Which of the following ECG disturbances indicates a need to limit exercise intensity?

The presence of a second-degree or third-degree atrioventricular (AV) block on an ECG is significant because it indicates a disruption in the electrical signals between the atria and ventricles of the heart. In these conditions, the heart's ability to properly conduct electrical impulses can be severely impaired, which may lead to a decreased heart rate and inadequate blood flow to the body during exercise.

When exercise intensity is increased, the heart needs to handle more demand for blood and oxygen. If the conduction system is compromised due to a higher-degree block, the heart may not be able to keep up with the physiological demands of exercise, potentially leading to symptoms like dizziness, fainting, or even more severe cardiac complications. Thus, detecting a second-degree or third-degree AV block on an ECG serves as a critical warning sign that exercise intensity should be limited to ensure patient safety.

In contrast, a normal sinus rhythm shows that the heart's electrical activity is normal and can typically support exercise. Sinus tachycardia, while elevated, usually indicates a physiological response to exercise rather than a malfunction. Atrial flutter, although it can be concerning, does not as directly indicate the need to restrict intensity as AV block does; it can still allow for adequate