Sir Rod Stewart has called off the remaining dates of his US tour after having what was described as a "routine" heart procedure. The 81-year-old singer has been advised by doctors to recuperate for the next four weeks following a coronary stent procedure. A stent is a small wire tube that can be inserted into coronary arteries, which supply blood to the heart, if they have become narrowed or blocked. "I'm already feeling better and very much on the mend," Sir Rod said on Instagram, external. "I want to thank the doctors, nurses and everyone who has taken such wonderful care of me.