A 68-year-old male with a background of AF is seen in the clinic following an episode of syncope. Prior to this, he described having a poor nights sleep and had some dizzy spells in the morning where he remembered suddenly feeling all the lights were dimmed around him. The event was witnessed by his wife who further added he appeared pale and suddenly dropped to the floor, with arms and legs shaking for a few seconds. He was able to regain consciousness in about 2 mins. He is on aspirin, furosemide and atenolol. What is the most likely cause of his syncope?