Why does ventricular rate increase with physical activity in atrial fibrillation?

In atrial fibrillation, the heart's atria are in a state of uncontrolled electrical activity, leading to an irregular and often rapid heartbeat. During physical activity, the body's demand for oxygen increases, and this requires the heart to pump blood more effectively. One way the body meets this increased demand is through the activation of the sympathetic nervous system.

When physical activity occurs, sympathetic nervous system activity increases, which stimulates the release of catecholamines, such as epinephrine. These hormones enhance the conduction through the AV node, allowing more impulses to be transmitted from the atria to the ventricles. As a result, the ventricular rate increases in response to this heightened sympathetic stimulation.

This mechanism explains why, during exercise or any physical exertion, the rate at which the ventricles contract can rise significantly, contributing to a faster heart rate even in the presence of atrial fibrillation. The increased activation of the AV node by the sympathetic nervous system directly influences the ventricular response to the rapid atrial firing.