Inclusion Criteria:1) Patient referred for elective coronary angiography becuase of a reasonable clinical suspicion of coronary ischemia.

2) Presence of angina or an anginal equivalent (including chest, back, shoulder, arm, neck, jaw discomfort, or shortness of breath brought on by physical exertion, emotional stress, or certain times of day/month).
 Exclusion Criteria:1) Asymptomatic (such as a pre-op cath)

2) Status-post heart transplant

3) Status-post coronary artery bypass grafting

4) Age <18

5) Renal insufficiency (creatinine >1.5)

6) Presence of an acute coronary syndrome (STEMI or NSTEMI), Tako-tsubo, an abnormal ejection fraction (EF<55%), cardiogenic shock, or recent VT/VF

7) Presence of another likely explanation of chest pain, such as pulmonary hypertension or aortic stenosis

8) History of adverse reaction to any of the medications being used (acetylcholine, nitroglycerin, adenosine, or heparin)

9) Currently taking vasoactive medication (such as nitroglycerin)

10) Inability to provide an informed consent, including an inability to speak, read, or understand English, Spanish, Chinese, Farsi, Japanese, Korean, Russian, or Vietnamese

11) A hearing impairment that won't allow for a typical verbal conversation or a visual impairment that won't allow for reading of the written consent

12) Participation in another study (with the exception of the Stanford Gene-PAD study)

13) A potentially vulnerable subject (including minors, pregnant women, economically and educationally disadvantaged, decisionally impaired, and homeless people)