A 42-ycar-old man with idiopathic cardiomyopathy is hospitalized after an episode of syncope 1 hour ago. The event occurred while he was sitting in a chair: he did not have any symptoms before the episode, and he was alert and had no incontinence afterward. He had previously been well and exercises regularly without difficulty. Idiopathic cardiomyopathy was diagnosed 6 months ago. Echocardiography at that time showed left ventricular enlargement and a left ventricular ejection fraction of 30%. Coronary angiography showed normal coronary arteries and decreased left ventricular systolic function. His medications are furosemide (diuretic), carvedilol (beta blocker), and lisinopril (ACEi). On physical examination, heart rate is 60/min and blood pressure is 100/60 mm Hg when supine, and heart rate is 66/ min and blood pressure is 45/58 mm Hg when standing. Cardiac examination reveals an enlarged point of maximal impulse that is laterally displaced and regular cardiac rhythm. Results of the neurologic examination are normal shows normal sinus rhythm without significant change compared with his last tracing Which of the following is the most likely cause of his syncope?