TEMPORAL BEHAVIOR OF LUNG AERATION IN PORCINE ARDS AS ANALYZED BY DYNAMIC MULTISCAN COMPUTERIZED TOMOGRAPHY (CT)
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Introduction: In the absence of lung disease, respiratory mechanics of ventilated patients may be described by a one-compartment model of the lung. In ARDS, perfused alveoli are to be recruited, as well as ventilated, in order to ensure adequate gas exchange. Lung aeration can be analyzed using fast dynamic multiscan CT, by following radiologic lung density during respiratory cycles. We studied the temporal behavior of radiologic lung density in- and deflation maneuvers, with the aim to identify the impact of ARDS upon dynamics of lung aeration. Methods: With IRB approval, five anesthetized pigs (27 +/- 1 kg) were imaged by dynamic multiscan CT (slice thickness = 1 mm, high resolution reconstruction, temporal resolution = 250ms). In one predefined transverse cross section of the lungs, fast repetitive imaging was performed before and after induction of saline lavage ARDS. Rectangular step-up and step-down maneuvers in airway pressures between 0 and 50 cm H2 O were performed during dynamic CT acquisitions (196 images). On consecutive images, the area of a density range of -910 to -300 Hounsfield Units was defined as aerated lung parenchyma, and determined planimetrically. The response of aerated lung area (A, % of total cross-sectional lung area at 50 cm H2 O) to pressure steps was plotted against time. Least-square fit procedures were used to describe two distinct time constants ("fast" or "slow", Tcfast, Tcslow) and their relative contribution to A (Afast, Aslow). Results: Prior to ARDS induction, a "fast" lung compartment dominated both inflation and also, although less so, deflation. In contrast, after establishment of lavage ARDS, a significantly larger portion of the lung responded to inflation as "slow compartment", and to deflation as "fast compartment (Table 1).Table 1Discussion: Dynamic multiscan CT allows to determine lung compartments whose aeration follows very distinct time constants prior to and after ARDS induction. The single, short Tc of healthy lungs may characterize ventilation of already aerated lung regions. In ARDS, the coexisting longer inspiratory Tc may represent alveolar recruitment, whereas, during expiration, a large compartment with short Tc reflects rapid alveolar collapse. Conventional ventilatory modes may not be able to recruit, and prevent collapse of, alveolar regions with this behavior. Thus, the detection of compartments with these characteristics could provide the rationale for the High Frequency Oscillatory Ventilation in severe ARDS. Funding: Deutsche Forschungsgemeinschaft (DFG TH 315/9)Keywords:
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High intensity focused ultrasound (HIFU) has gained clinical interest as a non-invasive local tumour therapy in many organs. In addition, it has been shown that lung cancer can be targeted by HIFU using One-Lung Flooding (OLF). OLF generates a gas free saline-lung compound in one lung wing and therefore acoustic access to central lung tumours. It can be assumed that lung parenchyma is exposed to ultrasound intensities in the pre-focal path and in cases of misguiding. If so, cavitation might be induced in the saline fraction of flooded lung and cause tissue damage. Therefore this study was aimed to determine the thresholds of HIFU induced cavitation and tissue erosion in flooded lung. Resected human lung lobes were flooded ex-vivo. HIFU (1,1 MHz) was targeted under sonographic guidance into flooded lung parenchyma. Cavitation events were counted using subharmonic passive cavitation detection (PCD). B-Mode imaging was used to detect cavitation and erosion sonographically. Tissue samples out of the focal zone were analysed histologically. In flooded lung, a PCD and a sonographic cavitation detection threshold of 625 Wcm − 2(p r = 4, 3 MPa) and 3.600 Wcm − 2(p r = 8, 3 MPa) was found. Cavitation in flooded lung appears as blurred hyperechoic focal region, which enhances echogenity with insonation time. Lung parenchyma erosion was detected at intensities above 7.200 Wcm − 2(p r = 10, 9 MPa). Cavitation occurs in flooded lung parenchyma, which can be detected passively and by B-Mode imaging. Focal intensities required for lung tumour ablation are below levels where erosive events occur. Therefore focal cavitation events can be monitored and potential risk from tissue erosion in flooded lung avoided.
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If two airways are sufficiently close, the parenchymal distortion forces between them may interfere with their abilities to narrow. This mechanism may act to homogenize airway narrowing throughout the lung. We investigated this in a 2-dimensional computational model of a pair of airways embedded in parenchyma represented as a linear elastic continuum (Fig. 1a). Airway contraction was achieved by imposing an inward radial force on the hole boundaries and calculated using the finite element method. We determined how airway lumen area was affected as a function of the separation distance between their centers when only one airway contracted versus when both airways contracted equally. We found that airway contraction was identical under both conditions until the two airways came within about 2 uncontracted diameters of each other (Fig. 1b) at which point a contracting airway narrowed less if its companion airway also narrowed. We also found that when airways were close they narrowed more than when they were far apart, no doubt due to the reduced parenchymal interdependence forces caused by the hole representing a nearby companion. These model results suggest that airway-parenchymal interdependence will only significantly affect the heterogeneity of airway narrowing if the airways are, on average, within about 2 diameters of each other, provided that the parenchyma behaves like an elastic continuum.
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The forces of mechanical interdependence between the airways and the parenchyma in the lung are powerful modulators of airways responsiveness. Little is known, however, about the extent to which adjacent airways affect each other's ability to narrow due to distortional forces generated within the intervening parenchyma. We developed a two-dimensional computational model of two airways embedded in parenchyma. The parenchyma itself was modeled in three ways: 1) as a network of hexagonally arranged springs, 2) as a network of triangularly arranged springs, and 3) as an elastic continuum. In all cases, we determined how the narrowing of one airway was affected when the other airway was relaxed vs. when it narrowed to the same extent as the first airway. For the continuum and triangular network models, interactions between airways were negligible unless the airways lay within about two relaxed diameters of each other, but even at this distance the interactions were small. By contrast, the hexagonal spring network model predicted that airway-airway interactions mediated by the parenchyma can be substantial for any degree of airway separation at intermediate values of airway contraction forces. Evidence to date suggests that the parenchyma may be better represented by the continuum model, which suggests that the parenchyma does not mediate significant interactions between narrowing airways.
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Postobstructive pulmonary vasculopathy (POPV) produced by chronic unilateral ligation of one pulmonary artery, results in perfusion of the pulmonary capillaries with systemic arterial blood. As a consequence, gas exchange occurs primarily in the contralateral nonligated lung. To determine whether the mechanical properties of the lung parenchyma are changed in POPV, we compared five dogs with chronic ligation of the left main pulmonary artery with five control dogs. Separate measurements of left and right lung airway flows, tracheal pressures, and alveolar pressures were made during mechanical ventilation at frequencies between 5 and 40 breaths/min. We calculated pulmonary elastance (EL) and pulmonary (RL), airway (Raw), and tissue (Rti) resistances. At all frequencies, dogs with POPV had higher left (ligated) EL and Rti and lower right (normal) lung Rti but similar EL compared with the respective lungs from control animals. Raw was the same in both lungs. Histology showed visceral pleura thickening and encroachment of new bronchial collaterals and lymphatics on the parenchyma of the ligated lungs. The contralateral lungs were entirely normal. We conclude that in POPV 1) there is an increase, in the ligated lung, of both EL and RL, the latter likely due to histological changes of the lung parenchyma, and 2) there is a reduction of Rti in the contralateral lung.
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