Welcome back! Today, we delve into a crucial topic: myocardial ischemia and injury, conditions that can be readily identified on an ECG. Understanding these patterns is critical for timely diagnosis and management of acute coronary syndromes (ACS).

Learning Objectives 🎯

• 💡 Define myocardial ischemia, injury, and infarction.
• 📝 Identify ECG changes associated with ischemia (T wave abnormalities).
• 📈 Recognize ECG changes associated with injury (ST segment elevation or depression).
• 📍 Understand the significance of reciprocal changes.
• ❤️ Correlate ECG findings with the underlying pathophysiology.

Step-by-Step Content 📝

Myocardial Ischemia occurs when the heart muscle doesn’t receive enough oxygen. This is often due to a blockage in a coronary artery. Ischemia is *reversible* if blood flow is restored quickly. The ECG changes associated with ischemia primarily involve the T wave.

T Wave Abnormalities

The most common ECG findings are T wave inversion or tall, peaked T waves. Remember that T wave inversion can also be a normal variant, so clinical context is vital.

• T Wave Inversion: Symmetrical, often deep inversions in the leads overlying the ischemic area.
• Tall, Peaked T Waves: Can be an early sign of ischemia, particularly in hyperacute cases. These are often broad-based and symmetrical.

Myocardial Injury represents a more severe degree of ischemia that, if prolonged, can lead to irreversible damage (infarction). The hallmark ECG finding for injury is ST segment abnormality.

ST Segment Abnormalities

ST segment changes are highly significant and require prompt action.

• ST Segment Elevation: Typically concave upwards (though a straight or convex shape is more concerning). This usually signifies transmural injury (injury across the entire wall of the heart). Think STEMI (ST-Elevation Myocardial Infarction).
• ST Segment Depression: Can indicate subendocardial ischemia or injury (injury to the inner layer of the heart muscle). Also seen in reciprocal changes.

Reciprocal Changes: These are ST segment and T wave changes seen in leads opposite the area of injury or ischemia. They are mirror images of the changes in the affected leads and provide stronger evidence of ACS. For example, ST elevation in anterior leads might be accompanied by ST depression in inferior leads.

ECG Evolution: ECG changes evolve over time during an acute MI. Initially, you might see peaked T waves, followed by ST elevation, then T wave inversion, and eventually, Q waves may develop (indicating infarction). Recognizing this evolution is crucial for understanding the stage of the event.

Summary 💡

Today, we learned that:

• Ischemia primarily affects the T wave (inversion or peaking).
• Injury primarily affects the ST segment (elevation or depression).
• Reciprocal changes enhance the specificity of ECG findings.
• ECG changes evolve over time in acute MI.

Review and Practice 📚

Review the ECG criteria for ischemia and injury. Practice identifying ST segment and T wave abnormalities on various ECG strips. Look for reciprocal changes.
Practice ECGs: Find ECGs online labeled “STEMI,” “NSTEMI,” and “Ischemia” to practice identifying these patterns. Websites like ECGpedia and Life in the Fast Lane are good resources.

Next Day Preview 📅

Tomorrow, we will discuss bundle branch blocks and their impact on ECG interpretation.

Quiz Time ❓

1. Which ECG finding is most indicative of myocardial injury?
   A. Peaked T waves
   B. Q waves
   C. ST segment elevation (Correct)
   D. T wave inversion
2. ST segment depression can indicate which of the following?
   A. Transmural injury
   B. Subendocardial ischemia (Correct)
   C. Hyperkalemia
   D. Pericarditis
3. T wave inversion typically indicates which of the following?
   A. Hyperacute myocardial infarction
   B. Myocardial ischemia (Correct)
   C. Atrial fibrillation
   D. Normal variant
4. Reciprocal changes are important because they:
   A. Decrease the specificity of ECG findings
   B. Increase the specificity of ECG findings (Correct)
   C. Indicate the presence of hyperkalemia
   D. Have no clinical significance
5. Which of the following is an early ECG finding in myocardial ischemia?
   A. Q waves
   B. Tall, peaked T waves (Correct)
   C. ST segment depression
   D. Pathologic Q waves

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