title: Enhanced photoacoustic gas analyser response time and impact on accuracy at fast ventilation rates during multiple breath washout.
abstract: BACKGROUND
  The Innocor device contains a highly sensitive photoacoustic gas analyser that has been used to perform multiple breath washout (MBW) measurements using very low concentrations of the tracer gas SF6. Use in smaller subjects has been restricted by the requirement for a gas analyser response time of <100 ms, in order to ensure accurate estimation of lung volumes at rapid ventilation rates.
  
  
  METHODS
  A series of previously reported and novel enhancements were made to the gas analyser to produce a clinically practical system with a reduced response time. An enhanced lung model system, capable of delivering highly accurate ventilation rates and volumes, was used to assess in vitro accuracy of functional residual capacity (FRC) volume calculation and the effects of flow and gas signal alignment on this.
  
  
  RESULTS
  10-90% rise time was reduced from 154 to 88 ms. In an adult/child lung model, accuracy of volume calculation was -0.9 to 2.9% for all measurements, including those with ventilation rate of 30/min and FRC of 0.5 L; for the un-enhanced system, accuracy deteriorated at higher ventilation rates and smaller FRC. In a separate smaller lung model (ventilation rate 60/min, FRC 250 ml, tidal volume 100 ml), mean accuracy of FRC measurement for the enhanced system was minus 0.95% (range -3.8 to 2.0%). Error sensitivity to flow and gas signal alignment was increased by ventilation rate, smaller FRC and slower analyser response time.
  
  
  CONCLUSIONS
  The Innocor analyser can be enhanced to reliably generate highly accurate FRC measurements down at volumes as low as those simulating infant lung settings. Signal alignment is a critical factor. With these enhancements, the Innocor analyser exceeds key technical component recommendations for MBW apparatus.
authors.affiliation: Institute of Inflammation and Repair, University of Manchester, Manchester, United Kingdom; Manchester Adult Cystic Fibrosis Centre, University Hospital of South Manchester, Manchester, United Kingdom.
authors.forename: Alex
authors.initials: A
authors.lastname: Horsley
authors.affiliation: Department of Respiratory Medicine, Great Ormond Street Hospital, London, United Kingdom.
authors.forename: Kenneth
authors.initials: K
authors.lastname: Macleod
authors.affiliation: School of Chemical Engineering and Analytical Science, University of Manchester, Manchester, United Kingdom.
authors.forename: Ruchi
authors.initials: R
authors.lastname: Gupta
authors.affiliation: School of Chemical Engineering and Analytical Science, University of Manchester, Manchester, United Kingdom.
authors.forename: Nick
authors.initials: N
authors.lastname: Goddard
authors.affiliation: Department of Respiratory Medicine, Bristol Royal Infirmary, University Hospitals Bristol NHS Foundation Trust, Bristol, United Kingdom.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Bell
mesh_terms: D005652:Functional Residual Capacity / Q000502:physiology*; D006801:Humans; D008176:Lung Volume Measurements; D011930:Reaction Time / Q000502:physiology