Cardiac allograft vasculopathy (CAV) is a progressive disease of the coronary arteries in transplanted hearts which is a significant cause of morbidity and mortality. The broad objective of this research study is to advance our ability to diagnose as early as possible the presence of CAV and to non-invasively predict those patients at increased risk of CAV with novel techniques. Optical coherence tomography (OCT) is a novel intracoronary imaging technique using an optical analog of ultrasound with a resolution 10 times greater resolution than intravascular ultrasound (IVUS). Endothelial progenitor cells (EPCs) in peripheral blood have been shown to play a role in the pathogenesis of atherosclerosis and peripheral arterial tonometry is a clinical tool used to predict endothelial dysfunction (a precursor of atherosclerosis) which has been validated in non-transplant patients. Patients scheduled for routine cardiac catheterization with IVUS at the Mayo Clinic Rochester, Minnesota (MN) that reach inclusion and exclusion criteria for the study will be approached on the day to get informed consent to perform OCT, blood sampling and peripheral endothelial function testing. The investigators aim to 1) compare the frequency and plaque type of CAV as defined with OCT versus IVUS-Virtual Histology (IVUS-VH), 2) predict the presence and severity of CAV with absolute counts of EPCs and 3) with peripheral endothelial function scores.