Final answer:
Chronic tachycardia is most likely related to a compromised sinoatrial (SA) node, which is responsible for initiating the heart's sinus rhythm and setting the pace for the heartbeat.
Step-by-step explanation:
When someone experiences chronic tachycardia, the structure of the heart most likely compromised is the sinoatrial (SA) node. The SA node has the highest inherent rate of depolarization and is known as the pacemaker of the heart, initiating the sinus rhythm. If the SA node is overstimulated or damaged, it could lead to an abnormally high heart rate, which is known as tachycardia. The atrioventricular (AV) node can be involved as it regulates the impulse transmission to the ventricles, ensuring that the heart beats in a coordinated fashion. If the AV node fails to control the impulses effectively due to overstimulation or damage, this can also contribute to tachycardia. However, the primary role in setting the pace of the heartbeat under normal conditions lies with the SA node.