International Study of Coronary Microvascular Angina (iCorMicA): a Randomised, Controlled, Multicentre Trial and Registry

Ischaemic heart disease (IHD) includes coronary artery disease (or 'coronary heart disease 'CHD') and ischaemic with no obstructive coronary arteries (INOCA). Coronary angiography is standard care for the evaluation of symptomatic patients and known or suspected coronary artery disease. A considerable proportion e.g. >1 in 3, of patients undergoing invasive coronary angiography for known or suspected angina do not have obstructive epicardial coronary artery disease.

The CorMicA pilot study involved prospective enrolment of patients referred for clinically indicated coronary angiography during a 12-month period (2016-2017) in the West of Scotland). 391 patients were enrolled, 185 (47%) had no obstructive coronary disease and 151 were randomised in a clinical trial of stratified medicine based on invasive tests of coronary vascular function. A high proportion (~4 in 5) of these patients had a diagnosis of INOCA due to a disorder of coronary vasomotion including microvascular- and/or vasospastic angina. The trial provided preliminary evidence that stratified medicine, as an adjunct to standard angiography-guided management, has potential to improve symptoms and quality of life. The mechanisms involved changes in diagnosis, treatment and lifestyle measures.

The CorMicA investigators were the first group to introduce stratified medicine for the management of ischaemic heart disease. Limitations of the CorMicA study included the setting (mainly single centre), partial blinding, short-term follow-up (primary outcome at 6 months) and the sample size. Acetylcholine reactivity testing was used to assess the susceptibility to coronary spasm (microvascular and/or macrovascular). Although acetylcholine is a naturally occurring substance, current formulations are not licensed for parenteral administration. Further, clinicians should have training and experience before implementing acetylcholine coronary reactivity testing. These considerations present a barrier to adoption in daily practice, which becomes all the more relevant given INOCA is generally under-recognised. A diagnostic coronary guidewire is already widely used in standard care and, unlike reactivity testing using acetylcholine, a guidewire has transferable potential to support stratified medicine during routine practice. This is especially the case for ad-hoc follow-on testing when obstructive coronary artery disease is excluded.

The vasoactive responses to acetylcholine reflect endothelial and vascular smooth muscle cell effects, which may overlap with the vasoactive responses to adenosine (non-endothelial dependent). Calcium channel blocker therapy is indicated for the functional endotypes associated with impaired vasodilatation and/or vasoconstriction. This raises the possibility that acetylcholine testing may not be routinely required as an adjunctive test to coronary angiography and instead could be reserved for selected cases, or in specialist centres. Instead, the diagnostic guidewire approach may be sufficient as a routine, first line test for the evaluation of INOCA during daily practice.

Anatomical imaging using coronary angiography is the standard of care and clinicians may determine any diagnosis based on all of the available information and their clinical judgement. This approach avoids the need for additional tests. With this approach, the patient's symptoms in response to empirical therapy can be assessed during follow-up and the treatment can be revised as clinically appropriate. In daily practice, adoption of adjunctive tests of coronary function is very low and, in the absence of large, multicentre trials, coronary angiography with or without adjunctive tests of coronary vascular function may be considered reasonable, reflecting equipoise.