DYNAMICS OF PULMONARY CIRCULATION 



I 70I 



portion of the dog's pressure-flow curve; un- 

 fortunately, in this study, sufficiently high flows to 

 define the steep portion of the curve were not 

 achieved. However, the original measurements by 

 Cournand and co-workers on human subjects after 

 pneumonectomy (open triangles) suggest that the rest 

 of the human pressure-flow curve may also resemble 

 that of the dog (89). More observations in both man 

 and dog at higher levels of flow are obviously needed; 

 unfortunately, patients with congenital left-to-right 

 shunts, who may, from a priori considerations of their 

 large pulmonary blood flows, appear to be logical 

 candidates for such measurements, are usually found 

 to be unsuitable for pressure-flow curves because of 

 complicating pulmonarv vascular disease and ana- 

 tomical defects which preclude precise measurements 

 of pulmonary blood flow. 



There are three interesting side lights to the curve 

 illustrated in figure 34. The first is the difference 

 between this parabolic curve of the normal subject 

 and the linear relationship between pressure and 

 flow which has been described for patients with ab- 

 normal vascular beds (132); this difference suggests 

 that those animal or isolated-lung experiments which 

 find a linear relationship between pulmonary arterial 

 pressure and flow may be dealing with abnormal, or 

 overfilled, lungs (132). The second is the relationship 

 between the sharp inflection of the curve and the 

 maximum diffusing capacity; it has yet to be estab- 

 lished whether maximal dilatation of the pulmonary 

 capillary bed, i.e., the achievement of the maximum 

 diffusing capacity, coincides with the abrupt increase 

 in the pulmonary arterial pressure (267, 407). The 

 third deals with the use of graded exercise to construct 

 the pressure-flow curve in intact animal or man. It 

 may be seen that during mild to moderate exercise in 

 man, the pressure-flow points overlap those obtained 

 during graded occlusion in the dog; during heavier 

 exercise, the coincidence of human and animal points 

 is not as exact. These discrepancies raise the possi- 

 bility that strenuous exertion may sufficiently alter 

 passive determinants, i.e., transmural pressures and 

 left atrial pressure, to invalidate the use of such exer- 

 cise for the construction of a reference curve which is 

 supposed only to depict the uncomplicated conse- 

 quence of increasing flow on pressure (132). On the 

 other hand, the use of mild to moderate exercise for 

 this purpose seems valid on several scores: a) the 

 mean left atrial pressure (104) and mean pleural 

 pressures are little affected by light exercise (132), 

 b) the pressure-flow curses obtained during light 

 exercise and the passive curves obtained from iso- 



lated lungs are quite similar (119, 416), and c) the 

 exercise points correspond to those obtained during 

 graded occlusion of the pulmonary artery (42, 53, 

 101). 



The second way of identifying pulmonary vasocon- 

 striction is particularly applicable to the use of 

 pharmacological agents; it has the advantage of circum- 

 venting many of the restrictions outlined for steady- 

 state measurements. It involves (fig. 33) the single 

 injection of a pharmacological agent into the pul- 

 monary circulation of the intact animal or man and 

 the continuous registration of the pressure drop across 

 the lungs, the heart rate, and the systemic blood pres- 

 sure during the single pulmonary circulation, i.e., 

 before recirculation. In this way, the effect of the 

 agent appears as a change in pulmonary arterial 

 pressure before flow can change and before the agent 

 can affect the systemic circulation (187). An alternate 

 way of accomplishing the same end for steady-state 

 experiments is the continuous infusion of an agent, 

 e.g., acetylcholine (192, 441) which is destroyed 

 within the lungs during the course of a single circula- 

 tion. 



Blood F/ozv Through Each Lung Separately 



After application of a unilateral stimulus, such as 

 hypoxia (135), or the unilateral infusion of acetylcho- 

 line (89), the partition of flow between the two lungs 

 is a measure of the relative resistances of the two sides 

 since the pressure gradient across the lungs is identical 

 on the two sides. Although the idea of using one lung 

 in this way, as a control for the other, is intuitively- 

 attractive, the experiments are generally technically 

 difficult, particularly if bronchospirometry is involved. 



Critical Closure 



Small muscular blood vessels of the systemic circu- 

 lation are believed to be inherently unstable so that 

 they are inclined to spring shut — concentrically and 

 completely — when their intraluminal pressure drops 

 below a critical value. This "critical closing pressure' 1 

 has been proposed as a measure of the tone of vascular 

 smooth muscle, i.e., of vasomotor activity: the level 

 of the "critical closing pressure" increases as wall 

 tension increases and as the size of the vessel decreases. 

 Critical closing pressure is manifested experimentally 

 by the arrest of flow despite an appreciable perfusion 

 pressure. By similar reasoning, the muscular small 

 vessels spring open when "critical opening pressures" 

 are exceeded (60, 165). 



