Medicine
Reading an ECG for Acute Myocardial Ischemia
Quick fact
ST-segment depression on an ECG is a classic sign of myocardial ischemia, and it can appear even before a heart attack occurs, offering a critical window for intervention.
Why this is interesting
Imagine a silent electrical storm in your chest that can be caught on paper—but only if you know where to look. How can a simple trace of squiggles tell doctors whether your heart is starved of oxygen?
Read the full explanation
Understanding Reading an ECG for Acute Myocardial Ischemia
An electrocardiogram (ECG) records the heart's electrical activity from the body surface. Each heartbeat produces a characteristic pattern: the P wave (atrial depolarization), the QRS complex (ventricular depolarization), and the T wave (ventricular repolarization). When blood flow to a part of the heart is reduced (ischemia), the affected muscle cells have altered electrical properties, which show up as changes in the ECG waveform. The most important changes for acute ischemia are ST-segment depression (the flat line between QRS and T becomes lower than baseline) and T-wave inversion (the T wave flips downward). These changes often occur in leads that overlie the ischemic region, helping to localize the affected artery.
A deeper explanation
The underlying mechanism involves the disruption of ion gradients across cardiac cell membranes. Under normal conditions, ventricular cells maintain a consistent resting membrane potential, and repolarization occurs uniformly. When ischemia sets in, reduced oxygen supply impairs the sodium-potassium ATPase pump, causing an accumulation of extracellular potassium and a loss of membrane potential. This creates a voltage gradient between the ischemic and normal myocardium during the ST segment, leading to ST-segment depression (in subendocardial ischemia) or ST elevation (in transmural ischemia). T-wave changes reflect abnormal repolarization. Recognizing these alterations is crucial because they indicate reversible damage—if blood flow is restored quickly, the muscle can recover; if not, infarction (cell death) follows. ECG interpretation thus not only diagnoses ischemia but also guides urgent interventions like angioplasty or thrombolysis.