DYNAMICS OF PULMONARY CIRCULATION 



1699 



200- 



150- 



P vs R 

 Q vs R 



R mmHg/ml/min 

 -I 1 1- 



FiG. 31. Passive changes in pulmonary' vascular resistance 

 (R) at different pulmonary arterial pressures (P) and at dif- 

 ferent pulmonary blood flows (Q). Pulmonary venous pressure 

 remains constant throughout. As How and pressure decrease, 

 resistance increases. [Based on Edwards (119).] 



which would be expected to obtain were it not for the 

 stimulus (fig. 32); and b) the continuous registration 

 of the pressure gradient across the pulmonary vascu- 

 lar tree and of the systemic blood pressure, before 

 and after the injection of a pharmacological agent 

 into the pulmonary artery (fig. 33). 



The use of pressure-flow points to recognize vaso- 

 motor activity requires that the passive pressure gra- 

 dient-flow relations be known or predictable. It is 

 difficult to compare the published relationships in the 

 pulmonary with those in the systemic circuit because 

 conventionally the data do not cover the same range. 

 Pressure-flow plots for systemic beds include zero 

 pressure and zero flow, while the conventional presen- 

 tation of pulmonary data start with "normal" pressure 

 and flow and plot the fractional excess of one against 

 the fractional excess of the other. Qualitatively, the 



modify vascular caliber and resistance. For example, 

 an increase in transmural pressure — arising from an 

 increase in either pulmonary- arterial or left atrial 

 pressure — passively widens the vessels and decreases 

 their resistance (fig. 31); conversely, a drop in trans- 

 mural pressure increases vascular resistance (36, 69, 

 366). Consequently, a change in resistance is not a 

 reliable sign of pulmonary vasomotricity when trans- 

 mural pressures change. Indeed, at different levels of 

 transmural pressure, calculated resistance may re- 

 main unaltered even though pulmonary vasomotor 

 tone has altered considerably (61). Considerations 

 such as these have had two major effects on experi- 

 mental design and interpretation: a) many have 

 urged that the use of ohmic resistance be abandoned 

 in favor of more straightforward presentation of 

 pressures and the corresponding flows, and b) others 

 have insisted on stringent experimental criteria, such 

 as constant flow (fig. 31 ), left atrial, alveolar and intra- 

 pleural pressures before interpreting a change in 

 pulmonary arterial pressure. 



Practical Recognition of Pulmonary Vasomotricity 



Dissatisfaction with the use of calculated resistance 

 (a ratio) to detect an active change in vascular caliber 

 has encouraged the use of graphic representations 

 which relate blood flow to the pressure gradient that 

 effects it (70). For example, the recognition of pul- 

 monary vasomotor activity has been attempted bv: 

 a) the comparison of experimentally determined pul- 

 monary vascular pressure-flow points, obtained after 

 applying a stimulus, with the pressure-flow curve 



20 -- 



10 -- 



2 I % . 



HI (- 



3.5 



4.5 



5.5 



0.3 



0. I -- 



E / 



12 



•/. 0. © 



EXERCISE 



21 V. 0. 



3.5 



4.5 

 Lit/min 



55 



fig. 32. Detection of a decrease in pulmonary vascular cali- 

 ber from pulmonary arterial flow-pressure curves and from 

 pulmonary vascular flow-resistance curves. For these curves, 

 mild exercise was used to increase pulmonary blood flow pas- 

 sively and acute hypoxia was used as the test stimulus. A. Dur- 

 ing exercise, pulmonary arterial pressure increased as blood 

 flow increased; during acute hypoxia, an equivalent increase 

 in pressure occurred without an appreciable increment in 

 blood flow. B. During exercise, calculated resistance decreased; 

 conversely, during acute hypoxia, calculated resistance in- 

 creased even though blood flow (and presumably all 

 other respiratory and circulatory parameters) remain un- 

 changed. [Based on Fishman et al. (132).] 



