Lung Ultrasonography May Provide an Indirect Estimation of Lung Porosity and Airspace Geometry

Background: Echographic vertical artifacts (B-lines) in chest\r\nultrasonography have often been associated with pathological\r\npatterns. A scientifically sound explanation of these\r\nartifacts has not yet been proposed. Objectives: The\r\n‘spongy’ nature of the lung in its liquid and solid components\r\nand the changes that take place in peripheral airspace\r\n(PAS) geometry might be the key point to understanding\r\nthese phenomena. Methods: Six excised right rabbit lungs\r\nwere obtained. Each lung underwent direct ultrasound\r\nevaluation in two different conditions: at complete tissue\r\nelastic recoil volume and at pulmonary expansion volume\r\nachieved by applying a constant positive pressure of 12 cm\r\nH 2 O. Lung volumes and densities were reported in both\r\nconditions. Histological examination was performed on\r\nthree naturally collapsed lungs and on three lungs under\r\npositive pressure inflation after having been fixed in forma-lin solution. Results: Mean volumes of naturally collapsed\r\nlungs and fixed expanded lungs were 11.2 ± 0.36 and 44.83\r\n± 3.03 ml, respectively. Mean densities were 0.622 ± 0.016\r\nand 0.155 ± 0.007 g/ml, respectively. Ultrasound evaluation\r\nof collapsed lungs showed dense vertical artifacts and a\r\n‘white lung’ pattern, while the evaluation of expanded lungs\r\nshowed hyperechoic line and horizontal artifacts of reflection.\r\nHistological evaluation showed a different PAS geometry\r\nin collapsed lungs caused by alveolar size reduction and\r\nshape changes with unfolded and closed units modifying\r\nthe peripheral porosity of the frothy nature of the lung. Conclusions:\r\nAirspace geometry, frothy nature and porosity are\r\nthe determinants of the different behavior of ultrasound interacting\r\nwith the subpleural lung parenchyma. Chest ultrasound\r\nmay thus be interpreted as an indirect ‘estimator’ of\r\nlung porosity.