What diagnostic tool is primarily used to assess the electrical activity of the heart?

The primary diagnostic tool used to assess the electrical activity of the heart is the electrocardiogram (ECG). An ECG records the heart's electrical signals, which allows healthcare providers to evaluate the heart's rhythm, identify any irregularities, observe how the heart responds to stress, and detect various cardiac conditions such as arrhythmias, ischemia, or infarction.

The ECG provides critical information about the timing and duration of each electrical phase in the heart's cycle. This is vital for diagnosing different types of heart disease and guiding appropriate treatments. In contrast, the other options serve different purposes: an echocardiogram uses sound waves to provide images of the heart structure and function; X-ray imaging shows the structure of bones and volumes of the heart but does not assess electrical activity; and a CT scan can create detailed cross-sectional images of the heart and surrounding tissues but, similarly to X-rays, is not designed to assess the heart's electrical functions. Thus, the electrocardiogram is uniquely suited for evaluating the electrical activity of the heart.