Functional Microvascular Dysfunction

Pathophysiological Distinction

• Functional microvascular dysfunction is characterized by increased resting coronary blood flow linked to enhanced nitric oxide synthase activity, with preserved minimal microvascular resistance during hyperemia (IMR <25), distinguishing it from structural microvascular disease where endothelial dysfunction reduces flow augmentation during exercise 1
• Impaired coronary flow reserve (CFR <2.5) despite normal hyperemic resistance, caused by the elevated baseline flow rather than fixed structural obstruction 1

Diagnostic Criteria

• Symptoms of myocardial ischemia (effort or rest angina, exertional dyspnea) are present in patients with functional CMD 2
• Absence of obstructive CAD (<50% diameter reduction or FFR >0.80) is required for the diagnosis of functional CMD 2
• Impaired coronary flow reserve (CFR <2.5 by thermodilution or Doppler) is a key diagnostic criterion for functional CMD 1, 2
• Normal microvascular resistance indices (IMR <25, HMR <2.5 mmHg/cm/s) during hyperemia are characteristic of functional CMD 1, 2
• Negative acetylcholine provocation test at high doses (100-200 μg intracoronary) rules out microvascular spasm in functional CMD 1

Clinical Significance

• Functional CMD represents approximately 62% of all patients with impaired coronary flow reserve, making it the predominant endotype of microvascular angina, and is characterized by enhanced nitric oxide synthase activity at rest that impairs the ability to further augment flow during stress 1

Therapeutic Implications

• The European Society of Cardiology recommends beta-blockers, such as nebivolol, to improve CFR by 24-33% in functional CMD, although this recommendation is based on a lower level of evidence 3, 4
• However, a more suitable recommendation is: The American College of Cardiology does not provide specific guidance on this topic, but the European Society of Cardiology suggests that ACE inhibitors enhance endothelium-dependent vasodilation and reduce left ventricular stiffness in patients with functional CMD, with a moderate level of evidence 3, 4
• Statins improve endothelial function through anti-inflammatory mechanisms beyond lipid-lowering in patients with functional CMD, with a high level of evidence 3, 4

Prognostic Considerations

• Functional CMD carries significant prognostic implications despite preserved microvascular structure, with an increased risk of major adverse cardiac events, myocardial infarction, heart failure with preserved ejection fraction, and death 3
• Impaired CFR <2.32 is associated with elevated hazard for major coronary events at 10-year follow-up, highlighting the importance of optimizing coronary flow reserve in patients with functional CMD 3
• Objective improvement in CFR to ≥2.5 should be targeted with repeat invasive coronary function testing at 3-6 months after medication optimization, as recommended by the European Society of Cardiology with a moderate level of evidence 4

Coronary Microvascular Dysfunction Diagnosis and Treatment

Pathophysiological Distinction

• The European Society of Cardiology (ESC) Guidelines state that microvascular angina results from both structural and functional changes that may co-exist and contribute to microvascular angina, with most patients having elements of both mechanisms 5, 6
• Structural coronary microvascular dysfunction is associated with cardiovascular risk factors, such as smoking, age, diabetes, hypertension, and dyslipidemia, as well as inflammatory conditions like systemic lupus erythematosus (SLE) and rheumatoid arthritis 5

Diagnostic Algorithm

• The 2024 ESC Guidelines emphasize that invasive coronary function testing is essential to determine the final endotype, allowing for appropriate treatment initiation 5, 6
• Impaired coronary flow reserve (CFR <2.5) is a required diagnostic criterion for coronary microvascular dysfunction, as stated in the ESC Guidelines 5

Clinical Implications

• The ESC Guidelines recommend acetylcholine provocation testing in all patients with angina and no obstructive coronary artery disease (ANOCA) to determine the final endotype and rule out vasospastic angina 5, 6
• If a vasospastic component is present, beta-blockers are absolutely contraindicated, and calcium channel blockers become first-line therapy, as suggested by the 2019 ESC Guidelines 7

Treatment Strategy

• For structural coronary microvascular dysfunction (IMR ≥25), the treatment strategy includes uptitrating beta-blockers, optimizing ACE inhibitor therapy, and continuing aggressive statin therapy, with the goal of improving coronary flow reserve (CFR) and reducing index of microvascular resistance (IMR) 5, 6
• For functional coronary microvascular dysfunction (IMR <25), beta-blockers can improve CFR, and ACE inhibitors can enhance endothelium-dependent vasodilation, as stated in the ESC Guidelines 5, 6

Coronary Microvascular Dysfunction and Non‑Obstructive CAD: Evidence‑Based Guideline Summary

Pathophysiology and Mechanisms

• Reduced epicardial coronary flow without obstructive disease reflects coronary microvascular dysfunction (CMD) or other functional abnormalities; systematic invasive coronary function testing is required to guide targeted therapy that improves symptoms and quality of life. 8, 9, 10
• The 2024 ESC guidelines state that myocardial ischemia frequently occurs in the absence of flow‑limiting epicardial stenosis. 8, 11
• Primary mechanisms include:
  
  • Coronary microvascular dysfunction (CMD) – functional and structural abnormalities of arterioles that regulate flow and resistance. 8, 12
  • Epicardial vasospasm – dynamic constriction of epicardial vessels without fixed stenosis. 8, 13
  • Myocardial bridging – extramural compression of a coronary segment during systole. 11, 13
  • Diffuse non‑obstructive atherosclerosis – endothelial dysfunction causing impaired flow‑mediated vasodilation. 8, 10
• These mechanisms often coexist; the presence of one does not exclude the others. 10, 12

Clinical Significance

• Persistent angina despite non‑obstructive coronary arteries markedly impairs quality of life. 9, 10
• The most dangerous diagnostic error is to reassure patients that ischemia is absent when angiography shows no obstructive lesions, leading to premature discharge, unnecessary pursuit of non‑cardiac diagnoses, and failure to initiate mechanism‑specific therapy. 11, 13

Guideline Recommendations (ESC)

• Class I recommendation: Perform invasive coronary function testing (CFT) in symptomatic patients with angina and non‑obstructive coronary arteries (ANOCA) or ischemia with non‑obstructive coronary arteries (INOCA). 9, 10

Invasive Diagnostic Measurements

Gender‑Specific Findings

• Women exhibit lower CFR than men (p = 0.02) and higher FFR than men (p = 0.03). 10
• Women more frequently present with INOCA/ANOCA. 10

Anatomical vs. Functional Testing

• Ischemia can occur without significant epicardial stenosis, and many obstructive lesions do not produce ischemia. 15
• Positive predictive value of anatomic lesions for ischemia is only 29–44 %. 15, 11

Management Algorithm (Evidence‑Based)

• Step 1 – Confirm non‑obstructive disease: FFR > 0.80. 14
• Step 2 – Perform comprehensive invasive CFT (measure CFR, IMR, TIMI frame count; conduct acetylcholine provocation). 9, 10
• Step 3 – Classify mechanism:
  
  • Vasospasm present → initiate calcium‑channel blockers; avoid beta‑blockers.
  • Structural CMD (IMR ≥ 25) → uptitrate beta‑blockers, add ACE inhibitors, continue statins.
  • Functional CMD (IMR < 25) → beta‑blockers (nebivolol preferred), ACE inhibitors, statins.
• Step 4 – Re‑assessment (3–6 months): repeat invasive CFT; target CFR ≥ 2.5 and document IMR improvement. 9, 10
• Outcome: This mechanism‑specific, stepwise approach improves symptoms, quality of life, and may favorably influence long‑term cardiovascular outcomes. 9, 10