DYNAMICS OF PULMONARY CIRCULATION 



I7II 



V *TI ON 



fio. 39. Schematic representation of the influence of positive pressure breathing on the right 

 ventricular (RV) and systemic arterial (BA) pressures of a normal human subject. The mask pressure 

 (M) appears above the pressure pulses. All pressures are in mm Hg. During inflation, as mask (and 

 pleural) pressures increase, the pulse pressure in the right ventricle progressively decreases; con- 

 comitantly, the systemic arterial pulse pressure increases. During expiration, the reverse occurs. The 

 prompt return of the mask pressure to ambient pressure accounts for the ability of expiration to com- 

 pensate for the inspiratory deficit in blood flow. [After Richards el al. (340). 



decreases (395) and the pulmonary vascular resist- 

 ance increases (49). Although the imposed pressure 

 raises pulmonary vascular luminal pressures the trans- 

 mural pressures are virtually unaffected (253), and 

 the pressure gradient along the length of the pulmo- 

 nary vascular tree remains essentially unchanged 

 (187, 256). The circulatory changes arising from the 

 imposed pressure reverse promptlv once the lungs 

 are vented to atmosphere (fig. 39). 



In systemic hypotensive states, positive pressure 

 breathing may precipitate circulatory collapse if 

 compensatory mechanisms are insufficient to sustain 

 the venous return to the right heart (49). Not only 

 are the output of the right heart and the pulmonary 

 blood volume reduced, but the normal balance be- 

 tween alveolar perfusion and alveolar ventilation is 

 also upset so that portions of the lung become ex- 

 cessively ventilated with respect to perfusion (160). 



A variety of mechanical devices are in common use 

 for intermittent positive pressure breathing (426). 

 Their effects on the circulation are functions of the 

 degree and duration of the cyclic swings which they 

 induce in intrathoracic pressure. The cardiac output 

 generally falls (11) in proportion to the mean increase 

 in intrathoracic pressure; in practice, the cardiac out- 

 put may be kept at control levels by using cycling 

 devices which operate to: a) inflate the lungs gradu- 

 ally to peak pressure; b) decompress the lungs sud- 

 denly by venting them to atmosphere; and c) allow a 

 longer period of exposure to atmospheric pressure 

 than to positive pressure. 



Negative Pressure Breathing (Pleural) 



As pressure around the collapsed isolated lung is 

 artificially decreased, its resistance to perfusion de- 

 creases (61, 354). The changes in resistance which 

 accompany further inflation of this type are unsettled. 



Thus, some have found only a continuing decrease in 

 resistance as the lung is expanded by progressive 

 decrements in "pleural pressure" (62); others have 

 found U-shaped curves in which the initial drop in 

 resistance as the lung begins to expand (pleural pres- 

 sure — 5 to — 10 cm H^O) is succeeded by an increase 

 in resistance as the pleural pressure decreases further 

 (pleural pressure —10 to —25 cm H 2 0) (354, 397). 

 The nadir in resistance occurs at half-maximal lung 

 volume. 



The mechanisms proposed to account for these 

 divergent results are enlightening. The initial de- 

 crease in resistance — to which all agree — has been 

 attributed to either an increase in transmural pressure 

 or to the unkinking of "gnarly" vessels (61 ). Different 

 explanations have been used to account for the di- 

 vergent results at high levels of inflation: those who 

 find a continued drop in resistance ascribe it to the 

 continued increase in transmural pressure as "pleural" 

 pressure drops (62); those who find that resistance 

 finally increases believe that at lung volumes exceed- 

 ing 50 per cent of maximal, mechanical distortion of 

 the pulmonary vessels — a function of lung volume 

 rather than of transmural pressure — is involved (354, 

 397). While these studies leave unsettled the question 

 of the behavior of the pulmonary vascular resistance 

 as the lung is progressively inflated, they do serve as 

 a reminder that the transmural pressures, which ac- 

 count satisfactorily for passive changes in caliber at 

 moderate degrees of inflation, may be supplanted by 

 other mechanical influences, e.g., stretching or col- 

 lapse, in determining vascular calibers at extreme 

 inflation or deflation. 



Negative Pressure Breathing (Intrapulmonary) 



"Snorkel" breathing is characterized by a lower 

 pressure within the lungs than around the body (49)- 



