How is a heart block commonly detected on an ECG?

A heart block is commonly detected on an ECG by observing a prolonged PR interval. In a heart block, the electrical signals that travel from the atria to the ventricles are delayed or disrupted. This results in a longer-than-normal duration of the PR interval, which is the time taken for the electrical impulse to travel from the atria to the ventricles.

A normal PR interval ranges from 120 to 200 milliseconds. If the PR interval exceeds this duration, it indicates a delay in conduction through the atrioventricular (AV) node, which is characteristic of first-degree heart block. In more severe forms of heart block, such as second-degree or third-degree, the pattern will also manifest through irregular PR intervals or dissociation between atrial and ventricular activity, respectively.

Focusing on the other options, increased heart rate might occur in various conditions but is not a definitive indicator of heart block. T wave height and ST segment height are more associated with myocardial ischemia or other cardiac conditions rather than conduction abnormalities like heart blocks. Therefore, the primary indicator of a heart block on an ECG is the elongation of the PR interval.