The advanced practice registered nurse (APRN) is examining a 70-year-old patient with pulmonary hypertension. On cardiac exam they auscultate a grade II/VI pansystolic (holosystolic) murmur of medium pitch at left lower sternal border which radiates to the right of the sternum (xiphoid area). The APRN asks the patient to inspire and notes an increase in the intensity of the murmur. Which is the most likely physiological cause of the murmur?