External work and arterial load in canine left ventricular ischaemic failure: evidence of ventricle-load mismatch.

It has been proposed that a normal ventricle and its arterial load constitute a matched system, and that such matching is not present during ventricular failure, i.e., failure represents a state of mismatch between ventricle and arterial load. To investigate this assumption we studied the relationship between external work and load in an equatorial segment of left ventricle (LV) in an intact canine preparation under the hypothesis that the LV works at the peak of the work versus load relationship during control conditions, but is shifted from this peak during LV failure. LV systolic wall force (F) and circumferential segment work (W) were calculated in eight pentobarbital anaesthetized, open-chest dogs from LV pressure (P) and anterior-posterior diameter (D) before and after induction of LV ischaemic failure (50 micron microspheres were injected into the left coronary vascular bed). Variations of arterial load were created by acute partial occlusions of the aorta, raising aortic pressure by 45 mm Hg before and 15 mm Hg during failure. Before failure W was unaffected by the variations of arterial load, but W decreased significantly during failure. From the relation between end-systolic F and D, theoretically optimal F (Fopt) corresponding to maximum W was estimated. Before failure the observed F did not differ significantly from Fopt, but the observed F was significantly greater than Fopt during failure. These findings support the notion that ventricular failure represents a state of mismatch between the ventricle and its systolic load.