What does the ECG primarily measure in terms of heart function?

The ECG, or electrocardiogram, primarily measures the electrical activity of the heart. This is crucial because the heart's ability to pump blood effectively is dependent on the coordinated contraction of its muscle fibers, which is triggered by electrical impulses generated by the heart's natural pacemaker, the sinoatrial (SA) node.

The ECG records these electrical signals by placing electrodes on the skin, which capture the depolarization and repolarization processes occurring in the atria and ventricles during each heartbeat. This activity can reveal the rhythm of the heart, identify areas of ischemia, detect arrhythmias, and help assess overall cardiac health.

While aspects like muscle strength, blood circulation, and heart size are important, they are not directly measured by the ECG. Instead, these factors can be inferred from the electrical patterns that the ECG displays or require different diagnostic methods for evaluation.