Lecture Notes: Myocardial Infarction and ECG Changes

Overview

Dr. Mikey explains how myocardial infarction can lead to ST elevation or ST depression on an ECG. The lecture focuses on the electrical changes in heart muscle cells and how these relate to ECG findings during ischemic events.

Normal Heart Muscle Cell Function

• Cardiomyocytes: Cells of heart muscle.
• Ion Concentration:
  • Sodium (Na+) is predominantly outside the cell.
  • Potassium (K+) is predominantly inside the cell.
• Ion Movement:
  • Sodium wants to enter the cell but can't due to closed channels.
  • Potassium exits the cell through open channels, making the inside of the cell more negative.
• Resting Membrane Potential:
  • Around -90 millivolts when the cell is at rest.
  • Depolarization occurs when the inside of the cell becomes more positive, usually via sodium influx.

Depolarization and Action Potential

• Depolarization Trigger: Positive sodium ions enter, changing the membrane potential.
• Threshold: Occurs around -70 millivolts, initiating a full depolarization with sodium influx.
• Calcium Role: Influx leads to muscle contraction.
• Repolarization: Occurs as calcium channels close and potassium continues to exit.

Potassium Concentration Changes

• Increased External Potassium: Results in less potassium leaving the cell.
• Effect on Membrane Potential: Potential becomes less negative (e.g., -50 millivolts), leading to early depolarization and repolarization.

ECG Changes in Myocardial Infarction

ST Elevation

• ST-Elevated Myocardial Infarction (STEMI):
  • Occurs when the ST segment is elevated on an ECG.
  • Represents full-thickness damage (transmural ischemia) across the heart muscle.

ST Depression

• ST Depression:
  • Indicates subendocardial ischemia (partial thickness).
  • Occurs when part of the heart muscle is underperfused, often due to limited blood flow from coronary artery blockage.
  • Potassium leakage causes early depolarization, affecting the ECG reading.

Mechanism of Ischemia and ECG

• Subendocardial Ischemia:

  • Affects inner layers first, leading to ST depression.
  • Potassium channel opening due to lack of ATP.
  • Early depolarization affects isoelectric point, causing apparent ST depression.

• Transmural Ischemia:

  • Affects the full thickness of heart muscle.
  • Depolarization moves away from the ECG lead, leading to a lower starting point for the isoelectric line before QRS, seen as ST elevation on ECG.

Conclusion

• Understanding potassium dynamics and ischemia types is crucial for interpreting ST segment changes in myocardial infarction.
• ST elevation and depression are indicators of the extent and severity of ischemia.