340 Pulmonic Interstitial Emphysema 



often been overlooked. Pneumoprecordium has been diagnosed occasionally, 

 but PIE as its source has not often been suspected. This condition is not in- 

 frequently called pneumopericardium, we think, erroneously, for the most part. 

 When the air has worked its way forward over the heart, a "crunching" sound, 

 audible several feet away from the patient, and synchronous with the heart 

 beat, may be heard.*' ^'^^ This is pathognomonic of air in the anterior medi- 

 astinum, but absence of this sign does not rule out the presence of air in other 

 parts of the mediastinum. 



Movement of Air Bubbles toward the Mediastinum 



Once rupture openings have been formed, the tendency is for leakage of air 

 into the interstitial tissue to continue until conditions arise to prevent it. The 

 air does not accumulate near the vascular-sheath invasion sites but tends to 

 move toward the mediastinum, in which it may develop a pressure gieater 

 than that of the atmosphere. The ordinary plus pressure of expiration woidd 

 seem insufficient to account for the impulsion of the bubbles against such 

 opposition, and elevation of the presstire head in the leaking alveoli, as by 

 coughing and straining with the glottis closed, must be postulated in explana- 

 tion. Newly entered bubbles push on those already in the connective tissue. 



Another factor in moving the bubbles along woidd seem to be a sort of 

 "pumping" or "milking" action on the part of the broncho-vascular rays. One 

 of the most interesting features of the functional anatomy of the lung is the 

 way all structures radiating out from the hilum, such as bronchi and blood 

 vessels, elongate in inspiration and shorten in expiration."'" Without this ac- 

 commodational length change, inflation and deflation of the lung would be 

 impossible.^' ""^ Yet this perpetual lengthening and shortening, particularly 

 of the pulmonary arteries and veins, is not an unmixed blessing when once 

 air has entered the sheaths, for it seems to have the effect of moving along the 

 train of air bubbles toward the lung root until stopped by the mass of packed 

 bubbles, and thus of augmenting the same propensity arising from their being 

 pushed from behind by new bubbles leaking in under the raised pressure head. 

 The stream does not seem to become easily dammed up; not, perhaps, at times, 

 until the pressure in the mediastinum rises to a prohibitive degree, in cases 

 where escape from that space is not readily provided. The path of least re- 

 sistance is toward the root. The way is thus constantly open for more air to 

 enter from the leakage points. The air bubbles increase in size through co- 

 alescence and the degree of their pressure damage increases as they move along, 

 for they constantly impinge on larger and larger trunks. Here, again, a knowl- 

 edge of the functional anatomy of the lung aids in the understanding of PIE. 



AlRBLOCK AND ItS EFFECTS 



Airblock is primarily a malady of the chest, but since the aberrant air may 

 extend through the superior and inferior thoracic apertures, a field beyond 

 the mediastinum may develop in addition to the ptilmonic and mediastinal 



