To investigate the mechanism by which increased pleural pressure (Ppl) assists left ventricular (LV) ejection, we compared the effects of phasic systolic or diastolic increases in Ppl (40-60 mmHg) with use of an isolated canine heart-lung preparation with constant venous return. Positive Ppl during systole (S) caused left atrial transmural pressure (Platm = Pla - Ppl) to decrease by 1.25 ± 0.46 (SE) mmHg (P < 0.025). Central blood volume (CBV), the volume of blood in the heart, lungs, and thoracic great vessels, decreased by 29 ± 4.0 (SE) ml (P < 0.001). When Ppl was raised for an equal duration during diastole (D), the decrease in Platm was not significant, but there was a significant decrease in CBV (10.5 ± 4.1 ml, P < 0.05). With constant venous return, these changes suggested that phasic elevations in Ppl in either S or D assisted LV ejection by decreasing LV afterload. To test the hypothesis that positive Ppl during D reduced afterload by emptying the thoracic aorta, we compared the effects of diastolic positive Ppl with a rigid aorta vs. a compliant aorta. Although there was no statistical difference in the effects of diastolic positive Ppl on Platm, the decrease in CBV was significantly greater when the aorta was compliant than when it was rigid (23 ± 2.2 vs. 17 ± 2.7 ml, P < 0.05). Furthermore, diastolic positive Ppl caused a greater fall in mean systolic left ventricular transmural pressure (Plvtm-s) (14.4 ± 1.0 vs. 9.7 ± 0.9 mmHg, P < 0.05) and left ventricular pressure at end ejection (Plvee) (16.3 ± 2.5 vs. 10.1 ± 2.3 mmHg, P < 0.005) when the aorta was compliant. We conclude that systolic positive Ppl reduced afterload by increasing the systolic extracardiac pressure and that diastolic positive Ppl reduces afterload by decreasing the systolic intracardiac pressure through a reduction in thoracic aortic blood volume.
ASJC Scopus subject areas
- Physiology (medical)