Abstract
BackgroundThe 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.MethodsA 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.Results10–90% 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.ConclusionThe 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.
Highlights
Multiple breath inert gas washout (MBW) is an old technique [1] that has been reinvigorated in the last decade by the recognition that this relatively simple test is a sensitive measure of lung physiology [2,3]
In order to derive functional residual capacity (FRC), expired gas volumes are calculated by integrating the gas signal with flow
In this study we have shown that can the response time of the Innocor analyser be reduced to well under the 100 ms target required by the multiple breath washout (MBW) technical guidelines [4], but that this results in a system with enhanced accuracy at a range of clinically relevant lung model settings
Summary
Multiple breath inert gas washout (MBW) is an old technique [1] that has been reinvigorated in the last decade by the recognition that this relatively simple test is a sensitive measure of lung physiology [2,3]. MBW systems need to be able to measure tracer gas concentrations with a high level of accuracy at both the start and end of a washout, the end concentration conventionally being 1/40th of that at the start. This requires a linear response profile of the analyser across a broad range, and a noise to signal ratio of at least 5% at end of washout [4]. In order to derive functional residual capacity (FRC), expired gas volumes are calculated by integrating the gas signal with flow This process requires accurate and stable alignment of the two signals [5]. 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
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