What is the time difference between a recent myocardial infarction (MI) and an old myocardial infarction (MI)?

Time Difference Between Recent and Old Myocardial Infarction

According to established guidelines, a recent myocardial infarction (MI) is considered to be one that occurred within 28 days, while an old MI is one that occurred 29 days or more after the event. 1

Pathological Classification of MI by Time

The temporal classification of myocardial infarction follows a well-defined progression based on pathological changes:

• Evolving MI: ≤6 hours after onset
• Acute MI: 6 hours to 7 days after onset
• Healing MI: 7-28 days after onset
• Healed MI: ≥29 days after onset 1

This classification is based on the cellular changes that occur in the myocardium following infarction:

• Acute MI is characterized by the presence of polymorphonuclear leukocytes (neutrophils) in the infarcted tissue
• Healing MI shows mononuclear cells and fibroblasts with absence of polymorphonuclear leukocytes
• Healed MI is manifested as scar tissue without cellular infiltration 1

Clinical vs. Pathological Timing

It's important to note that clinical and electrocardiographic timing may not correspond exactly with pathological timing:

• The ECG may still show evolving ST-T changes
• Cardiac biomarkers may remain elevated (implying a recent infarct)
• Pathologically, the infarction may already be in the healing phase 1

Epidemiological Classification for Research Purposes

For epidemiological and research purposes:

• Incident MI: The individual's first MI
• Recurrent MI: An MI that occurs after 28 days following an incident MI
• Reinfarction: An acute MI that occurs within 28 days of an incident or recurrent MI 1

Biomarker Considerations

Cardiac troponin values may remain elevated for 7-10 days or longer after myocardial necrosis, which can complicate the timing assessment of recent events 1. This is particularly important when evaluating patients with suspected reinfarction.

Healing Process Timeline

The complete healing process of an infarcted myocardium typically requires at least 5-6 weeks 1. During this time, the myocardium progresses through the following stages:

1. Cell death begins within minutes of ischemia (as little as 20 minutes)
2. Complete necrosis of myocardial cells requires 2-4 hours or longer
3. Macroscopic or microscopic evidence of necrosis becomes visible after several hours
4. The entire healing process leading to scar formation takes 5-6 weeks 1

Clinical Implications

Understanding the time difference between recent and old MI has important clinical implications for:

• Determining appropriate treatment strategies
• Assessing risk of complications
• Evaluating prognosis
• Planning cardiac interventions

For example, the risk of operative mortality for coronary artery bypass grafting is significantly higher when performed between 6 hours and 7 days after MI, compared to operations performed more than 30 days after MI 2.

The 28-day cutoff between recent and old MI provides clinicians with a standardized framework for patient management and research classification, aligning with the natural progression of myocardial healing.