Medicine
ECG Interpretation of Ischemic Changes
Quick fact
In a heart attack (STEMI), the ST segment becomes elevated, while during angina (ischemia without cell death), it typically becomes depressed.
Why this is interesting
A routine ECG can be the difference between a mild warning and a life-threatening emergency. Why do subtle squiggles on a paper strip hold such power to predict heart attacks?
Read the full explanation
Understanding ECG Interpretation of Ischemic Changes
Think of the heart as a muscular pump powered by tiny electrical signals. An electrocardiogram (ECG) records these signals as waves on a moving strip of paper. The most important parts for spotting ischemia are the ST segment (the flat line between the QRS complex and T wave) and the T wave (the bump after the ST segment). When blood flow to a part of the heart is reduced (ischemia), the electrical properties of that region change. This causes the ST segment to shift up or down compared to the normal baseline. If a coronary artery is completely blocked, the ST segment often elevates (ST elevation), which is a red flag for a heart attack. If the block is partial, you might see ST depression instead. These changes can also affect the T wave, making it inverted (pointing down).
A deeper explanation
The ST segment and T wave reflect the repolarization phase of the cardiac action potential. During ischemia, the affected heart muscle cells have an altered resting membrane potential and action potential duration. This creates a voltage difference between the ischemic and healthy regions, producing a current that shifts the ST segment. In transmural ischemia (full-thickness wall involvement, often due to complete occlusion), the ST vector points toward the injured area, causing ST elevation on the leads facing that area. In subendocardial ischemia (inner layer of the heart, often due to partial blockage), the ST vector points away, leading to ST depression. T-wave changes arise from altered repolarization timing and shape. These ECG patterns not only confirm ischemia but also localize the affected artery and guide immediate interventions like angioplasty or thrombolysis.