Effects of Blood Flow and Left Atrial Pressure on Pulmonary Venous Resistance

Pulmonary venular pressure was measured in 11 thoracotomized dogs and in 8 isolated lung lobes by introducing a small catheter (O.D. 0.6 or 0.9 mm.) as far as possible retrograde into the pulmonary veins. In thoracotomized dogs, insufflated with a maximal air pressure of 6 mm. Hg, the mean pulmonary arterial, venular, and left atrial pressures averaged 18.4, 9.1, and 4.2 mm. Hg, respectively. An almost linear relationship was found between venular pressure and pulmonary blood flow, except in the low flow ranges; venular pressures exceeded left atrial pressure by one-fourth to one-half of the gradient from pulmonary artery to left atrium. The calculated pulmonary venous resistance decreased with increased blood flow. The fall in pulmonary vascular resistance resulting from elevation of the left atrial pressure was shown to reside primarily in the venous system, especially when left atrial pressure was low; arterial and capillary resistance also fell, but to a lesser extent. Pulmonary venular pressure is higher than left atrial pressure by a factor which varies with blood flow. The effect of such mechanical forces on the venous resistance is discussed.