Heterogeneous Airway Versus Tissue Mechanics and Their Relation to Gas Exchange Function During Mechanical Ventilation

Abstract

We have advanced a commercially available ventilator (NPB840, Puritan Bennett/Tyco Healthcare, Pleasanton, CA) to deliver an Enhanced Ventilation Waveform (EVW). This EVW delivers a broadband waveform that contains discrete frequencies blended to provide a tidal breath, followed by passive exhalation. The EVW allows breath-by-breath estimates of frequency dependence of lung and total respiratory resistance (R) and elastance (E) from 0.2 to 8 Hz. We hypothesized that the EVW approach could provide continuous ventilation simultaneously with an advanced evaluation of mechanical heterogeneities under heterogeneous airway and tissue disease conditions. We applied the EVW in five sheep before and after a bronchial challenge and an oleic acid (OA) acute lung injury model. In all sheep, the EVW maintained gas exchange during and after bronchoconstriction, as well as during OA injury. Data revealed a range of disease conditions from mild to severe with heterogeneities and airway closures. Correlations were found between the arterial partial pressure of oxygen (PaO2) and the levels and frequency-dependent features of R and E that are indicative of mechanical heterogeneity and tissue disease. Lumped parameter models provided additional insight on heterogeneous airway and tissue disease. In summary, information obtained from EVW analysis can provide enhanced guidance on the efficiency of ventilator settings and on patient status during mechanical ventilation.

Dynamic Instability of Central Airways and Peripheral Airspace in Rat Lungs Perfused with Cold Preservation Solutions

“…4b, f), and both E low and E het became higher during cold storage. These findings suggest that peripheral capacitive heterogeneity after cold storage for 9 h resulted from peripheral airspace closure and asymmetrical suspended areas of the bronchial tree with upper airway shunts [16,17,18] induced by the fluid in the distal airspaces and/or loss of surfactant function. Furthermore, the hyperosmotic characteristic of EC might have caused the increasing elasticity which resulted from extracellular matrix and/or its constitutional cell swelling after cold storage.…”

“…Reciprocally, when reinflating collapsed regions of the lung, the inflation process conforms to an avalanche-like power law behavior [29], which results in heterogeneous aeration. Bellardine et al [16,30] suggested that once the peripheral heterogeneity in lungs reaches a critical level, arterial oxygen tension rapidly decreases. Thus, keeping peripheral airways and alveoli homogeneous plays a key role in oxygenation and prevention of VILI.…”

“…Other indexes were the absolute difference between E low and E max (E het ), and the absolute difference between R low and R high (R het ). These low-frequency features should be sensitive to tissue stiffness, the amount of recruitment, those heterogeneities (R L <2 Hz and E het ) and airway closure (E low ) [16,18], whereas the high-frequency features should be sensitive to the mean constriction level (e.g. R L at a high frequency range represents mean airway resistance) and airway wall shunting (E at high frequency range).…”

“…Furthermore, it was shown that the impedance frequency characteristic reflected the heterogeneity in the respiratory system [16,17]. …”

Reliability of Estimating Stochastic Lung Tissue Heterogeneity from Pulmonary Impedance Spectra: A Forward-Inverse Modeling Study

“…16,17,27,41,52 Heterogeneous conditions that may coexist in different regions of the same lung during positive pressure ventilation, such as intra-tidal alveolar overdistention and cyclic opening and closing of lung units, result in the release of cytokines and other inflammatory mediators that can worsen existing lung pathology. 4,5,33,52 Since the degree of mechanical heterogeneity may depend on lung volume, 27 it has recently been proposed that ventilating over pressure ranges that minimize such heterogeneity may result in better outcomes for patients, particularly those with acute lung injury (ALI) or the acute respiratory distress syndrome (ARDS).…”