What information does a 12-lead ECG provide?

• A. Only heart rhythm
• B. Electrical activity of the ventricles only
• C. Contraction and relaxation of atria and ventricles
• D. Blood oxygenation

Answer: (C)

The main idea is that a 12-lead ECG maps the heart’s electrical activity, which drives the contraction and relaxation of the chambers. The P wave shows atrial depolarization (leading to atrial contraction), the QRS complex shows ventricular depolarization (ventricular contraction), and the T wave shows ventricular repolarization (ventricular relaxation). Because the ECG records these electrical events from multiple viewpoints, it provides information about the timing and coordination of both atrial and ventricular contraction and relaxation. It does not measure blood oxygen levels, and it covers more than just rhythm by reflecting how the electrical activity propagates through the atria and ventricles.