Heartbeat is about the exhaustion of feeling trapped inside your own emotions. The speaker begins by rejecting the usual image of a heart that "thrums" or "races." Instead, theirs is heavy, weighed down, and that choice tells us this isn't excitement or fear but something closer to despair.
The central image is a wild creature hurling itself against a cage. What stands out is the contradiction in how the speaker describes it: the cage is "newly formed," yet also "familiar." It is still "unsafe," but the heart has been in it so long that leaving feels impossible. The speaker says it wants freedom but lacks the "sanity nor strength" to reach it, which suggests a mind and body both worn thin.
The repeated "again / again / again" gives the poem its pulse, mimicking both a heartbeat and the pointless battering against bars. The ending is bleak: the heart simply stops having the will to "stand or think to fight." The mood is tired, trapped, and quietly hopeless.