:physiopathology; D016022:Case-Control Studies; D045853:Exhalation / Q000502:physiology*; D005260:Female; D017709:Fractals; D006801:Humans; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D012135:Respiratory Sounds / Q000502:physiology*; D014132:Trachea / Q000502:physiology*