Angina is form of chest pain that is due to a lack of blood to the heart muscle. Angina is commonly triggered by stress and exertion, and is a common health problem worldwide. The diagnosis and treatment of angina is usually focused on detection of blockages in heart arteries, and relief of this problem with drugs, stents or bypass surgery. However, about one third of all invasive angiograms that are performed in patients with angina do not reveal any blockages. Many of such patients may have symptoms due to narrowings in the very small micro vessels (too small to be seen on an angiogram). The purpose of this research is to undertake a 'proof-of-concept' clinical trial to gather information as to whether routine tests of small vessel function in the heart might help identify patients with a stable coronary syndrome due to a disorder of coronary function (vasospastic or microvascular angina), and appropriately rule out this problem in patients with normal test results. The diagnostic strategy enables stratification of patient sub-groups to optimized therapy (personalised medicine). Evidence of patient benefits in this study would support the plan for a larger study that would be designed to impact on healthcare costs and patient reported outcome measures (PROMS).

Background: Patients with a stable coronary syndromes include those with obstructive coronary artery disease (CAD) or ischaemia with no obstructive CAD (INOCA). Disorders of coronary vasomotion leading to microvascular and vasospastic angina are debilitating, prognostically important health problems. Use of coronary function tests with thresholds (normal/abnormal) that are linked to evidence-based treatment (start/stop) could be useful to diagnose (rule-in/rule-out) these conditions, but, evidence is lacking.

Design: (1) A proof-of-concept, randomised controlled stratified medicine trial of a clinical strategy informed by invasive tests of coronary function and linked guideline-based treatment decisions vs. standard care using angiography only in 150 patients; (2) A nested observational imaging sub-study using quantitative stress perfusion cardiac magnetic resonance (CMR). CONSORT guidelines will be followed.

Objectives: (1) To assess whether a diagnostic strategy involving tests of coronary function changes the diagnosis and treatment and improves health and economic outcomes; (2) To assess the diagnostic accuracy of novel MRI methods for abnormal perfusion due to microvascular disease.

Methods: Patients undergoing invasive coronary angiography for the investigation of known or suspected angina and who do not have either structural heart disease or a systemic health problem that would explain those symptoms will be invited to participate. Written informed consent is required for participation. Eligibility is further confirmed at the time of the coronary angiogram by exclusion of obstructive (>50% stenosis, fractional flow reserve <=0.80) coronary artery disease (CAD). After the angiogram, eligible participants will be randomised immediately in the catheter laboratory to test disclosure (intervention group) or measurement without disclosure (control group). Coronary function will be assessed using a diagnostic guidewire and intra-coronary infusions of acetylcholine (10-6M, 10-5M, -10-4M) and a bolus of glyceryl trinitrate (300 micrograms). The guidewire-derived parameters include fractional flow reserve (FFR), coronary flow reserve (CFR), index of microvascular resistance (IMR) and the resistance reserve ratio (RRR). Participants who are enrolled but not randomised will enter a follow-up registry. The endotypes (diagnostic strata) are: obstructive CAD, coronary vasospastic angina, microvascular angina, endothelial dysfunction (no angina), normal (non-cardiac, normal coronary function results, no angina). Thus, a diagnosis may be ruled-in or ruled-out based on the test results. Microvascular disease will be characterised as structural (abnormal IMR) and/or functional (abnormal CFR, RRR). Primary outcome: Between-group difference in Seattle Angina Questionnaire (SAQ) scores at 6 months; Secondary: Reclassification of the treatment decision; certainty of the diagnosis; health status (EQ-5D, Illness Perception, Treatment Satisfaction & Patient Health questionnaires); angina medication and adherence; health economics; reference clinical decisions as evaluated by an independent expert panel of clinicians. Follow-up will continue in the longer term, including through electronic health record linkage.

Value: To our knowledge, the study is the first to assess the clinical value of invasive management guided by routine use of adjunctive tests of coronary function in appropriately selected patients. The study will provide new insights into disease mechanisms and provide pilot data to inform the rationale and design of a larger clinical trial. The CMR substudy will provide information on the diagnostic utility of quantitative non-invasive imaging methods in this patient population. Should our hypotheses be confirmed, the research will bring new knowledge with potential benefits to patients and healthcare providers.

Inclusion Criteria:

A clinically-indicated plan for invasive coronary angiography. Symptoms of angina or angina-equivalent (according to the Rose- and Seattle Angina questionnaires).

Exclusion Criteria:

A non-coronary indication for invasive angiography e.g. valve disease During the angiogram: obstructive disease evident in a main coronary artery (diameter >2.5 mm), i.e. a coronary stenosis>50% or a fractional flow reserve (FFR) ≤0.80 Lack of informed consent.

Data access may be possible pending Sponsor approval

Participants are randomised to a 'Disclosed group' (Intervention group) or a 'Not disclosed group' (Standard Care). In the standard care group, coronary function parameters are measured but the results are not disclosed to the attending clinician or the participant.