DYNAMICS OF PULMONARY CIRCULATION 



1689 



in various ways, including direct measurements from 

 gas pockets and balloons within the pleural or 

 mediastinal spaces (82) and indirect estimates from 

 the esophagus (287). It is generally conceded that 



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fig. 20. Effect of occluding the right pulmonary artery on 

 blood pressures distal and proximal to the occlusive balloon. 

 Before occlusion, blood pressures are identical in the main and 

 right pulmonary arteries. After occlusion, the distal pressure 

 falls to the level of pulmonary wedge pressures (left atrial 

 pressure), pressure in the main pulmonary artery proximal to 

 the balloon increases by approximately 5 mm Hg. ( Unpub- 

 lished observations of M. Brandfonbrenner, A. Himmelstein, 

 G. M. Turino, and A. P. Fishman.) 



even the direct methods may fail to provide precise 

 measurements of the pressures which are operating 

 at the surface of the particular pulmonary vessels 

 under consideration: the pressure within the pleura 

 may not be entirely uniform (82, 127); the extramural 

 pressures along the length of the vascular tree may 

 differ from segment to segment and from the pleural 

 pressure, depending on the location of the segment, 

 i.e., intrapericardial, intrapulmonary, or juxta- 

 alveolar. The use of indirect measures, which provides 

 reliable measures of pleural pressures in some experi- 

 mental and clinical conditions, fails in others (287). 



Transmural \ersus Luminal Pressures 



During each respiratory cycle, the changing pleural 

 pressures (fig. 22) affect all intrathoracic vessels except 

 those apposed to alveoli. Consequently, for the 

 alveolar capillaries, the pressure which determines 

 their caliber, i.e., transmural pressure, is customarily 

 calculated as the difference between (estimated) 

 intracapillary and alveolar pressure; the transmural 

 pressure of all other vessels is calculated as the differ- 

 ence between the luminal and the pleural pressure 

 (fig. 23) (61, 233). The practical difficulties in esti- 

 mating perivascular pressure from pleural pressure 

 have been indicated above; pericapillary pressures 

 also have an element of uncertainty because of the 

 prospect that tissue forces, such as alveolar surface 

 tension, may decrease pericapillary pressure to sub- 

 atmospheric levels. 



Depending on the purpose of the observation, 

 pulmonary vascular pressures are referred either to 

 atmospheric or to pleural pressure. Considerable 



fic. 21. Differential pressure record of a "cough."' The lowest tracing is from a mouthpiece into 

 which a forcible expiration was made. The middle record is that of luminal systemic arterial pressure. 

 The upper record is a differential record of the middle minus the lower record. The mouthpiece 

 record is assumed to show pressure changes nearly identical with intrathoracic pressure changes; 

 the differential record indicates the stresses which the intrathoracic arteries undergo. [After Hamilton 

 etal. (190).] 



