The purpose of this study is to determine if regadenoson is as safe and effective as adenosine when used in the cardiac catheterization lab during measurement of coronary flow reserve and fractional flow reserve. The study hypothesis is the assessment of Fractional Flow Reserve (FFR) in the catheterization lab can be performed with equivalent accuracy when hyperemia is induced with IV Regadenoson compared with IV Adenosine without compromising patient safety.

Patients with clinical indications to be assessed by fractional flow reserve (FFR) will be included. Following diagnostic angiography in the catheterization lab, the patient will have a guide catheter placed in the artery to be assessed followed by placing a pressure guidewire down the coronary vessel and past the lesion of interest. FFR will be measured in the traditional manner, with intravenous adenosine infusing at 140 mcg/kg/min until maximal steady state hyperemia is reached. At this point, the FFR will be recorded as Pd (distal pressure from the pressure wire in the coronary artery) divided by Pa (proximal pressure from the guide catheter). The adenosine infusion will then be stopped and the patient's hemodynamics allowed to return to baseline. Once baseline state is again achieved, an intravenous bolus of regadenson (0.4 mg) will be given and FFR will be remeasured as the lowest steady state Pd/Pa. Patients will be enrolled at two sites, The University of Florida - where FFR will be measured alone, and Saint Louis University, where coronary flow will be measured simultaneously to evaluate the effect of regadenoson on coronary flow velocity as compared to adenosine. The primary endpoint of the study will be to compare the FFR achieved with adeonsine to that obtained with regadenoson.

Inclusion Criteria:

• male or female patients greater than 18 years old (If female, pt. must be post-menopausal, surgically sterile, or be non-pregnant as determined by a negative urine or serum beta human chorionic gonadotropin (B-HCG) pregnancy test within 24 hours prior to enrollment.
• Provided written consent approved by Institutional Review Board and provided Health Insurance Portability and Accountability Act (HIPAA) authorization
• Have at least one coronary stenosis (greater or equal to 40% but less than 70& narrowing by visual inspection) and technically accessible coronary artery into which the pressure wire may be introduced.

Exclusion Criteria:

• ST elevation myocardial infarction
• Cardiogenic shock
• Pregnancy
• Total vessel occlusion
• Extremely tortuous coronary arteries
• Second and third degree heart block without pacemaker
• Severe chronic obstructive pulmonary disease and active bronchospasm
• Less than age 18 years
• Have received theophylline, aminophylline, pentoxifylline or dipyridamole within 12 hours of FFR measurement.
• Has severe 3 vessel disease defined by >80% luminal narrowing by visual inspection
• Known hypersensitivity to adenosine or regadenoson
• Recent uncontrolled ventricular arrhythmia
• History of greater than Type I atrioventricular block, symptomatic resting bradycardia, sick sinus syndrome (without permanent pacemaker)
• History of heart transplantation

Patients with intermediate to severe, single vessel or multi-vessel, angiographic coronary artery disease(visual 40%-70% angiographic stenosis) whose lesion assessment with FFR is planned, were eligible for enrollment. A total of up to 50 subjects(25 at University of Florida Health Science Center and 25 at St. Louis University) will be enrolled.