Introduction

Exercise electrocardiographic stress testing (EST) represents a ubiquitous, noninvasive, and low-cost functional test for the evaluation of patients with new-onset angina. However, its use has declined over the past decade because of the higher sensitivity of other noninvasive stress imaging modalities and the perceived high false positive rate of EST. In view of this, EST has been downgraded to a Class 2b recommendation in the latest European Society of Cardiology guidelines. It is important to remember that the accuracy of EST has historically been assessed and validated against its ability to detect the presence of obstructive coronary artery disease (CAD), with the reference standard being visual diameter stenosis on coronary angiography. However, we now know that myocardial ischemia can, and indeed in nearly one-third of cases does, occur in the absence of obstructive CAD due to coronary microvascular dysfunction (CMD). Therefore, it is conceivable that historical false positive EST results were due not to the poor specificity of EST as a diagnostic test but rather to the limitations of obstructive CAD as a reference standard for myocardial ischemia. The aim of this study was to examine the specificity of EST in detecting an ischemic substrate compared against the robust reference standard of coronary endothelium-independent and endothelium-dependent microvascular function in patients with angina and nonobstructive coronary arteries (ANOCA).

Methods

Study population

We prospectively enrolled consecutive patients presenting with angina who were referred for further assessment (Figure 1). Inclusion criteria were ANOCA (fractional flow reserve >0.80) and preserved left ventricular ejection fraction (>50%). Exclusion criteria were inability to undergo adenosine or acetylcholine assessment, chronic kidney disease (estimated glomerular filtration rate <30 mL/min/m2), significant valvular disease, history of acute coronary syndrome, previous revascularization, cardiomyopathy, limitation by nonanginal symptoms, existing bundle branch block, poor electrocardiographic (ECG) traces during exercise, and paced rhythm hindering ECG interpretation. All patients provided written informed consent in accordance with the protocol, which was approved by the UK National Research Ethics Service (20/LO/1294).

Figure 1
Consolidated Standards of Reporting Trials Diagram Demonstrating Study Flow
This diagram demonstrates the number of patients assessed for eligibility and the reasons for exclusion. Overall, 262 patients with stable angina were assessed for eligibility, of whom 102 were excluded because of nonanginal symptoms or absence of symptoms. A total of 160 patients underwent coronary angiography with physiological assessment, of whom 38 were excluded. A total of 122 patients with comprehensive coronary physiological assessment (in response to both adenosine and acetylcholine [ACh]) underwent the mandated exercise ECG stress test, of whom 102 were included in the final analysis. BBB = bundle branch block; CAD = coronary artery disease; LBBB = left bundle branch block.

Intracoronary physiological assessment

Our protocol for systematic evaluation of patients with ANOCA has been described in full previously. Briefly, all coronary physiological measurements were made in the left anterior descending coronary artery. A 0.014-inch dual sensor–tipped intracoronary guidewire was used for the measurement of distal coronary pressure and average peak flow velocity (APV). Aortic pressure was measured using the fluid-filled guide catheter. All patients received 1 mg intravenous midazolam, 200 μg intracoronary glyceryl trinitrate, and 70 U/kg unfractionated heparin prior to angiography and physiological assessment. We first assessed endothelium-independent microvascular function using intravenous adenosine (140 μg/kg/min), followed by endothelium-dependent microvascular function using graded intracoronary infusions of acetylcholine (18 μg/mL acetylcholine solution delivered at 1 mL/min followed by 2 mL/min) via the guide catheter. All intracoronary acetylcholine measurements were made at least 15 minutes after the intracoronary nitrate injection. Patients, researchers, and physiologists were blinded to the results of the coronary physiological assessment.

Physiological data analysis

Signals were sampled at 200 Hz, with data exported into a custom-made study manager program (Academic Medical Center, University of Amsterdam) and analyzed using custom-made software (Cardiac Waves, King’s College London). Coronary flow reserve (CFR) was derived as adenosine-mediated hyperemic APV/basal APV; endothelium-independent microvascular dysfunction was defined as CFR <2.5. Hyperemic (minimal) microvascular resistance (hMR) was calculated as distal coronary pressure/APV during hyperemia. Elevated hMR was defined as hMR ≥2.5 mm Hg/cm/s. Acetylcholine flow reserve (AChFR) was calculated as the ratio of coronary blood flow (CBF) in response to acetylcholine infusion compared with basal CBF; endothelium-dependent microvascular dysfunction was defined as AChFR ≤1.5. The estimation of volumetric flow from Doppler flow velocity also incorporates vessel diameter. Given that acetylcholine can cause either epicardial vasodilation or vasoconstriction, volumetric CBF was calculated as quantitative coronary angiography–derived cross-sectional area × APV × 0.5, with quantitative coronary angiography performed 5 mm distal to the tip of the guidewire. CMD was defined as endothelium-independent and/or endothelium-dependent microvascular dysfunction (ie, CFR <2.5 and/or AChFR ≤1.5). Figure 2 describes our coronary physiological assessment protocol in patients with ANOCA.

Figure 2
Coronary Physiological Assessment Protocol
This is our standard clinical protocol that is used in all patients with angina and nonobstructive coronary arteries to identify an ischemic substrate. All patients undergo coronary angiography followed by intravenous (IV) adenosine assessment. Only patients with fractional flow reserve (FFR) >0.80 were included in this study. Coronary flow reserve (CFR) was calculated as the ratio of hyperemic average peak velocity (APV) in response to adenosine (140 μg/kg/min) and resting APV, with a value of <2.5 used to diagnose endothelium-independent microvascular dysfunction. Patients then underwent assessment with intracoronary (IC) acetylcholine (ACh) infusion (18 μg/mL), and ACh flow reserve (AChFR) was calculated as the ratio of volumetric coronary blood flow (CBF) during ACh infusion and CBF during rest. Volumetric CBF, in turn, was calculated as 0.5 × APV × cross-sectional area 5 mm distal to the Doppler sensor. AChFR ≤1.5 was diagnostic of endothelium-dependent microvascular dysfunction. Patients with CFR <2.5 and/or AChFR ≤1.5 were defined as having coronary microvascular dysfunction (CMD).