DAY 20: Evolution of Myocardial Infarction & Q Waves 🩺

Welcome back! Today, we’re diving deep into the fascinating and crucial topic of how myocardial infarctions (heart attacks) evolve over time and how this is reflected in the ECG. Understanding these changes allows us to not only diagnose a heart attack but also to estimate how long it has been going on. Let’s get started!

Learning Objectives 🎯

• 💡 Understand the stages of myocardial infarction (MI) on ECG.
• 📝 Recognize the significance of Q waves in MI diagnosis.
• 📈 Identify the changes in ST segment and T wave morphology during MI evolution.
• 🕰️ Estimate the age of an MI based on ECG findings.
• 🤝 Differentiate between acute, evolving, and old MIs on ECG.

Step-by-Step Content 📝

1. Acute Phase (Minutes to Hours): This is the initial phase where blood flow is suddenly blocked to a portion of the heart. The ECG changes are most dramatic during this period.

• Tall, peaked T waves: These are often the earliest sign of ischemia (lack of oxygen).
• ST-segment elevation: This is the hallmark of an acute STEMI (ST-segment elevation myocardial infarction). The ST segment is elevated above the baseline. Think of it as a “tombstone” appearance. 🪦
• Reciprocal ST-segment depression: ST depression in leads opposite the infarct area can be seen.

2. Evolving Phase (Hours to Days): As time progresses, further ECG changes occur.

• Q wave formation: This is a significant development. Q waves represent the electrical silence of necrotic (dead) tissue. Q waves are considered significant when they are > 0.04 seconds in duration and/or > 1/3 the height of the R wave in that lead.
• R wave decrease: The R wave amplitude may decrease as the heart muscle dies.
• T wave inversion: The T wave becomes inverted, usually after ST-segment elevation has resolved.

3. Chronic/Old Phase (Days to Weeks/Months): In this phase, the ECG shows signs of a completed infarction.

• Persistent Q waves: These remain indefinitely, indicating that a myocardial infarction has occurred.
• Normalized ST segment: The ST segment returns to the baseline.
• T wave may remain inverted or return to normal.

Important Note: Not all MIs develop Q waves. These are called Non-Q wave MIs or NSTEMIs. Also, remember that the location of the infarction (anterior, inferior, lateral, etc.) will determine which ECG leads show the characteristic changes. This is why understanding ECG lead placement is critical! 📍

Q Waves Deep Dive: Q waves represent infarcted tissue. Not ALL Q waves are pathological, though! Small Q waves in leads I, aVL, V5, and V6 are considered normal. Pathological Q waves are wide (>= 0.04 seconds) and deep (>= 1/3 the height of the R wave) in leads where Q waves are NOT normally seen, like V1-V4.

Example: An ECG showing ST-segment elevation in leads II, III, and aVF indicates an inferior MI in the acute phase. Over time, Q waves develop in these leads, and the ST segment normalizes, indicating an evolving/old inferior MI.

Practice Exercise: Look at different ECG strips and try to identify the stage of the MI (acute, evolving, or old) based on the ST segment, T wave, and Q wave morphology.

Quiz Time ❓

1. Which ECG finding is most indicative of an acute STEMI?
   A. T wave inversion
   B. Q wave formation
   C. ST-segment elevation (Correct)
   D. Normal ECG
2. A wide and deep Q wave is most indicative of:
   A. Acute ischemia
   B. Old myocardial infarction (Correct)
   C. Hyperkalemia
   D. Pericarditis
3. ST-segment depression in leads opposite the area of ST-segment elevation is called:
   A. Q wave formation
   B. Reciprocal changes (Correct)
   C. T wave inversion
   D. Bundle branch block
4. In which phase of MI evolution would you expect to see tall, peaked T waves?
   A. Acute phase (Correct)
   B. Evolving phase
   C. Chronic phase
   D. Recovery phase
5. Which of the following ECG changes suggests an older, completed MI?
   A. ST-segment elevation
   B. T wave inversion without Q waves
   C. Persistent Q waves (Correct)
   D. Normal ECG

Summary 💡

Today, we learned how myocardial infarctions evolve on ECG. Recognizing the characteristic changes in ST segments, T waves, and Q waves is crucial for accurate diagnosis and management. Remember the importance of Q waves as markers of past infarctions and the significance of ST-segment elevation in acute STEMIs.

Review and Practice: Review ECG strips showing different stages of MI. Focus on identifying Q waves, ST-segment changes, and T wave morphology. Practice identifying the infarct location based on lead involvement. Consider using online ECG simulators for more realistic practice.

Next Day Preview: Tomorrow, we will explore Bundle Branch Blocks (BBB). We’ll learn how to identify Left Bundle Branch Blocks (LBBB) and Right Bundle Branch Blocks (RBBB) on ECG and their clinical significance.