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Enhanced photoacoustic gas analyser response time and impact on accuracy at fast ventilation rates during multiple breath washout.

Horsley A, Macleod K, Gupta R, Goddard N, Bell N - PLoS ONE (2014)

Bottom Line: 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.Signal alignment is a critical factor.With these enhancements, the Innocor analyser exceeds key technical component recommendations for MBW apparatus.

View Article: PubMed Central - PubMed

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.

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.

Conclusion: 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.

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Measurement of flow gas delay (FGD) and response time.1A: An instantaneous flow signal and square wave of SF6 was generated using an electronic solenoid-activated valve to direct a stream of 0.2% SF6 past the gas sample needle and onto the flowmeter mesh. 1B: Flow signal (red) showing a sudden rise when the solenoid is activated. Point A is the zero point for start of FGD measurement. SF6 signal is shown in purple with zero point (B) and SF6 plateau (C) identified. The software then identifies the 50% rise point, as the end of FGD, and the 10–90% rise time (T90).
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pone-0098487-g001: Measurement of flow gas delay (FGD) and response time.1A: An instantaneous flow signal and square wave of SF6 was generated using an electronic solenoid-activated valve to direct a stream of 0.2% SF6 past the gas sample needle and onto the flowmeter mesh. 1B: Flow signal (red) showing a sudden rise when the solenoid is activated. Point A is the zero point for start of FGD measurement. SF6 signal is shown in purple with zero point (B) and SF6 plateau (C) identified. The software then identifies the 50% rise point, as the end of FGD, and the 10–90% rise time (T90).

Mentions: The same method was used to measure flow gas delay (FGD) and T90 simultaneously. An electrically operated rapidly responding solenoid-activated valve (Clippard Inc, Ohio, USA) was connected to a supply of 0.2% SF6 in air (BOC special gases, Surrey, UK) at the inlet side, with gas flow of 4 Lmin−1. The outlet was connected to a custom-made nylon plug that fitted into the exhaust port of the flowmeter. Gas exiting the valve was directed over the gas sample needle and onto the mesh of the flowmeter. When the valve was activated, this produced an instantaneous spike in pressure, detected by the flowmeter, and a square wave change in SF6 from 0 to 0.2%. Custom built software, written using Igor Pro (Wavemetrics Inc., Oregon, USA), was used to identify FGD from the first up-spike in flowmeter pressure to the 50% maximum SF6 signal, minus time taken for the SF6 to traverse the 0.02 ml deadspace of the nylon plug, and with a fixed adjustment of −20 ms to allow for passage of gas through the valve and time to flow peak. T90 was defined as the time in ms for the SF6 signal to rise from 10% to 90% of plateau SF6 concentration. The apparatus and analysis software are illustrated in Figure 1. To separately assess the impact of oxygen on gas transit time, the same process was repeated using a mix of 1% SF6 in 94% oxygen and 5% N2O supplied from Innocor's own on-board gas supply (Innovision ApS, Odense, Denmark) at 8 L/min−1.


Enhanced photoacoustic gas analyser response time and impact on accuracy at fast ventilation rates during multiple breath washout.

Horsley A, Macleod K, Gupta R, Goddard N, Bell N - PLoS ONE (2014)

Measurement of flow gas delay (FGD) and response time.1A: An instantaneous flow signal and square wave of SF6 was generated using an electronic solenoid-activated valve to direct a stream of 0.2% SF6 past the gas sample needle and onto the flowmeter mesh. 1B: Flow signal (red) showing a sudden rise when the solenoid is activated. Point A is the zero point for start of FGD measurement. SF6 signal is shown in purple with zero point (B) and SF6 plateau (C) identified. The software then identifies the 50% rise point, as the end of FGD, and the 10–90% rise time (T90).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4043749&req=5

pone-0098487-g001: Measurement of flow gas delay (FGD) and response time.1A: An instantaneous flow signal and square wave of SF6 was generated using an electronic solenoid-activated valve to direct a stream of 0.2% SF6 past the gas sample needle and onto the flowmeter mesh. 1B: Flow signal (red) showing a sudden rise when the solenoid is activated. Point A is the zero point for start of FGD measurement. SF6 signal is shown in purple with zero point (B) and SF6 plateau (C) identified. The software then identifies the 50% rise point, as the end of FGD, and the 10–90% rise time (T90).
Mentions: The same method was used to measure flow gas delay (FGD) and T90 simultaneously. An electrically operated rapidly responding solenoid-activated valve (Clippard Inc, Ohio, USA) was connected to a supply of 0.2% SF6 in air (BOC special gases, Surrey, UK) at the inlet side, with gas flow of 4 Lmin−1. The outlet was connected to a custom-made nylon plug that fitted into the exhaust port of the flowmeter. Gas exiting the valve was directed over the gas sample needle and onto the mesh of the flowmeter. When the valve was activated, this produced an instantaneous spike in pressure, detected by the flowmeter, and a square wave change in SF6 from 0 to 0.2%. Custom built software, written using Igor Pro (Wavemetrics Inc., Oregon, USA), was used to identify FGD from the first up-spike in flowmeter pressure to the 50% maximum SF6 signal, minus time taken for the SF6 to traverse the 0.02 ml deadspace of the nylon plug, and with a fixed adjustment of −20 ms to allow for passage of gas through the valve and time to flow peak. T90 was defined as the time in ms for the SF6 signal to rise from 10% to 90% of plateau SF6 concentration. The apparatus and analysis software are illustrated in Figure 1. To separately assess the impact of oxygen on gas transit time, the same process was repeated using a mix of 1% SF6 in 94% oxygen and 5% N2O supplied from Innocor's own on-board gas supply (Innovision ApS, Odense, Denmark) at 8 L/min−1.

Bottom Line: 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.Signal alignment is a critical factor.With these enhancements, the Innocor analyser exceeds key technical component recommendations for MBW apparatus.

View Article: PubMed Central - PubMed

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.

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.

Conclusion: 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.

Show MeSH