Analysis of the method for ventilation heterogeneity assessment using the Otis model and forced oscillations

Assessment of Heterogeneity in Lung Structure and Function During Mechanical Ventilation: A Review of Methodologies

The mammalian lung is characterized by heterogeneity in both its structure and function, by incorporating an asymmetric branching airway tree optimized for maintenance of efficient ventilation, perfusion, and gas exchange. Despite potential benefits of naturally occurring heterogeneity in the lungs, there may also be detrimental effects arising from pathologic processes, which may result in deficiencies in gas transport and exchange. Regardless of etiology, pathologic heterogeneity results in the maldistribution of regional ventilation and perfusion, impairments in gas exchange, and increased work of breathing. In extreme situations, heterogeneity may result in respiratory failure, necessitating support with a mechanical ventilator. This review will present a summary of measurement techniques for assessing and quantifying heterogeneity in respiratory system structure and function during mechanical ventilation. These methods have been grouped according to four broad categories: (1) inverse modeling of heterogeneous mechanical function; (2) capnography and washout techniques to measure heterogeneity of gas transport; (3) measurements of heterogeneous deformation on the surface of the lung; and finally (4) imaging techniques used to observe spatially-distributed ventilation or regional deformation. Each technique varies with regard to spatial and temporal resolution, degrees of invasiveness, risks posed to patients, as well as suitability for clinical implementation. Nonetheless, each technique provides a unique perspective on the manifestations and consequences of mechanical heterogeneity in the diseased lung.

Quantitative Assessment of the Airway Response to Bronchial Tests Based on a Spirometric Curve Shift

OBJECTIVE

Although spirometry is the most common pulmonary function test, there is no method to quantitatively infer about airway resistance or other properties from the flow-volume curves. Recently, an identifiable inverse model for forced expiration was proposed, as well as the idea to deduce changes in airway resistances and compliances from spirometric curve evolution. The aim of this work was to combine the above advances in a method for assessing the airway response to bronchial tests from a spirometric curve shift.

METHODS

The approach is based on the differential measurement of the degree, site of maximal effect and width of changes, further recalculated into relative changes in the distribution of airway resistances (δR_g) and compliances (δC_g) along the bronchial tree. To this end, appropriate models were identified using the pre- and post-test spirometry data. The accuracy was validated using sets of data simulated by the anatomy and physiology based models. Finally, the method was used to analyze the bronchodilation tests of three asthmatic subjects.

RESULTS

The expected errors in assessing the degree, site and width of changes in the zone of conducting airways were 6.3%, 2.4 generations and 22%, respectively, and for δR_g and δC_g were 5-10% and 13-16%, respectively. The analyses of clinical data indicated a significant reduction in resistances and an increase in compliances of airway generations 8-12, consistent with clinical knowledge.

CONCLUSION

An unprecedented method to plausibly transforming the spirometry data into the site and degree of changes in airway properties has been proposed.

SIGNIFICANCE

The method can be used to deduce about the effects of bronchial tests, as well as to monitor changes in the airways between visits or to investigate how inhaled pharmaceuticals affect the bronchi.

Effects of homogeneous and heterogeneous changes in the lung periphery on spirometry results

BACKGROUND AND OBJECTIVES

The most widespread chronic pulmonary disorders are associated with heterogeneous changes in the lung periphery and spirometry is the most commonly used test to monitor these diseases. So far only a few attempts have been undertaken to investigate the effects of lung inhomogeneity on spirometry results. The aim of this work was to evaluate whether the spirometric curve and indexes are sensitive to parallel peripheral inhomogeneities, and if the level of heterogeneity can be deduced from this test.

METHODS

To this end, an enhanced computational model for forced expiration, taking into account a heterogeneous structure and properties of the respiratory system, was used. Two main phenomena were mimicked: small airways narrowing and the loss of tissue elastic recoil. Numerical simulations were performed with the model having 76 separate peripheral compartments. For a given degree of mean change, three heterogeneity levels were investigated and compared to the effects of homogeneous alterations.

RESULTS

All spirometric curves representing different patterns of inhomogeneous constriction, computed for each of the investigated cases, almost coincided with the curve originating from homogeneous changes, regardless of the heterogeneity level. Also the differences between the spirometric indexes obtained for heterogeneous and homogeneous alterations were negligible in comparison to their values.

CONCLUSION

The main finding is that the spirometry results are insensitive to the level of heterogeneity in the lung periphery and that it is practically impossible to distinguish between the homogeneous or heterogeneous nature of pathological processes occurring in this lung region.