Hemodynamic responses of dog lung lobe to lobar venous occlusion

We perfused the left lower lobe of the dog lung with constant flow. When the lobar venous outflow was occluded, the lobar venous pressure rose suddenly to a level somewhere below the arterial pressure, and then the arterial and venous pressures began to rise more slowly. A possible explanation for this response is that, when the outflow was occluded, flow through some downstream segment of the bed ceased. Because flow into the lung continued, the arteriovenous pressure difference after occlusion represents the pressure drop across some upstream segment through which the flow continued. We designated the arteriovenous pressure difference just after outflow occlusion as the upstream pressure drop. The arteriovenous pressure difference before occlusion minus the upstream pressure drop was designated the downstream pressure drop. In an attempt to better understand the meaning of the upstream and downstream pressure drops, we examined the influence of pulmonary vasoconstriction and flow direction on the size of the upstream and downstream pressure drops. We also compared these pressure drops with the pressure drops occurring upstream and downstream from the midpoint of the lobar vascular volume, using the low-viscosity bolus technique. The results indicate that changes in the upstream and downstream pressure drops, as evaluated by outflow occlusion, reflect changes in the lobar arterial and venous resistances.