DYNAMICS OF PULMONARY CIRCULATION 



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been claimed that alveolar hypoventilation elicits 

 pulmonary arterial hypertension not only by wa\ of 

 acute hypoxia but also by an hypothetical "alveolo- 

 vascular reflex." Since mechanical underventilation 

 of the lungs involves an element of mechanical col- 

 lapse of alveoli as well as a change in the alveolar gas 

 composition, the operation of this special alveolar 

 reflex is difficult to prove. Nonetheless, this hypotheti- 

 cal reflex is consistent with the consensus that pulmo- 

 nary vasomotor effects of acute hypoxia (by airway) 

 are independent of systemic arterial hypoxemia (204). 



Pulmonary Vasomotor Reflexes 



The plentiful supply of autonomic nerves to the 

 lungs and of nerve fibers to the pulmonary blood 

 vessels has stimulated the search for direct evidence 

 of pulmonary reflex activity. This search has dis- 

 closed that numerous afferent vagal fibers and baro- 

 receptor endings exist in the large pulmonary arteries, 

 that the impulse activity of the pulmonary barorecep- 

 tor fibers varies with the pulsatile blood pressure in 

 the pulmonary artery and that the receptors are active 

 at the usual levels of pulmonary arterial pressure (80). 

 On the other hand, since the efferent limbs have not 

 yet been traced either to their conjunction with affer- 

 ent limbs or to their endings in effector cells, the 

 proof of their existence consists entirely of indirect 

 physiologic observations, e.g., systemic vasodilation as 

 pulmonary arterial pressure increases (6). 



The pulmonary circulation is believed to participate 

 in a wide variety of mechano- and chemoreflexes 

 (207, 393). Some of these hypothetical reflexes are 

 conceived to be purely local, e.g., pulmonary veno- 

 arteriolar (6, 101, 231, 409) or alveolar-vascular 

 (359); these local reflexes are inaccessible for direct 

 appraisal. Much more tangible are the remote reflexes. 



Three types of remote reflexes have been exten- 

 sively studied. The first is a reflex from the pulmo- 

 nary vessels to the systemic circulation. With rare 

 exception (261), this type of reflex has been ''depres- 

 sor" in nature, evoking bradycardia and systemic 

 arterial hypotension in response to a wide variety of 

 stimuli; the stimuli have included an increase in static 

 pressure at either end of the pulmonary vascular tree 

 or along its whole length (6, 110), chemoreflexes of 

 different kinds (99), and pulmonary vascular hypo- 

 thermia (159)- 



The second type of remote reflex is a combined or 

 chain reflex from the pulmonary arteries to the small 

 pulmonary vessels on the one hand (4, 32 1 ) and to the 

 respiratory apparatus on the other (432). With few 



exceptions (101), such a reflex has customarily been 

 invoked to account for the dramatic clinical syndrome 

 which follows multiple pulmonary emboli, i.e., the 

 pulmonary hypertension, the rapid shallow breathing, 

 the bronchoconstriction, and the decrease in periph- 

 eral arterial oxygenation (227). However, many of the 

 links in this reflex chain reaction remain speculative. 

 More precisely defined, but much less meaningful 

 with respect to function, are the chemoreflex path- 

 ways which connect the pulmonary arterial tree with 

 the respiratory apparatus (99). 



The third type of remote reflex runs from the 

 reflexogenic areas of the carotid arterial bifurcations 

 and aortic arch to the pulmonary circulation (206). 

 To create the proper experimental setting for the 

 demonstration of these feeble reflexes, Daly and Daly 

 were obliged to resort to the "vasosensory controlled 

 perfused living animal" preparation in which the 

 pulmonary and systemic circulations could be sepa- 

 rately controlled. In this special preparation, intense 

 pressor stimulation of the systemic baroreceptors 

 evoked pulmonary vasodilatation; perfusion of the 

 carotid chemoreceptors with hypoxic or venous blood 

 (during interrupted bronchial arterial flow) evoked 

 pulmonary vasoconstriction (96). The authors are 

 careful to point out that the elaborate controls re- 

 quired to demonstrate the existence of these reflex 

 pathways obscure the meaning of these reflexes for the 

 live, intact organism (95). 



Pulmonary Vasomotor Waves 



Rhythmic oscillations in systemic arterial blood 

 pressure (Traube-Hering-Mayer waves) were first 

 described toward the close of the nineteenth century 

 (404). Although the consensus since then has favored 

 the view that these systemic waves reflect the rhythmic 

 activity of the medullary vasomotor center, not always 

 has irradiation from the respiratory to the vasomotor 

 center been excluded. Most often, the Traube- 

 Hering-Mayer waves have been encountered in ab- 

 normal or deteriorating experimental preparations; 

 even in the same preparation the pattern of the waves 

 tends to vary with respect to frequency and to ampli- 

 tude (229). 



Infrequently, the swings in systemic arterial blood 

 pressure were found to be associated with swings in 

 pulmonary arterial blood pressure (125). And, on 

 rare occasion, the pulmonary arterial swings occurred 

 either without (379), or with barely perceptible (125), 

 systemic arterial waves. In these few instances, other 

 passive effects were not entirely excluded. 



