The speaker lies awake at night, listening to their own heartbeat, and feels completely empty. The poem opens by admitting that this hollowness is almost unbearable, and the rest of it follows from that confession. In the quiet, the heartbeat becomes impossible to ignore, and the speaker hears it not as something alive but as a machine going up and down with no real purpose.
That comparison is the heart of the poem. A heart usually stands for love, warmth and life, but here it is just a "monotonous organ" doing its job. Words like "metallic" and "drudging" make the beat sound cold, heavy and tired. The speaker's body keeps working even though they can't find any meaning in going on.
The mood is bleak, lonely and numb. Nothing dramatic happens. The poem's power comes from its flat, repetitive feeling, which mirrors the dull rhythm it describes.