DYNAMICS OF PULMONARY CIRCULATION 



n i 3 



interpreted as reflecting the gradual increase in 

 peripheral venous blood volume and pressure during 

 the sustained expiration until adequate filling of the 

 right, and then the left, heart is restored (187, 253). 



Forced Expiration (Valsalva) 



The effects of the Valsalva maneuver (forced ex- 

 piration against a closed glottis or a column of water 

 30 to 40 cm high) (fig. 41 ), has been more intensively 

 studied in the systemic circulation than in the pulmo- 

 nary circulation (255). Shortly after the start of the 

 maneuver, the distending pulmonary arterial pressure 

 falls abruptly as the filling pressure of the heart is 

 reduced by the increased intrathoracic pressure (187, 

 253); it remains low during the period of strain. Upon 

 release of the expiratory effort, pulmonary arterial 

 mean and pulse pressures "overshoot" the prestrain 

 level, but to a lesser extent than in the systemic 

 arteries. During the maneuver, considerable quanti- 

 ties of blood may be displaced from the thorax to the 

 periphery (253). The systemic arterial overshoot 

 seems to involve a combination of an increased 

 cardiac output and vasoconstriction; although some 

 believe that these same mechanisms are involved in 

 the pulmonary arterial overshoot, the evidence for 

 pulmonary vasoconstriction is much more tenuous 

 than for systemic vasoconstriction (255). 



OCCLUSION OF A PULMONARY ARTERY 



In principle, occlusion of larger and larger portions 

 of the pulmonary arterial tree provides a simple tool 



BEFORE 



LEFT 



RIGHT 



DURING 



fig. 42. Effect of complete occlusion of one pulmonary artery 

 on ipsilateral oxygen uptake. Before inflation of the occlusive 

 balloon in the right pulmonary artery (left panel), both lungs 

 share almost equally in the oxygen uptake. The middle panel 

 shows the occlusive balloon, inflated with Diodrast, positioned 

 at the end of a cardiac catheter in the right pulmonary artery. 

 After complete occlusion, the oxygen uptake by the right lung 

 ceases. [After Fishman et al. (139J.J 



for testing the passive effects of an increase in pulmo- 

 nary blood flow on pulmonary arterial pressures 

 (fig. 34) and pulmonary vascular resistance. In the 

 open-chest dog, graded occlusion of the pulmonary 

 vascular tree is easily performed (194). The situation 

 is much more complicated in the closed-chest dog or 

 man in whom balloon-tipped, venous catheters are 

 guided, under fluoroscopic control, into a pulmonary- 

 artery; in this experimental situation additional tech- 

 niques, such as bronchospirometry, are required to 

 establish the degree of occlusion which has been 

 accomplished (fig. 42). 



In wondrous contrast to the catastrophic effects 

 occluding the pulmonary vascular tree by emboli, in- 

 flation of a balloon in one pulmonary artery is entirely- 

 innocuous: the metabolic rate, the total cardiac out- 

 put, the systemic arterial and left atrial blood pres- 

 sures, and the heart rate remain unchanged; the total 

 ventilation rarely increases by more than 10 per cent 

 (53, 101). However, even this slight change in total 

 minute ventilation helps to adapt the alveolar 

 ventilation to the altered pulmonary capillary per- 

 fusion (390). 



The first studies of the pulmonary circulation fol- 

 lowing occlusion of one pulmonary artery- were made 

 in 1876 on the open-chest dog (132). These indicated 

 that the pulmonary arterial pressure, measured 

 proximal to the site of occlusion, increased by 50 per 

 cent following interruption of the blood flow to one 

 lung. Subsequently, a similar procedure in other 

 open-chest animals found lesser increases, ranging from 

 zero in the rabbit ( 1 25) to 20 per cent in the cat ( 1 25). 

 More puzzling than these divergent results in the dif- 

 ferent species is the fact that in the intact dog, oc- 

 clusion of one pulmonary artery by a balloon-tipped 

 catheter has also produced variable effects: on the 

 one hand are results indicating that pulmonary ar- 

 terial pressure remains essentially unchanged (68, 

 101); other results indicate an increase in pressure of 

 the order of 33 per cent (252, 259). However, since 

 none of the experiments in the dog verified the degree 

 of pulmonary arterial occlusion produced by the in- 

 flated balloon, it seems reasonable to assume that in- 

 flation of the balloon was not equally successful in 

 producing complete occlusion in the different dogs, 

 and that the larger increments in pulmonary arterial 

 pressure — i.e., of the order of 33 per cent (252, 259) — 

 represent the more complete occlusions. 



In man the results have been more consistent: after 

 complete occlusion of one pulmonary artery, the 

 pulmonary arterial pressure (primarily systolic) 

 proximal to the occlusion increases, the pulmonary- 



