Abstract
BackgroundThis work presents a generalized technique to estimate pulmonary ventilation-to-volume (v/V) distributions using the multiple-breath nitrogen washout, in which both tidal volume (VT) and the end-expiratory lung volume (EELV) are allowed to vary during the maneuver. In addition, the volume of the series dead space (vd), unlike the classical model, is considered a common series unit connected to a set of parallel alveolar units.MethodsThe numerical solution for simulated data, either error-free or with the N2 measurement contaminated with the addition of Gaussian random noise of 3 or 5 % standard deviation was tested under several conditions in a computational model constituted by 50 alveolar units with unimodal and bimodal distributions of v/V. Non-negative least squares regression with Tikhonov regularization was employed for parameter retrieval. The solution was obtained with either unconstrained or constrained (VT, EELV and vd) conditions. The Tikhonov gain was fixed or estimated and a weighting matrix (WM) was considered. The quality of estimation was evaluated by the sum of the squared errors (SSE) (between reference and recovered distributions) and by the deviations of the first three moments calculated for both distributions. Additionally, a shape classification method was tested to identify the solution as unimodal or bimodal, by counting the number of shape agreements after 1000 repetitions.ResultsThe accuracy of the results showed a high dependence on the noise amplitude. The best algorithm for SSE and moments included the constrained and the WM solvers, whereas shape agreement improved without WM, resulting in 97.2 % for unimodal and 90.0 % for bimodal distributions in the highest noise condition.ConclusionsIn conclusion this generalized method was able to identify v/V distributions from a lung model with a common series dead space even with variable VT. Although limitations remain in presence of experimental noise, appropriate combination of processing steps were also found to reduce estimation errors.Electronic supplementary materialThe online version of this article (doi:10.1186/s12938-016-0213-y) contains supplementary material, which is available to authorized users.
Highlights
This work presents a generalized technique to estimate pulmonary ventilation-to-volume (v/V) distributions using the multiple-breath nitrogen washout, in which both tidal volume (VT) and the end-expiratory lung volume (EELV) are allowed to vary during the maneuver
Note that for the most general case, the FNet2 can increase along the washout when VTE > VTI
The best result was achieved with fixed λ, and with alveolar unit weight
Summary
This work presents a generalized technique to estimate pulmonary ventilation-to-volume (v/V) distributions using the multiple-breath nitrogen washout, in which both tidal volume (VT) and the end-expiratory lung volume (EELV) are allowed to vary during the maneuver. The multiple-breath nitrogen washout (MBN2W) and the single-breath washout are classical pulmonary function tests, based on the measurement of the concentration or fraction of N2 in the breathing gases, to evaluate the ventilation inhomogeneity. Among measures and indices obtainable from the MBN2W, one of the most useful is the functional residual capacity (FRC) or end-expiratory lung volume (EELV), meaning the relaxed lung functional volume comprising all ventilated alveolar units plus the series dead space. Several indices developed to evaluate the ventilation inhomogeneity are derived from the MBN2W, such as the lung clearance index [2] and the multiple-breath alveolar mixing inefficiency [3]. Despite being relatively benign and non-invasive, the MBN2W needed, when it was being experimentally explored, costly, bulky machinery such as respiratory mass spectrometers (RMS); it could not map anatomy nor the spatial distribution of the quantities it measured, and it saw a decline of interest as a technique for general, day-to-day clinical application
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