Many patients with acute respiratory failure suffer recurrent pulmonary air leaks. Loculation of pleural air may occur as a result of pleural inflammation, bleeding, or the presence of intercostal catheters. The lung may be tethered at many points, and localization of the pneumothorax, either clinically or with anteroposterior radiography, may not prove possible. In addition, pneumothorax distribution in supine patients may be atypical; anteromedial and subpulmonic collections were most common in one study (Tocino eL§L 1985).
Patients with clinically significant pneumothoraces may already have several functioning intercostal catheters, and the site of new collections may be unclear. Lateral shoot-through decubitus films may be very helpful in guiding therapy. The improved definition of computerized phosphor-plate radiology has much increased the yield from lateral films in adults and even in obese patients. Radiographs from one such patient with acute respiratory distress syndrome ( Fig 2) show the anteroposterior radiograph, which suggested an anterior subpulmonary collection, to be misleading. The lateral film confirms a large posterior left-sided collection in an area which was previously densely consolidated.
Fig. 2 The anteroposterior radiograph (a) suggests an anterior subpulmonary collection, whereas the lateral radiograph (b) shows a large left posterior collection.
High-definition CT scanning is the definitive imaging tool. Significant pneumothoraces may be completely missed on anteroposterior films. Fig 3(a) shows a significant posterior collection which was not identified on the anteroposterior radiograph ( Fig 3(b)). The risk versus the benefit of transfer to the CT scanner is controversial, but in our experience the potential benefit in patients with refractory respiratory failure outweighs the logistic difficulties in safely transporting such patients.
Fig. 3 Important posterior collection not identified on the anteroposterior film.
Another unusual problem is the large pneumothorax not associated with ongoing air leaks. Figure..4(a) shows the CT scan of a patient with acute respiratory distress syndrome who, despite an excellent oxygenation response to nitric oxide therapy, exhibited deterioration of anteroposterior radiograph. A patent chest drain is seen in the left side. Direct pressure measurements of anterior air collections indicated no tension. An overenthusiastic reduction in mean airway pressure in response to selective vasodilator therapy had resulted in a loss of lung volume over several days. Aggressive lung volume maintenance was reinstituted, and Fig 4(b) shows re-expansion, without further chest drains, 6 days later. In this case, lung volume expansion, rather than further pleural drainage, represented definitive treatment.
Fig. 4 CT scan of a patient with acute respiratory distress syndrome in whom rapid reduction in mean airway pressure resulted in loss of lung volume. Re-expansion was obtained by increasing inflation pressure, not by further pleural drainage.
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