Concurrently Low Coronary Flow Reserve and Low Index of Microvascular Resistance Are Associated With Elevated Resting Coronary Flow in Patients With Chest Pain and Nonobstructive Coronary Arteries

Background: Coronary microvascular function can be distinctly quantified using the coronary flow reserve (CFR) and index of microvascular resistance (IMR). Patients with low CFR can present with low or high IMR, although the prevalence and clinical characteristics of these patient groups remain unclear. Methods: One hundred ninety-nine patients underwent coronary microvascular assessments using coronary thermodilution techniques. A pressure-temperature sensor-tipped guidewire measured proximal and distal coronary pressure, whereas the inverse of the mean transit time to room temperature saline was used to measure coronary blood flow. The CFR and IMR were quantified during adenosine and acetylcholine hyperemia. Results: Low adenosine and acetylcholine CFR was observed in 70 and 49 patients, respectively, whereas low CFR/low IMR to adenosine and acetylcholine was observed in 39(56%) and 19(39%) patients, respectively. Despite similar adenosine CFR, patients with low CFR/low IMR had increased resting (2.8 +/- 1.2 versus 1.3 +/- 0.4s(-1)) and hyperemic coronary blood flow (4.8 +/- 1.5 versus 2.1 +/- 0.5s(-1)) compared with patients with low CFR/high IMR (both P<0.01). The same pattern was observed in response to acetylcholine. Patients with low CFR/low IMR to adenosine were younger (56 +/- 12 versus 63 +/- 10 years), women (84% versus 66%), had fewer coronary risk factors (1.1 +/- 1.0 versus 1.6 +/- 1.1), lower hemoglobin A1c (5.8 +/- 0.7 versus 6.1 +/- 0.9 mmol/L), and thinner septal thickness (8.5 +/- 2.5 versus 9.9 +/- 1.6 mm) compared with patients with low CFR/high IMR to adenosine (all P<0.05). Conclusions: Low CFR/low IMR to adenosine and acetylcholine are associated with elevated resting coronary blood flow and preserved hyperemic coronary blood flow. These patients present with distinct phenotypic characteristics. Simultaneous CFR and IMR measures appear necessary to differentiate these endotypes.