DYNAMICS OF PULMONARY CIRCULATION 



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confined to dogs fitted with angiostomy cannulae 

 (150, 225); in recent years, these pressures have 

 been measured in both intact animal and man by 

 every conceivable route: right heart catheterization 

 in patients with congenital atrial defects, direct 

 cardiac puncture, transbronchial puncture, trans- 

 thoracic puncture, and intracardiac transseptal 

 puncture (45, 299). 



Pulmonary Arteriovenous Pressure Gradient 



In man, cat, and dog, the pressure drop across the 

 pulmonary vascular bed is of the order of one-tenth 

 of the pressure drop across the systemic circulation. 

 The pulmonary arterial-left atrial pressure gradient 

 is maximal early in systole (fig. 18); it decreases late 

 in systole and may even approach zero if diastole is 

 sufficiently prolonged (182). Unfortunately, since 

 both the pulmonary arterial and pulmonary venous 

 pressure pulses have different origins (right and left 

 sides of the heart, respectively), it is not possible to 

 predict the shape of the pressure pulses of the inter- 

 vening vascular bed from the pulmonary arterial- 

 pulmonary venous pressure gradient. 



Pulmonary Wedge Pressures 



The pulmonary arterial wedge pressure is recorded 

 by advancing a cardiac catheter until its tip occludes 

 a terminal branch of the pulmonary artery; flow then 



fig. 18. The pulmonary vascular pressure gradient. Upper 

 curve: record of the pulmonary arterial pressure pulse of an un- 

 anesthetized, unoperated dog; blood pressure 35/12 mm Hg, 

 mean 20 mm Hg. Lower curve : record of the pulmonary venous 

 pressure pulse; blood pressure 2 to 12 mm Hg. Middle curve: 

 differential manometer record of pulmonary arterial minus 

 pulmonary venous pressure, i.e., the gradient of pressure driving 

 blood through the pulmonary vascular system. [After Hamilton 

 (182).] 



stops in the vascular segment beyond the tip of the 

 catheter: the pressure transmitted by the intervening 

 static column of blood presumably approximates 

 closely the pressure in the first communicating 

 pulmonary veins in which flow persists. Pulmonary 

 venous wedge pressure is recorded by impacting a 

 catheter (passed retrograde) in a pulmonary vein. 



Originally (200), it was believed that the wedged 

 pulmonary arterial pressure could be used as a meas- 

 ure of pressure in the pulmonary capillary bed. It is 

 now clear that the wedged arterial catheter registers 

 more remote events, i.e., events in the large pulmonary 

 veins and, unless the "throttles" actually operate, in 

 the left atrium (83). In both dog and man — with 

 normal pulmonary circulation or with pulmonary 

 venous congestion — the mean pulmonary arterial 

 wedge pressure and the mean left atrial pressure are 

 nearly identical (83). In the normal animal and man, 

 the level of the arterial wedge pressure is of the order 

 of 5 to 9 mm Hg (103); in patients with pulmonary 

 venous congestion from mitral stenosis, it parallels the 

 left atrial and pulmonary venous pressure. 



The validity and meaning of the arterial wedge 

 pressure have been the subjects of considerable debate 

 (26). Various criteria have been adopted for deciding 

 if a wedge pressure is a reliable measure of the level of 

 the left atrial pressure; these include higher pulmonary 

 arterial mean and diastolic pressures than the re- 

 corded wedge pressure, the withdrawal of fully oxy- 

 genated blood from the impacted catheter, the snap of 

 the catheter as it is withdrawn from the wedge posi- 

 tion and a characteristic configuration of the wedge 

 tracing (83, 103). No single one of these criteria en- 

 sures a reliable measure of left atrial pressure, 

 particularly when pressure is changing rapidly (26). 

 Indeed, even when all criteria are met, the left atrial 

 pressure may be poorly transmitted due to a faulty 

 wedge position of the catheter (fig. 19) (22). 



The use of the arterial wedge pressure as a measure 

 of the level of left atrial pressure is on sounder footing 

 than its use to record the contour of the left atrial 

 pressure pulse. Only in states of pulmonary venous 

 congestion is the wedge catheter apt to reproduce 

 cyclic events in the left atrium (83, 113). Interpreta- 

 tion of changes in contour is particularly troublesome 

 when artifacts are present; these artifacts tend to be 

 most pronounced during exercise and deep breathing. 



Blood pressure falls in the pulmonary artery distal 

 to an occlusive balloon and assumes the nondescript 

 character of a wedge pressure (fig. 20) (49, 42). The 

 level of this distal pulmonary arterial pressure 

 corresponds to that in the left atrium and fluctuates 



