A 48-year-old woman has a new holosystolic murmur and exertional dyspnoea. Echocardiography shows a regurgitant volume from the left ventricle into the left atrium during ventricular systole, with an end-diastolic volume of 180 mL and an end-systolic volume of 50 mL. The pressure-volume loop has no vertical segment at either end. Which mechanical change accounts for this loop?
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