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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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Related in: MedlinePlus

Importance of accurate flow-gas signal alignment in different lung model scenarios.The effect of increasing ventilation rate (red joining lines) and signal alignment on accuracy of a lung model, generated using the speeded Innocor analyser. Slope of error versus signal misalignment was increased by smaller lung volumes and faster ventilation rates. Horizontal dotted lines represent the 5% limits of acceptability for functional residual capacity (FRC) determination; vertical dotted line represents the correct signal alignment. RR: respiratory rate, FGD: flow gas delay.
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pone-0098487-g007: Importance of accurate flow-gas signal alignment in different lung model scenarios.The effect of increasing ventilation rate (red joining lines) and signal alignment on accuracy of a lung model, generated using the speeded Innocor analyser. Slope of error versus signal misalignment was increased by smaller lung volumes and faster ventilation rates. Horizontal dotted lines represent the 5% limits of acceptability for functional residual capacity (FRC) determination; vertical dotted line represents the correct signal alignment. RR: respiratory rate, FGD: flow gas delay.

Mentions: The washouts presented in Table 3 were also analysed with adjustments to the FGD in order to explore the impact under different conditions of flow and gas signal mis-alignment on washout accuracy. Each washout was analysed at 10 additional FGD alignments in 10 ms steps from −50 ms to +50 ms of the measured FGD. The most challenging scenario from a technical point of view is that of the small FRC (0.5 L) and fast ventilation rate (30 min−1), whereas the least technically challenging scenario is the 2 L FRC ventilated at 10 min−1. A comparison of the effect of FGD misalignment on these two scenarios, for both the speeded (T90 = 88 ms) and slow (T90 = 154 ms) systems is presented in Figure 6. The slope of the graphs in Figure 6 represent the error sensitivity of the system, i.e. the degree to which errors in signal alignment affect accuracy of FRC determination [5]. This is much steeper when the model is ventilated at a fast rate. Figure 6 also illustrates how slower response time can also be compensated for by shifting the FGD of the standard system by around 30–40 ms. Figure 7 illustrates the performance of the speeded system at different FRC and fast and slow ventilation rates. Although the absolute error remains small at faster ventilation rates, the slope of the error-FGD misalignment curve increases with increasing rate. This error sensitivity is summarised in Figure 8 at fast and slow ventilation rates for both the speeded and slow analyser systems. Error sensitivity was low for both systems at low ventilation rates, but increased with smaller FRC and faster ventilation, and was greater in the “slow” system with longer T90.


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)

Importance of accurate flow-gas signal alignment in different lung model scenarios.The effect of increasing ventilation rate (red joining lines) and signal alignment on accuracy of a lung model, generated using the speeded Innocor analyser. Slope of error versus signal misalignment was increased by smaller lung volumes and faster ventilation rates. Horizontal dotted lines represent the 5% limits of acceptability for functional residual capacity (FRC) determination; vertical dotted line represents the correct signal alignment. RR: respiratory rate, FGD: flow gas delay.
© Copyright Policy
Related In: Results  -  Collection

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

pone-0098487-g007: Importance of accurate flow-gas signal alignment in different lung model scenarios.The effect of increasing ventilation rate (red joining lines) and signal alignment on accuracy of a lung model, generated using the speeded Innocor analyser. Slope of error versus signal misalignment was increased by smaller lung volumes and faster ventilation rates. Horizontal dotted lines represent the 5% limits of acceptability for functional residual capacity (FRC) determination; vertical dotted line represents the correct signal alignment. RR: respiratory rate, FGD: flow gas delay.
Mentions: The washouts presented in Table 3 were also analysed with adjustments to the FGD in order to explore the impact under different conditions of flow and gas signal mis-alignment on washout accuracy. Each washout was analysed at 10 additional FGD alignments in 10 ms steps from −50 ms to +50 ms of the measured FGD. The most challenging scenario from a technical point of view is that of the small FRC (0.5 L) and fast ventilation rate (30 min−1), whereas the least technically challenging scenario is the 2 L FRC ventilated at 10 min−1. A comparison of the effect of FGD misalignment on these two scenarios, for both the speeded (T90 = 88 ms) and slow (T90 = 154 ms) systems is presented in Figure 6. The slope of the graphs in Figure 6 represent the error sensitivity of the system, i.e. the degree to which errors in signal alignment affect accuracy of FRC determination [5]. This is much steeper when the model is ventilated at a fast rate. Figure 6 also illustrates how slower response time can also be compensated for by shifting the FGD of the standard system by around 30–40 ms. Figure 7 illustrates the performance of the speeded system at different FRC and fast and slow ventilation rates. Although the absolute error remains small at faster ventilation rates, the slope of the error-FGD misalignment curve increases with increasing rate. This error sensitivity is summarised in Figure 8 at fast and slow ventilation rates for both the speeded and slow analyser systems. Error sensitivity was low for both systems at low ventilation rates, but increased with smaller FRC and faster ventilation, and was greater in the “slow” system with longer T90.

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
Related in: MedlinePlus