Influence of the Diaphragm Upon Portal Blood Flow and Venous Return

Abstract

A modified prepn. for studying portal hemodynamics in the anesthetized dog is descr. which minimizes trauma and features the measurement of the extra-vascular as well as the intravascular portal pressure so as to yield the dynamically significant "effective portal pressure." With this prepn., detns. were made of portal pressure and splanchnic blood flow during normal respiration and accompanying descent of the diaphragm evoked by stimulation of the phrenic nerve. The results indicate that, with descent of the diaphragm, there is a dynamically significant compression of the hepatic and splanchnic vascular beds. This initially acts to squeeze blood from these beds so as to yield a transient augmentation in portal blood flow, even though maintained tetanus of the diaphragm was found to impede blood flow in both the splanchnic and hepatic vascular beds. On the basis of these and other data, it is concluded that diaphragmatic contraction is capable of effectively squeezing blood from the abdominal viscera and thereby augmenting venous return. Under normal conditions with a slow respiratory rate, the quantitative significance of this augmented venous return appears to be slight. However, under conditions of hyperpnea and possibly in the state of shock, this respiratory augmentation of venous return should be of greater importance.