Assessing the calculation of conductive and acinar ventilatory heterogeneity indices S cond and S acin from multiple-breath washout data.

Sensor errors resulting in elevated values of N 2 concentration [N 2 ] 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 [N 2 ], 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 N 2 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 .