Role of luminal volume changes in the increase in pulmonary blood flow at birth in sheep

The mechanism by which pulmonary blood flow increases and pulmonary vascula r resistance decreases after birth is not fully understood. The aim of this study was to simulate the decrease in lung volume caused by the onset of a ir-breathing at birth and determine whether it can duplicate the changes in pulmonary blood flow and vascular resistance that occur at this time. In c hronically catheterized fetal sheep near term (145 days of gestation), feta l pulmonary arterial blood flow was measured, using coloured microspheres, before and after fetal lung liquid volumes were reduced from 52.2 +/- 2.7 t o 21.2 +/- 1.6 ml kg(-1). During the 30 min period following the reduction in lung liquid volume, the pulmonary-to-systemic arterial pressure differen ce decreased from 6.8 +/- 1.2 mmHg (pulmonary > systemic) to 1.6 +/- 0.5 mm Hg. Reducing the volume of fetal lung liquid increased pulmonary blood flow from 59.1 +/- 10.5 to 204.2 +/- 40.4 mi min(-1) (100 g tissue)(-1) and red uced pulmonary vascular resistance from 0.53 +/- 0.20 to 0.14 +/- 0.04 mmHg min ml(-1) (100 g tissue)(-1). We conclude that a reduction in fetal lung liquid volume, which simulates the reduction in lung volume that occurs at birth, causes a 3- to 4-fold increase in pulmonary blood flow and a reducti on in pulmonary vascular resistance of a similar magnitude. Thus, the reduc tion in lung volume associated with the lung changing from a liquid- to an air-filled organ, may partly account for the increase in pulmonary blood fl ow and decrease in pulmonary vascular resistance at birth.