Effects of blood flow and left atrial pressure on pulmonary venous resistance.

• The contribution of the pulmonary venous system to the pulmonary vascular resistance has been a subject of increasing interest." The pulmonary veins are sometimes considered to consist of wide inactive vessels; however, histological studies have revealed a layer of smooth muscle in these veins, especially at their junction with the left atrium. The veins may also respond histologieally by venous intimal fibrosis in chronically increased blood flow, as in atrial septal defect. Some workers have, without measuring flow rates, interpreted an increase in the pressure gradient from the pulmonary vein or the pulmonary arterial wedge position to the left atrium as evidence for venous constriction.' However, a change in gradient from pulmonary capillary or vein to left atrium does not necessarily indicate active venomotion, since mechanical factors, such as blood flow or left atrial pressure, may also account for this increasing gradient. To be certain of such an interpretation, it would be necessary to measure the capillary pressure directly, but no means are available for such a direct measurement. The arterial wedge pressure, which approximates the pressure in the left atrium or the large pulmonary veins," has been utilized as an estimate of the pulmonary capillary pressure.' Theoretical objections and technical difficulties' ~ have raised serious questions concerning the use of arterial wedge pressure measurements as estimates of the pulmonary capillary pressure. Direct measurement of venular pressures with a small