Controlled versus free breathing for multiple breath nitrogen washout in healthy adults

Assessing the calculation of conductive and acinar ventilatory heterogeneity indices Scond and Sacin from multiple-breath washout data

Sensor errors resulting in elevated values of N₂ concentration [N₂] in commercial multiple-breath washout (MBW) devices have been shown to prolong the washout and result in erroneously high functional residual capacity (FRC) and lung clearance index (LCI) values. The errors also affect the indices of conductive and acinar ventilatory heterogeneity (S_cond and S_acin) although the mechanism by which this change in values occurs remains unclear. Exploring these effects also provides a timely opportunity to examine the appropriateness of the algorithm used to calculate these indices. Using a two-compartment model with differing specific ventilation (SV) such that the lower SV unit empties late, noise-free MBW were simulated both corrected and uncorrected for the recent sensor error. S_cond was calculated using regression of normalized phase III slope (Sn_III) against lung turnover (TO) from a TO range starting at 1.5 and ending at an upper turnover (TO_upper) between 4 and 8 (default 6) over a range of simulated values. The principal effect of the sensor error was that as the MBW proceeded the phase III slope of successive breaths was normalized by an increasingly overestimated [N₂], resulting in Sn_III values that fell precipitously at high TO, greatly reducing S_cond. Reanalysis of previously published data and of simulated data showed a large proportional bias in S_cond, whereas S_acin was only minimally affected. In adult subject data, reducing TO_upper below 5.5 was associated with a large drop of up to ∼60% in S_cond calculated from data corrected for sensor error. Raising the upper TO limit elevated S_cond by ∼20% but with a large concomitant increase in variability. In contrast to S_cond, S_acin was relatively unaffected by changes in TO_upper with changes of <3%. This work serves to emphasize that the upper limit of TO of 6 represents an appropriate upper limit for the calculation of S_cond.NEW & NOTEWORTHY Sensor errors that elevated values of N₂ concentration in commercial multiple-breath washout (MBW) devices resulted in errors in calculated parameters including S_cond and S_acin. We examined the mechanism of the change in values produced by these errors and explored the appropriateness of the calculation of S_cond and S_acin. This work serves to emphasize that the current algorithm in use is appropriate for the calculation of S_cond and S_acin.

Model analysis of multiple breath nitrogen washout data: robustness to variations in breathing pattern

We recently developed a model-based method for analyzing multiple breath nitrogen washout data that does not require identification of Phase-III. In the present study, we assessed the effect of irregular breathing patterns on the intra-subject variabilities of the model parameters. Nitrogen fraction at the mouth was measured in 18 healthy and 20 asthmatic subjects during triplicate performances of multiple breath nitrogen washout, during controlled (target tidal volume 1 L at 8-12 breaths per minute) and free (unrestricted) breathing. The parameters S_cond, S_acin and functional residual capacity (FRC) were obtained by conventional analysis of the slope of Phase-III. Fitting the model to the washout data provided functional residual capacity (FRC_M), dead space volume (V_D), the coefficient of variation of regional specific ventilation ([Formula: see text]), and the model equivalent of S_acin (S_acin-M). Intra-participant coefficients of variation for the model parameters for both health and asthma were FRC_M < 5.2%, V_D < 5.4%, [Formula: see text] < 9.0%, and S_acin-M < 45.6% for controlled breathing, and FRC_M < 4.6%, V_D < 5.3%, [Formula: see text] < 13.2%, and S_acin-M < 103.2% for free breathing. The coefficients of variation limits for conventional parameters were FRC < 6.1%, with S_cond < 73.6% and S_acin < 49.2% for controlled breathing and S_cond < 35.0% and S_acin < 74.4% for free breathing. The model-fitting approach to multiple breath nitrogen washout analysis provides a measure of regional ventilation heterogeneity in [Formula: see text] that is less affected by irregularities in the breathing pattern than its corresponding Phase-III slope analysis parameter S_cond.

The effect of O2 and CO2 cross-sensitivity sensor error in the Eco Medics Exhalyzer D device on measures of conductive and acinar airway function

Introduction

The multiple breath nitrogen washout (MBNW) test provides important clinical information in obstructive airways diseases. Recently, a significant cross-sensitivity error in the O₂ and CO₂ sensors of a widely used commercial MBNW device (Exhalyzer D, Eco Medics AG, Duernten, Switzerland) was detected, which leads to overestimation of N₂ concentrations. Significant errors in functional residual capacity (FRC) and lung clearance index (LCI) have been reported in infants and children. This study investigated the impact in adults, and on additional important indices reflecting conductive (S_cond) and acinar (S_acin) ventilation heterogeneity, in health and disease.

Methods

Existing MBNW measurements of 27 healthy volunteers, 20 participants with asthma and 16 smokers were reanalysed using SPIROWARE V 3.3.1, which incorporates an error correction algorithm. Uncorrected and corrected indices were compared using paired t-tests and Bland–Altman plots.

Results

Correction of the sensor error significantly lowered FRC (mean difference 9%) and LCI (8–10%) across all three groups. S_cond was higher following correction (11%, 14% and 36% in health, asthma and smokers, respectively) with significant proportional bias. S_acin was significantly lower following correction in the asthma and smoker groups, but the effect was small (2–5%) and with no proportional bias.

Discussion

The O₂ and CO₂ cross-sensitivity sensor error significantly overestimated FRC and LCI in adults, consistent with data in infants and children. There was a high degree of underestimation of S_cond but minimal impact on S_acin. The presence of significant proportional bias indicates that previous studies will require reanalysis to confirm previous findings and to allow comparability with future studies.

O₂ and CO₂ cross-sensitivity sensor error in the Exhalyzer D device significantly overestimates FRC and LCI in adults, consistent with infants and children. Importantly, there was a high degree of underestimation of S_cond, but minimal impact on S_acin.https://bit.ly/3HcH3Tp

Controlled versus free breathing for multiple-breath nitrogen washout in asthma

Multiple-breath nitrogen washout (MBNW) is an emerging clinical test for assessing ventilation heterogeneity [1], often characteristically increased in asthma. MBNW indices both indicate and predict response to asthma treatment [2–4], and therefore may be an important tool for guiding treatment decisions [2]. Two established breathing protocols are currently in use: 1-L tidal volume (V_T) controlled breathing (CB) [5, 6] and unrestricted free breathing (FB) [7]. The CB protocol requires targeted V_T and respiratory rate, whereas the FB protocol encourages relaxed tidal breathing, making it more suitable for paediatrics [8]. Two recently published studies in healthy adults showed that indices of conductive and acinar ventilation heterogeneity (S_cond and S_acin, respectively) and, to a lesser extent, lung clearance index (LCI), were not comparable between breathing protocols [9, 10]. Importantly, differences between the protocols were dependent on the magnitude of ventilation heterogeneity. Thus, the assumption is that these effects would be amplified in disease, where ventilation heterogeneity is greater and clinical utility is most relevant. However, this has not been confirmed to date. We hypothesised that people with asthma, where ventilation heterogeneity is greater, would exhibit greater differences between the two protocols than the differences seen in healthy adults.

The lack of comparability in indices of ventilation heterogeneity between free- and controlled-breathing MBNW protocols is confirmed in asthmahttps://bit.ly/3lmri4A