Summary of "ST Elevation vs ST Depression"

Summary of “ST Elevation vs ST Depression” by Dr. Mike

The ECG uses 12 leads (6 precordial on the chest and 6 limb leads) to view the heart’s electrical activity from different angles.
Each lead detects electrical signals moving towards or away from it, producing characteristic waves on the ECG trace.
Key waves explained:
- P wave: atrial depolarization (electrical activity moving towards the lead → upward bump).
- Q wave: interventricular septum depolarization (electrical activity moving away from the lead → downward dip).
- R wave: ventricular depolarization (mainly left ventricle, thick muscle → large upward bump).
- S wave: final depolarization moving away from the lead → downward dip.
- T wave: ventricular repolarization.

Occurs when heart muscle (myocardium), especially ventricles, does not receive enough oxygen/nutrients due to blocked coronary arteries.
Focus on the Left Anterior Descending (LAD) artery, which supplies the left ventricle and interventricular septum.
Blood supply is layered:
- Epicardium (outer)
- Myocardium (middle)
- Endocardium (inner)
Oxygen/nutrient delivery is greatest to epicardium and least to endocardium.
Tissue dies progressively: endocardium first, then myocardium, then epicardium.

Subendocardial infarct: damage limited to inner layers (endocardium).
Transmural infarct: full thickness damage through the entire myocardial wall.

Resting membrane potential is maintained by the sodium-potassium ATPase pump (3 Na⁺ out, 2 K⁺ in) and potassium leak channels.
Healthy cells maintain a charge difference: positive outside, negative inside.
Depolarization occurs when the inside of the cell becomes more positive.

In infarcted (dying) cells, ATP-dependent channels fail, causing potassium to leak out excessively into the extracellular space.
This extracellular potassium accumulation affects neighboring healthy cells:
- Healthy cells surrounded by high extracellular potassium cannot leak potassium out easily.
- This causes these cells to become more positive inside (early depolarization at rest).

The ECG baseline represents the isoelectric line when no electrical activity occurs.
Early depolarization of healthy cells near infarcted tissue shifts the baseline due to altered resting potentials.
In Subendocardial Infarct:
- Potassium leaks extracellularly on the side facing the lead.
- Nearby healthy cells depolarize early towards the lead.
- This causes the baseline (isoelectric line) to elevate.
- The apparent “ST depression” is actually an elevation of the baseline (everything else segment).
In Transmural Infarct:
- Potassium leaks extracellularly on the side away from the lead.
- Nearby healthy cells depolarize early away from the lead.
- This causes the baseline to depress.
- The apparent “ST elevation” is actually a depression of the baseline.
The terms ST elevation or depression refer to changes relative to the shifted baseline caused by these early depolarizations.

ST segment changes in ECG during heart attacks are due to altered resting membrane potentials caused by potassium leakage from dying cells.
The direction of potassium leakage and subsequent early depolarization relative to the ECG lead determines whether the ST segment appears elevated or depressed.
Subendocardial infarcts cause baseline elevation (seen as ST depression).
Transmural infarcts cause baseline depression (seen as ST elevation).

Review normal ECG waveforms and their relation to electrical activity direction relative to leads.
Understand heart anatomy and coronary blood supply, focusing on the LAD artery and myocardial layers (epicardium, myocardium, endocardium).
Explain the progressive nature of myocardial infarction (subendocardial vs transmural).
Describe resting membrane potential maintenance and how ATP depletion affects potassium channels in infarcted cells.
Detail how potassium leakage from dying cells alters extracellular potassium concentration.
Explain how elevated extracellular potassium causes early depolarization in adjacent healthy cells.
Relate direction of early depolarization relative to ECG lead to baseline shifts (elevation or depression).
Clarify that ST segment changes represent shifts in baseline due to these ionic changes, not just simple elevation or depression of the ST segment itself.