Abstract
BackgroundThe multiple-breath washout (MBW) is able to provide information about the distribution of ventilation-to-volume (v/V) ratios in the lungs. However, the classical, all-parallel model may return skewed results due to the mixing effect of a common dead space. The aim of this work is to examine whether a novel mathematical model and algorithm is able to estimate v/V of a physical model, and to compare its results with those of the classical model. The novel model takes into account a dead space in series with the parallel ventilated compartments, allows for variable tidal volume (VT) and end-expiratory lung volume (EELV), and does not require a ideal step change of the inert gas concentration.MethodsTwo physical models with preset v/V units and a common series dead space (vd) were built and mechanically ventilated. The models underwent MBW with N2 as inert gas, throughout which flow and N2 concentration signals were acquired. Distribution of v/V was estimated—via nonnegative least squares, with Tikhonov regularization—with the classical, all-parallel model (with and without correction for non-ideal inspiratory N2 step) and with the new, generalized model including breath-by-breath vd estimates given by the Fowler method (with and without constrained VT and EELV).ResultsThe v/V distributions estimated with constrained EELV and VT by the generalized model were practically coincident with the actual v/V distribution for both physical models. The v/V distributions calculated with the classical model were shifted leftwards and broader as compared to the reference.ConclusionsThe proposed model and algorithm provided better estimates of v/V than the classical model, particularly with constrained VT and EELV.
Highlights
The multiple-breath washout (MBW) is able to provide information about the distribution of ventilation-to-volume (v/V) ratios in the lungs
Motta‐Ribeiro et al BioMed Eng OnLine (2018) 17:3 lungs as a set of all-parallel units, including a dead space, whose contributions to the total lung ventilation are the unknowns. This approach has some limitations. It disregards the effects of the series dead space (vd), whose volume may be estimated via the Fowler’s method [4] throughout the washout; the end-expiratory lung volume (EELV) and the tidal volume (VT) must remain constant during the multiple-breath nitrogen washout (MBN2W); the inspired fraction of tracer gas should decrease instantaneously to zero
VolJ, is an ideal mixer characterized by the fraction γ of V T that enters and leaves it at each cycle, and its specific ventilation (S(J) = γVT/VolJ), the sum of all compartmental volumes being equal to EELV-vd
Summary
The multiple-breath washout (MBW) is able to provide information about the distribution of ventilation-to-volume (v/V) ratios in the lungs. The novel model takes into account a dead space in series with the par‐ allel ventilated compartments, allows for variable tidal volume (VT) and end-expiratory lung volume (EELV), and does not require a ideal step change of the inert gas concentration. The alternative to impose a priori constraints determined along the M BN2W limits the set of the v/V parameters estimates Since this same novel model drove the simulated MBN2W, the results could have favored the algorithm in some form. It is arguable, that bench tests with well-known physical models would allow for a better, less biased assessment of the effects of modelling the series dead space in the estimates of v/V distributions
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