title: Sources of error in noninvasive pulmonary blood flow measurements by partial rebreathing: a computer model study.
abstract: BACKGROUND
  Partial rebreathing is a noninvasive method for measuring pulmonary blood flow (PBF). This study examines the systematic errors produced by the partial rebreathing technique utilizing a comprehensive mathematical model of the cardiorespiratory system of a healthy, 70-kg adult male.
  
  
  METHODS
  The model simulates tidal breathing through a branched respiratory tree and incorporates the effects on carbon dioxide dynamics of lung tissue mass, vascular transport delays, multiple body compartments, and realistic blood-gas dissociation curves. Four studies were performed: (1) errors produced under standard conditions, (2) effects of recirculation, (3) effects of alveolar-proximal airway partial pressure of carbon dioxide (Pco(2)) differences, and (4) effects of rebreathing time.
  
  
  RESULTS
  Systematic errors are less than 10% when the simulated PBF is between 3 and 6 l/min. At 2 l/min, PBF is overestimated by approximately 35%. At 14 l/min, PBF is underestimated by approximately 40%. At PBF of greater than 6 l/min, recirculation causes approximately 60% of the systematic error, alveolar-proximal airway differences cause approximately 20%, and alveolar-arterial differences cause approximately 20%. The standard rebreathing time of 50 s is shown to be excessive for PBF of greater than 6 l/min. At PBF of less than 3 l/min, errors are caused by inadequate rebreathing time and alveolar-arterial gradients.
  
  
  CONCLUSIONS
  Systematic errors in partial rebreathing cardiac output measurements have multiple causes. Our simulations suggest that errors can be reduced by using a variable rebreathing time, which should be increased at low PBF so that quasi-equilibrium in the alveoli can be achieved and decreased at high PBF to reduce the effects of recirculation.
authors.affiliation: Department of Anaesthetics, University of Sydney, Camperdown, Australia.
authors.forename: Johnny S
authors.initials: JS
authors.lastname: Yem
authors.forename: Yongquan
authors.initials: Y
authors.lastname: Tang
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Turner
authors.forename: A Barry
authors.initials: AB
authors.lastname: Baker
mesh_terms: D000328:Adult; D000399:Air Pressure; D002245:Carbon Dioxide / Q000032:analysis / Q000378:metabolism; D003198:Computer Simulation; D006801:Humans; D008297:Male; D008953:Models, Anatomic; D015233:Models, Statistical; D011650:Pulmonary Alveoli / Q000502:physiology; D011652:Pulmonary Circulation / Q000502:physiology*; D015203:Reproducibility of Results; D012121:Respiration, Artificial; D015656:Respiratory Mechanics
chemical_list: D002245:Carbon Dioxide