REGIONAL MECHANICS OF INJURED LUNGS

There is a great deal of evidence from computerized tomography (CT) that the lungs of patients with acute respiratory distress syndrome (ARDS) are inhomogeneously affected. We use the parenchymal marker technique to measure the distribution of regional volume expansion in normal and oleic acid injured canine lungs and study the effects of positive end-expiratory pressure, frequency, and posture on these distributions. Our motivation is the absence of experimental data on the regional dynamics of injured lungs. Without such data, mechanical ventilation strategies that are to protect aerated lung from overdistension yet promote recruitment of flooded or collapsed units must be based on untested assumptions. CT technology, which has shaped many current hypotheses in the field, has a limited temporal and spatial resolution and cannot discern the displacements of specific structures in the lung from which to compute regional volume and deformation. Currently accepted models of regional mechanics in ARDS view dependent lung units as compressed by the weight of overlying edematous tissue. Our data suggest otherwise. The volume of dependent lung is increased, not reduced. Alveoli may be airless, but they are not collapsed; they are flooded and expanded by edema. This distinction is important because it implies a different mechanism of alveolar recruitment during mechanical ventilation.

Compare the dimensions of aerated alveoli with those of flooded and partially aerated alveoli in the images to the right. Note that the alveoli of edematous lungs are not collapsed, that they contain air pockets of different sizes and shapes (implying non-uniform pressure). Maintenance of non-uniform pressure requires that air be trapped by liquid and foam. We believe that the impedance offered by interfacial phenomena is a much more likely determinant of regional recruitment of injured lungs than is the so-called superimposed hydrostatic pressure.

Selected Publications

1. Martynowicz MA, Minor TA, Walters BJ, Hubmayr RD. Regional expansion of oleic acid injured lungs. Am J Respir Crit Care Med, 1999; 160:250-258
2. Martynowicz MA, Walters BJ, Hubmayr RD. Mechanisms of recruitment in oleic acid injured lungs. J Appl Physiol 90: 1744-1753, 2001
3. Wilson TA, Anafi R, Hubmayr RD. Mechanics of edematous lungs. J Appl Physiol 90: 2088-2093, 2001
4. Hubmayr RD, Perspective on lung injury and recruitment: A skeptical look at the opening and collapse story. Am J Respir Crit Care Med 165:1647-165, 2002  Download the .pdf version of this publication
5. De Chazal I, Hubmayr RD. Novel aspects of pulmonary mechanics in intensive care. British J. of Anesthesia. 2003; 91: 81-91  Download the .pdf version of this publication