1 688 HANDBOOK OF PHYSIOLOGY ■*■ CIRCULATION II 





fig. 19. Various positions of the "wedged" catheter redrawn from pulmonary wedge arterio- 

 grams. A: the catheter is wedged in an artery which is slightly smaller than the catheter tip; the 

 lumen of the artery is in direct line with the lumen of the catheter. B: the catheter is wedged at a 

 bifurcation of an artery of the same size as the catheter tip. C: the tip of the catheter impinges against 

 the wall of a sharply angulated artery. D: The catheter is wedged at a point where the artery divides 

 into three or more branches. E: the catheter is incompletely wedged. The injected dye regurgitates 

 around the catheter outlining the artery proximal to the catheter tip. Positions A, B, and D are 

 favorable for recording wedge pressure; positions C and E are not. [After Bell el al. (22).] 



with changes in the left atrial pressure. The tracing 

 shows no left atrial or pulmonary venous events but 

 does display respiratory swings. 



Pulmonary venous wedge pressures have also been 

 recorded in the dog (435), in normal human subjects 

 (84, 248), and in patients with atrial septal defects. In 

 the normal dog the pulmonary venous wedge pressure 

 approximates mean pressure in the pulmonary artery 

 (435) '■> m patients, with pulmonary hypertension, the 

 pulmonary arterial mean pressure is much higher 

 than the pulmonary venous wedge pressure, pre- 

 sumably due to the interposed high vascular resistance 

 (84). 



In brief, neither the pulmonary arterial wedge 

 pressure nor the pulmonary venous wedge pressure 

 provides a measure of the pulmonary capillary 

 pressure. However, with care and under appropriate 

 circumstances, the pulmonary arterial wedge pres- 

 sure does provide an approximate measure of the 

 pulmonary venous, and usually, of the mean left 

 atrial pressure; it can then be used to estimate the 

 driving pressure across the entire pulmonary vascular 

 bed and to calculate the resistance to perfusion offered 

 by the small pulmonary vessels. 



Influence of Intrathoracic Pressure on Pulmonary 

 Vascular Pressure 



Pressure in an intrathoracic vessel is not a simple 

 concept. In order for such a pressure to have meaning, 

 it must be related to a reference level, i.e., atmospheric 

 or pleural pressure. If the manometer which records 

 the pressure is balanced against atmospheric pressure, 

 all pressure changes within the thorax arising from the 

 ventilation — for example, a cough (fig. 2 1 ) — will be 

 immediately propagated across the walls of the pul- 

 monary vessels and heart to the incompressible blood 

 which they contain; the intrathoracic pressure changes 

 will, therefore, be registered as an integral part of 

 the pressure pulse. However, pressures recorded in 

 this way ("luminal" pressures) provide no measure 

 of the pressure which distends the vessels ("trans- 

 mural" pressures) : during a cough, while the pressure 

 recorded by a manometer balanced against atmos- 

 pheric pressure rises precipitously, a manometer 

 balanced against pleural pressure shows that the 

 transmural pressure has remained virtually unchanged 

 (190). 



Values for the pleural pressures have been obtained 



