title: Assessment of breath-by-breath alveolar gas exchange: an alternative view of the respiratory cycle.
abstract: OBJECTIVE
  Breath-by-breath (BbB) determination of the O2 flux at alveolar level implies the identification of the start and end points of each respiratory cycle; Grønlund defined them as the times in two successive breaths showing equal expiratory gas fractions. Alternatively, the start and end points of each breath might be linked to the ratio between the exchangeable and non-exchangeable gases. The alternative algorithm is described and evaluated with respect to the algorithm proposed by Grønlund.
  
  
  METHODS
  Oxygen and carbon dioxide fractions, and ventilatory flow at the mouth were continuatively recorded in 20 subjects over 6 min at rest and during a cycloergometer exercise including 4 increasing intensities lasting 6 min each. Alveolar BbB oxygen uptake was calculated from the gas and flow traces by means of the two methods at stake.
  
  
  RESULTS
  Total number of analysed breaths was 14,257. The data obtained with the two methods were close to the identity line (average slope 0.998 ± 0.004; R > 0.994; n > 334 in all subjects). Average difference between the O2 uptake data obtained by the two methods amounted to -0.27 ± 1.29 mL/min, whilst the standard deviation of the differences was 11.5 ± 4.6 mL/min. The relative percentage difference was independent from the O2 uptake and showed an average bias amongst subjects close to zero (-0.06 ± 0.15 %).
  
  
  CONCLUSIONS
  The alternative timing of the respiratory cycle provided congruent O2 uptake data and made the identification of the start and end points of each breath more robust without introducing systematic errors.
authors.affiliation: Department of Medical and Biological Sciences, University of Udine, P.le Kolbe 4, 33100, Udine, Italy.
authors.forename: V
authors.initials: V
authors.lastname: Cettolo
authors.forename: Maria Pia
authors.initials: MP
authors.lastname: Francescato
mesh_terms: D000328:Adult; D000465:Algorithms; D001944:Breath Tests / Q000379:methods; D003198:Computer Simulation; D015444:Exercise / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008954:Models, Biological; D010101:Oxygen Consumption / Q000502:physiology*; D011650:Pulmonary Alveoli / Q000502:physiology*; D011659:Pulmonary Gas Exchange / Q000502:physiology*; D015203:Reproducibility of Results; D015656:Respiratory Mechanics / Q000502:physiology*; D012680:Sensitivity and Specificity