Impedance and transmission properties of the pulmonary arterial system’

Authors ’ synopsis Simultaneous measurements of instantaneous pulmonary arterial input pressure and flow were compared with lung capillary blood flow in 13 closed-chested dogs. Vasoconstrictor effects of’hypoxia and serotonin injiusion were shown to cause major altera-tions in the normal input impedance spectrum of the pulmonary arterial system. In spite of this, the transmission ratio of flow pulsatility from the main pulmonary artery to the lung capillaries remained unaltered, indicating that the time constant of the pulmonary arterial system is self-regulating. Now that electromagnetic flowmeters are commercially available, physiologists are able t o make simultaneous measurements of instantaneous blood flow and pressure at many sites in the circulation. Previously, the concept of resistance had to be used in order to relate the mean pressure drop across a vascular bed t o the mean flow through it. This concept, although useful, is limited in its application because it fails to take into ac-count the pulsatile nature of blood flow. More recently, McDonald (1960) and Bergel, Caro, and McDonald (1960) have introduced the concept of vascular impedance to describe the relationship between oscillatory blood flow and pressure at a particular site in the circulation. Furthermore, the ratio of oscil-latory pressure to oscillatory flow a t the origin of the pulmonary artery- namely, the modulus of the pulmonary arterial input impedance- not only defines the relationship between these two parameters, but also gives dynamic information about the properties of the pulmonary arterial system. Bergel and Milnor (1965) have studied pulmonary vascular impedance in the dog, and our work re-examines this and extends it t o a study of the effects of hypoxia and inotropic agents. We have previously defined pulmonary capillary flow pulsatility as the ratio of th