Seven short lines, and the first thing that happens in them is a sound: "A great knocking in me." Before we know whose body this is or what has gone wrong, we hear a pulse turned violent, as if someone were pounding on a door from the inside. The title names the event flatly, and the poem then declines to describe it in medical terms. Instead it offers wood, a shoreline and a falling tide.
The second line is where things get strange: "and I am down, I am wood." Being "down" is literal enough, a collapse. But "wood" does double duty. A body gone rigid and heavy might feel like timber, and since something is knocking, the word also recalls the habit of knocking on wood to ward off bad luck. Here the charm has failed, or become a grim joke, because the speaker has turned into the wood being knocked on. The repeated "I am" gives the line the flat rhythm of someone stating facts they can barely take in.
Then the imagery moves to the sea. "Hopes have to be settled on shore" is a puzzling sentence, and it is richer for that. "Settled" suggests accounts being squared or an estate put in order, but also a boat brought to rest on land. The phrasing suggests the speaker is still out on the water, and that hope can only be dealt with once the crossing is over. That is my reading, and the poem never spells it out.
What it does say is that "the water being almost / at low water, fear is sure." The line break after "almost" leaves the sentence hanging, and the stiff, slightly legalistic grammar ("the water being") sounds like a mind slowed down and working through its situation one clause at a time. Low water is the ebb, the point where a boat loses what holds it up, and a reader can hear life draining away in it. Yet "almost" leaves a small margin, while fear gets no qualifier at all. Hope is something that "has to be" managed. Fear is simply "sure." The poem has no end rhymes, but "shore" and "sure" sit two lines apart and nearly echo each other, so the place of safety and the certainty of dread are bound together in sound.
After that comes the poem's only plain, practical moment: "I urge the surgeon on." Another person finally enters, and the vocabulary stops being figurative. The line is short and brisk, like a command given in a hurry. It also shows how little the speaker can do. All that remains of his power is his voice, and even the urging may be silent. The poem doesn't say.
The last line is the longest, ten syllables against the six or seven of most of its neighbours, and it slows everything down: "my heart heavy for not hearing my heart." The repetition of "heart" works as wordplay. The first is the emotional heart of the idiom "heavy-hearted," and the second is the organ itself. The h-sounds in "heart heavy... hearing... heart" make the line breathy, almost a sigh. There is a reversal here too. The poem began with a knocking so loud it was terrifying, and it ends in grief at not hearing that same sound. Perhaps the silence comes from anaesthesia or the operation itself, though that is only an inference. Either way, the speaker has learned that the thing he feared hearing is the thing he most needs to hear.
The poem is significant for how much it leaves out. It uses no hospital jargon and gives no account of pain, only a few old, physical images (wood, shore, tide) that make a medical emergency feel elemental. The speaker is a body being acted upon, listening for itself. The poem doesn't tell us whether it comes from lived experience, and it doesn't need to. In seven lines it shows that fear can be certain while hope stays uncertain, and that a heart is at its loudest when it is failing and most missed when it goes quiet.