Makindo Medical Notes

🫀 Myocardial Perfusion Imaging (MPI) relies on stressing the heart to unmask regions with impaired blood flow. By raising oxygen demand—through 🏃 exercise or 💊 pharmacological stress—ischemic areas supplied by stenotic coronary arteries appear underperfused (“cold”) compared to normal myocardium. This contrast helps detect coronary artery disease (CAD) before overt infarction.

🔎 About

• Based on the principle that stenoses >70% limit flow during stress, but not always at rest.
• Well-perfused myocardium = normal uptake/motion ✅; Ischemic myocardium = reduced uptake or abnormal contraction ❌.
• Techniques vary (nuclear, echo, angiography), but all test the supply-demand mismatch.

📡 Common Diagnostic Modalities

• 🏃 Exercise Stress Test: ECG monitoring looks for ST depression as a marker of ischemia. Non-invasive, but less sensitive/specific than imaging modalities.
• 💉 Dobutamine Stress Echocardiography: Dobutamine ↑ HR and contractility → ↑ O₂ demand. New wall motion abnormalities = ischemia.
• ☢️ Technetium Radionuclide Imaging: Radioactive tracer (e.g., Tc-99m sestamibi) injected → distribution imaged. “Cold spots” after stress = ischemic myocardium.
• 🧪 Thallium / MIBI / Myoview Scans: Thallium behaves like potassium → taken up by viable cells. Reversible ischemia = stress defect that fills in on delayed imaging. Technetium agents → sharper images, shorter half-life.
• 💊 Adenosine or Dipyridamole Stress: Potent vasodilators → normal vessels dilate, stenotic ones do not. Creates “steal phenomenon,” highlighting perfusion defects. Preferred if patient cannot exercise.
• 🩻 Coronary Angiography: Gold standard → direct contrast imaging of coronary lumen. Invasive, but defines exact location/severity of stenosis.
• 📊 MUGA Scan: Radionuclide ventriculography assessing LV function. Provides EF and wall motion info but not perfusion directly.

📊 Comparison Table

💡 Teaching Pearls: - MPI detects flow-limiting stenosis, not early plaque. - Reversible defect = ischemia; Fixed defect = infarct. - Adenosine stress = beware in asthmatics (bronchospasm). - Always check renal function before angiography due to contrast.