Effects of pleural pressure and abdominal pressure on diaphragmatic blood flow.

The aim of this experiment was to determine if blood flow to the diaphragm is affected by generating the transdiaphragmatic pressure mainly with positive abdominal pressure or mainly with negative pleural pressure, during both sustained and intermittent diaphragmatic contractions. Diaphragmatic blood flow was measured in anesthetized mongrel dogs by the radionuclide-labeled microsphere method. Sustained and intermittent tetanic diaphragmatic contractions were produced with 1) free abdomen and closed chest (high negative pleural pressure) and 2) bound abdomen and open chest (high positive abdominal pressure). During sustained contractions, diaphragmatic blood flow at maximum levels of activation was significantly higher with negative pleural pressure (P less than 0.05). In contrast with this, intermittent diaphragmatic contractions did not yield a significant difference between diaphragmatic blood flow with negative pleural pressure and with positive abdominal pressure at maximal levels of transdiaphragmatic pressure. During both sustained and intermittent contractions, blood pressure, as measured from the carotid artery, did not vary significantly between negative pleural pressure and positive abdominal pressure. We conclude that during sustained tetanic diaphragmatic contractions, diaphragmatic blood flow is obstructed by high positive abdominal pressures, but during intermittent diaphragmatic contractions, high positive abdominal pressures do not affect total blood flow, since any inhibition of blood flow during the contractile period can be compensated for during the relaxation period between contractions.