Pulmonary vascular congestion selectively potentiates airway responsiveness in piglets

The influence of pulmonary vascular congestion on the response of the airwa ys and lung tissue to low doses of inhaled methacholine (MCh) was studied b y inflating a balloon catheter in the left atrium of the heart in six pigle ts, with an additional five piglets serving as control animals. Congestion alone resulted in small increased in baseline airway (Raw) (14.6 +/- 3.7%) and tissue (Rti) resistance (8.1 +/- 6.5%). Low-dose inhaled MCh (0.3 mg/ml ) increased Raw and Rti in the control group by 10.8 +/- 10.3% and 42.2 +/- 29.5%, respectively. The increase in Raw with MCh in the presence of vascu lar engorgement was significantly greater (67.8 +/- 18.9%) but the increase in Rti (38.1 +/- 13.2%) was similar to that seen in the control group. Mor phometric measurements were performed on transverse sections of large and s mall airways from nine additional piglets (three congested only, three MCh only, and three congestion plus MCh). The thickness of the inner airway wal l was similar in all groups. Compared with MCh only piglets, the thickness of the outer airway wall (between the outer border of the smooth muscle and the surrounding lung parenchyma) was increased (p < 0.05) in engorged only and engorged plus MCh piglets. Compared with MCh only and engorgement only , the amount of airway smooth muscle shortening was greater (p < 0.05) in a ll airway size groups in piglets that underwent engorgement plus MCh challe nge. The results of this study demonstrate that pulmonary vascular engorgem ent, induced by increased left atrial pressure, selectively enhances the ai rway, but not the parenchymal, response to inhaled MCh. These changes are a ssociated with increased thickness of the outer airway wall in response to vascular congestion, suggesting that uncoupling of the mechanical interdepe ndence between the airway smooth muscle and the lung parenchyma may have oc curred. Mechanical uncoupling may reduce the load opposing smooth muscle sh ortening resulting in increased airway narrowing in response to low doses o f inhaled methacholine.