This single-arm, multi-center, prospective study will assess the coronary flow reserve (CFR), index of microcirculatory resistance (IMR), and determinants thereof (transit time, Pa, Pd) in patients before and after intracoronary nitroglycerine.

This will be a single arm, multi-center, prospective interventional study. We will approach, consent, and enroll patients referred to the cardiac catheterization laboratory for coronary angiography for the indication of angina without obstructive coronary artery disease (CAD; lesions <50%, fractional flow reserve (FFR) >0.80, or non-hyperemic pressure index (RFR) <0.89). Patients will undergo invasive assessment with CRT first without IC nitrates followed by a dose of 200-500 mcg of intracoronary nitrates and repeat CRT.

Approximately 50 patients undergoing coronary angiography for evaluation of ANOCA or INOCA will be consented for this study.

The primary outcome will be comparing the variables assessed by CRT with the primary outcome being CFR before and after nitroglycerin administration.

The secondary outcomes will be IMR, Pd, Pa, RRR, and transit times before and after nitrates.

These variables will be compared to one another using paired t-tests and generalized estimating equations.

Inclusion Criteria:

1. Aged 18 or older
2. Patient being evaluated for ANOCA or INOCA
3. Clinical suspicion for angina
4. Patient should be able to comply with the protocol.
5. Provide written informed consent before study participation.

Exclusion Criteria:

1. Existing coronary artery disease
2. Previous percutaneous interventions within the coronaries
3. Current use (within 48 hours) of long-acting nitrate therapies
4. Current use (within 48 hours) of PDE-5 inhibitors (sildenafil, tadalafil)
5. Any other condition or co-morbidity which, in the opinion of the investigator or operator, may pose a significant hazard to the subject if he or she is enrolled in the study.
6. Children below 18 years, prisoners, pregnant people and patients who are unable to provide consent are excluded.