Flow-mediated vasodilatation of the main pulmonary artery.

This study compares the independent effects of flow (Qpa) and transmural pressure (Ppa) on the diameter (D) and length of a segment (L) of the main pulmonary artery (MPA) in anesthetized open-chested dogs. Qpa was increased by a shunt from the left lower lobe pulmonary artery to an external jugular vein. Ppa was increased with positive end-expiratory pressure (n = 5), or with a femoral arteriovenous shunt (n = 5). Mean Ppa, Qpa, D and L were calculated by averaging over a single cardiac cycle selected at end expiration of successive breaths. The independent effects of Ppa and Qpa on MPA dimensions were determined by multiple linear regression. The effects of Ppa delta D%/delta P% = 0.153 +/- 0.03 SE) and Qpa (delta D%/delta Q% = 0.021 +/- 0.004 SE) on D were similar in both groups of dogs. Changes of L (delta L%/delta Ppa% = 0.168 +/- 0.042 SE and delta Qpa% = 0.033 +/- 0.006 SE, n = 5) were similar to the corresponding changes of D. We conclude that the effects of flow are small relative to pressure and that flow not only causes vasodilatation but also vasoelongation of the MPA.