A 73-year-old man presents with worsening shortness of breath with activity over the last few months. He is not able to complete as many physical activities during the day as he could manage 3 months ago. Physical examination reveals hypotension, tachycardia, extremities that are cool to the touch, expiratory wheezing, and rhonchi auscultated during the pulmonary exam; there is also a diminished first heart sound with an S3 gallop heard during the cardiac exam. Question What medication should you begin in this patient to help alleviate his symptoms?