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The question of how best to deal with alveolar instability in ARDS has remained unsettled for more than 50 years.1 Potential advantages of increasing ventilating capacity include improved efficiency of gas exchange, better tidal compliance, avoidance of tidal reversals of atelectasis (atelectrauma), and less concentration of global mechanical power within the “baby” lung.2 Unfortunately, although the addition of normally functioning lung units is clearly desirable, attempts to optimize recruitment may also impose hazards. The study by Lhermitte, Daubin, and colleagues3 in this issue of Respiratory Care brings to light several difficulties of striking an appropriate balance between gaining the potential benefits and minimizing the hazards associated with lung unit recruitment. To fully understand its value, it may be useful to first address the relevant physiologic background.
Sufficient transpulmonary pressure, the difference between the air space and pleural pressures, is needed first to reopen and then to maintain patency of unstable alveoli. The regionally varied flexibility of the lung’s chest-wall enclosure and local compression by overlying edematous lung increase the airway pressures (Paw) needed to accomplish and maintain alveolar inflation in dependent zones.4 The single pressure measured in the ventilator circuit, therefore, is associated with multiple local states of inflation and collapse. Consequently, elevating Paw to seek recruitment tends simultaneously to overdistend some lung units while leaving others collapsed. The prone position improves the uniformity of alveolar dimensions, reducing the disparity of such responses.5
Units of ARDS that remain open exist alongside those with collapse and consolidation, and these are also vulnerable to elevations of transpulmonary pressure. The proportions of impaired and normal alveoli, as well as the pressures necessary to accomplish recruitment of eligible units, vary with disease stage and local chest-wall flexibility or stiffness. Fortunately, the pressure needed to sustain …
Correspondence: John J Marini MD, Pulmonary and Critical Care Medicine, University of Minnesota, Minneapolis/St Paul, Minnesota. E-mail: marin002{at}umn.edu
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