Contemporary left ventricular assist device (LVAD) therapy improves survival during advanced heart failure but vascular aging develops rapidly leading to major adverse events including stroke and bleeding in nearly half of patients. In this study, the study team aims to investigate whether sildenafil pharmacotherapy, which has anti-fibrotic effects, can reduce vascular aging during LVAD support. An aim of this study is to compare changes in small blood vessels in the gastrointestinal tract between participants receiving sildenafil or placebo. Video capsule endoscopy (VCE) will be used to assess these changes in small blood vessels.