Enhanced Photoacoustic Gas Analyser Response Time and Impact on Accuracy at Fast Ventilation Rates during Multiple Breath Washout

<div><p>Background</p><p>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.</p><p>Methods</p><p>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.</p><p>Results</p><p>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.</p><p>Conclusion</p><p>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.</p></div

Improvements to Innocor gas analyser rise time.

<p>Effect of modifications to Innocor hardware on rise time (T₉₀) and flow gas delay of SF₆ signal. Steps were performed sequentially in the numbered order. FGD - flow gas delay.</p

Error sensitivity of lung volume calculation.

<p>Error sensitivity of the two different Innocor systems (speeded with T₉₀ = 88 ms, and slow with T₉₀ = 154 ms) at two different lung model ventilation rates. Performances of the speeded system are joined by red lines, those of the slow system by black lines. Error sensitivity was defined as the % error in FRC that would be caused by a 10 ms (single sample step) mis-alignment in flow gas delay.</p

Measurement of flow gas delay (FGD) and response time.

<p>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).</p

Importance of accurate flow-gas signal alignment in different lung model scenarios.

<p>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.</p

Impact of hardware improvements on SF₆ response time (T₉₀).

<p>Aligned flow and SF₆ signals are shown for a standard Innocor (T₉₀ = 154 ms), and an improved version, labelled “Speeded SF₆” (T₉₀ = 88 ms). Double headed arrows indicate the time between 10% and 90% plateau SF₆ response.</p

Diagram of lung model.

<p>A high-precision computer-controlled linear motor was used to drive a calibration syringe. This moved air into and out of the ventilation compartment of the lung tank. Water level in the lung compartment determined functional residual capacity (FRC) of the model whilst ventilation rate and volume were controlled by speed and excursion of the linear motor. A flowmeter and gas sample needle were connected to the lung compartment. During washin, a T-piece connected the flowmeter to the open circuit 0.2% SF₆.</p

Scatter plot with Spearman correlation of the % change in the ‘alveolar’ Vt_alv/FRC_alv with the % change in ‘alveolar’ LCI_alv for both cystic fibrosis (CF) and healthy control (HC) subjects combined.

<p>The Spearman correlation suggests a positive relationship between increasing % change Vt_alv/FRC_alv and increasing % change LCI_alv.</p

A comparison of the % change in ‘alveolar’ multiple breath washout outcomes between healthy controls and cystic fibrosis subjects.

<p>A comparison of the % change in ‘alveolar’ multiple breath washout outcomes between healthy controls and cystic fibrosis subjects.</p

Demographics and baseline lung function data for healthy control and cystic fibrosis subjects.

<p>Demographics and baseline lung function data for healthy control and cystic fibrosis subjects.</p