Fig. 1.

Diagnostic strategy for suspected coronary microvascular dysfunction. When coronary microvascular dysfunction (CMD) is suspected based on signs and/or symptoms of ischemia in the absence of explanatory obstructive epicardial coronary artery disease (CAD), the following non-invasive and invasive methodologies are recommended for further evaluation. Measurement of coronary flow reserve (CFR) by positron emission tomography (PET) and cardiac magnetic resonance imaging (CMR) received a 2a recommendation in the recent AHA/ACC 2021 Chest Pain Guidelines [62], while stress echocardiography (TTE) with Doppler assessment of left anterior descending artery (LAD) flow velocity reserve received a 2b recommendation for patients with persistent stable chest pain and nonobstructive CAD. CFR or coronary flow velocity reserve values below 2 suggest CMD. For those patients that also have at least mild ischemia on imaging, invasive testing was given a 2a recommendation. On invasive coronary function testing, in addition to CFR, the index of myocardial resistance (iMR) can be calculated based on the distal pressure measured by the intracoronary wire and the transit time of the saline bolus between the proximal and distal temperature sensors. This is a measure that specifically targets the microcirculation (the hyperemic microvascular resistance, or HMR is an analogous calculation with measurements obtained using a Doppler-pressure intracoronary wire). Advantages and disadvantages of the different modalities are discussed