Effect of edema on segmental vascular resistance in isolated lamb lungs determined by micropuncture

We have studied the mechanical effects of fluid accumulation on the pulmonary vasculature in 28 isolated blood perfused lungs of newborn lambs. Vascular resistance in the pulmonary arteries, microvessels, and veins was determined by micropuncture measurement of microvascular pressures, and regional distribution of blood flow in the lungs was determined using radiolabelled microspheres both before and after the development of varying degrees of hydrostatic edema. Edema was induced by raising venous pressure. During measurements, alveolar and venous pressures were kept constant at 7 and 8 cm H2O, respectively, as well as lung blood flow (540 +/- 107 ml/min). All vascular pressures were referenced to the superior surface of the lung, site of all micropunctures. Active vasomotor changes were eliminated by addition of papaverine to the perfusate. Under baseline nonedematous conditions in the absence of vasomotor tone, 17% of the total pressure drop was in arteries, 41% was in microvessels, and 42% was in veins. With the development of alveolar edema (80 +/- 13% weight gain), there was no change in total or segmental vascular resistance, but after 148 +/- 97% weight gain, total pulmonary vascular resistance increased by 74%. Segmental pressure drop increased in arteries by 172% and in microvessels by 132% but decreased by 22% in the venous segment. Regional distribution of blood flow remained unchanged. Possible mechanisms for increased resistance to blood flow may be compression of small arterioles and venules (less than 20 micron diameter) by liquid cuffs and/or occlusion of microvessels by the weight of alveolar liquid.