Allograft Dysfunction in Heart Transplant

Inclusion Criteria:

* Age greater than or equal to 18 years old.
* At least three months status post heart transplantation.

Exclusion Criteria:

* Biopsy proven acute rejection episode in the past 3 months.
* Patients with symptoms or signs of acute myocardial ischemia or recent acute coronary syndrome in the past 3 months.
* Uncontrolled obstructive ventilatory disease including asthma and COPD.
* History of generalized tonic-clonic seizures.
* Second or third degree AV nodal block.
* Sinus node dysfunction.
* Contraindications to MRI including cardiovascular implantable electronic devices.
* Renal dysfunction with an estimated GFR less than 30 mL/min/1.73m2.
* Prior adverse reaction to either regadenoson or gadolinium contrast. Prior adverse reaction to adenosine or dipyridamole will be assessed on a case-by-case basis.
* Systolic blood pressure greater than 180 or less than 85 mmHg.
* Diastolic blood pressure greater than 120 or less than 40 mmHg.
* Resting heart rate greater than 120 or less than 45 beats per minute. - Severe claustrophobia.