Use of negative intrathoracic pressure to obtain end-systolic pressure volume relations in dogs.

Left ventricular contractility can be assessed from the end-systolic pressure-volume relationship (ESPVR). In this study we test the hypothesis that the same ESPVR can be obtained by varying LV loading with different levels of negative intrathoracic pressure as by varying LV filling. In six dogs mean aortic transmural pressure was used to approximate LV end-systolic pressure and LV volume was determined from data gathered from biplane cineradiograms of multiple markers placed in the LV midwall. In each preparation right heart bypass allowed control of cardiac output while the thoracic pressure was varied with a box surrounding a midsternal thoracotomy. Reflex effects were minimized by ganglionic blockade and bilateral vagotomy. ESPVRs were obtained by varying the cardiac output at constant thoracic pressure or by changing intrathoracic pressure at constant cardiac output. The slopes of the ESPVRs were not significantly different. This result implies that LV loading by negative intrathoracic pressure, in this highly controlled preparation, can be used to generate a systolic LV elastance similar to that obtained by varying LV filling.