A 54-year-old man presents after a syncopal episode. The patient has no recollection of the event; according to bystanders, he awakened about 45 seconds after he "fell out." The patient has a history of bipolar disorder managed with quetiapine. He recently had an episode of prostatitis treated with ciprofloxacin. His other medications include lisinopril and hydrochlorothiazide (HCTZ) for hypertension and cyclobenzaprine and a hydrocodone/acetaminophen combination pill for low back pain. On examination, the patient is alert and oriented. Neurological examination is nonfocal. Cardiac examination is unremarkable, without murmur, gallop, or jugular distention. ECG shows nonspecific ST and T wave changes and a prolonged QT interval (QTc of 540 milliseconds). What is the best initial management approach?