Introduction to Myocardial Infarction (MI)

• Myocardial infarction (MI), commonly known as a heart attack, is a serious medical condition that occurs when blood flow to the heart muscle is severely reduced or completely blocked.
• This blockage is typically caused by the build-up of plaque in the coronary arteries, a condition known as atherosclerosis.
• Plaques consist of fat, cholesterol, and other substances that can accumulate and harden within the artery walls.

Etiology of Myocardial Infarction

• The most common cause of MI is coronary artery disease (CAD).
• The INTERHEART study identifies several modifiable risk factors for CAD, including:
  1. Smoking
  2. Abnormal lipid profile (high ApoB/ApoA1 ratio)
  3. Hypertension
  4. Diabetes mellitus
  5. Abdominal obesity (waist-hip ratio > 0.90 for men, > 0.85 for women)
  6. Psychosocial factors (stress, depression, major life events)
  7. Low fruit and vegetable consumption
  8. Lack of physical activity
• Other risk factors include:
  • Family history of MI
  • Moderately high plasma homocysteine levels, which can be treated with B vitamins and folate.

Pathophysiology of Myocardial Infarction

• MI occurs when one or more coronary arteries are blocked for an extended period, typically 20–40 minutes.
• This blockage is usually due to a blood clot forming at the site of a ruptured plaque within the artery.
• The interruption of blood flow leads to oxygen deprivation (ischaemia) in the affected area of the myocardium.
• Prolonged ischaemia disrupts vital cellular processes:
  • Sarcolemma (cell membrane) is damaged.
  • Myofibrils (muscle fibres) relax.
  • Mitochondria malfunction, leading to a loss of energy production.
• Without adequate oxygen, myocardial cells die, a process called necrosis.
• Necrosis begins in the subendocardium (innermost layer of the heart wall) and progresses towards the subepicardium (outermost layer).
• The infarcted area heals by forming scar tissue, which is less functional than healthy myocardium and can lead to long-term complications like heart failure.

Types of Myocardial Infarction

1. ST-Elevation Myocardial Infarction (STEMI):
   • Caused by a complete blockage of a major coronary artery.
   • Diagnosed by characteristic changes on an electrocardiogram (ECG), including ST-segment elevation.
2. Non-ST Elevation Myocardial Infarction (NSTEMI):
   • Often caused by a partial blockage but can also occur with a complete blockage.
   • Diagnosed by elevated cardiac biomarkers (e.g., troponin) without ST-segment elevation on ECG.

Myocardial Infarction with Non-Obstructive Coronary Arteries (MINOCA)

• MINOCA is diagnosed when a patient meets the criteria for MI but does not have significant blockage (<50% narrowing) in the major coronary arteries on angiography.
• MINOCA accounts for approximately 6% of all MI cases.
• Causes of MINOCA include:
  • Plaque disruption, leading to micro-clots or spasm.
  • Coronary thromboembolism.
  • Spontaneous coronary artery dissection (SCAD).
  • Coronary artery spasm.
  • Coronary microvascular dysfunction (CMD).
  • Myocardial bridging.

Diagnosing MINOCA

• A multi-modality approach is required to identify the underlying cause:
  • Cardiac magnetic resonance (CMR) is crucial for:
    • Confirming MI and excluding other conditions that mimic MI (e.g., myocarditis, Takotsubo cardiomyopathy).
    • Identifying the pattern of myocardial damage, which can suggest the mechanism of injury.
  • Intra coronary imaging, particularly optical coherence tomography (OCT), is important for:
    • Visualising plaque disruptions not visible on angiography.
    • Characterising plaque morphology (e.g., rupture, erosion).
  • Coronary function testing (e.g., acetylcholine provocation test) may be needed to diagnose coronary spasm or microvascular dysfunction.

Prognosis of MINOCA

• While generally considered more favourable than MI due to CAD, MINOCA is not benign.
• There are long-term risks of death, recurrent MI, stroke, and heart failure.
• Risk stratification using CMR, OCT, and functional testing is essential for guiding treatment.

Conclusion

• Understanding the pathophysiology of MI and MINOCA is crucial for pharmacists involved in the care of these patients.
• Accurate diagnosis and identification of the underlying cause are essential for developing appropriate treatment and secondary prevention strategies.

References

1. Cleveland Clinic. (n.d.). Heart Attack (Myocardial Infarction). Retrieved from Cleveland Clinic.
2. Mayo Clinic. (n.d.). Heart Attack: Symptoms and Causes. Retrieved from Mayo Clinic.
3. Eltahawy, E., & Sharma, S. (2021). Myocardial Infarction. In StatPearls. StatPearls Publishing. Retrieved from StatPearls.
4. Benjamin, E. J., Muntner, P., & Bittencourt, M. S. (2015). Heart disease and stroke statistics—2015 update: A report from the American Heart Association. Circulation, 131(4), e29-e322. Retrieved from PubMed.
5. Ogawa, H., An, Y., & Morimoto, T. (2023). Advances in antithrombotic therapy for patients with acute myocardial infarction: A review. Journal of Cardiology, 82(3), 229-238. Retrieved from ScienceDirect.