Since Pasteur's1 original study of this phenomenon as a postdiphtheritic and postoperative complication, this subject has been receiving increasing attention. Numerous case reports and reviews have appeared containing many observations on the etiology, incidence and mechanism of this phenomenon. Its probable mechanism has particularly interested the various authors, and various theories have been advanced to explain it. I shall report five additional cases of massive pulmonary atelectasis. One of these was studied in considerable detail over a period of years, and presents some unique features not previously reported. All the cases illustrate the various factors which are responsible for this phenomenon, suggesting its probable mechanism, and emphasizing its clinical significance.
DEFINITION AND CLASSIFICATION
By massive collapse, atelectasis or apneumatosis is understood the deflation of large portions of lung tissue to a retracted airless state. While these terms are trequently used interchangeably, Scott, Pancoast, Jackson2 and others prefer the term atelectasis