There may be many great poems about being an overweight kid at the seashore, but I am aware of only one: “On Home Beaches,” by Les Murray. The line that destroys me is the one in which the poet recalls being a “red boy, holding his wet T-shirt off his breasts.”

The unnamed narrator of Garth Greenwell’s third novel, “Small Rain,” loves Murray’s poem, too. He’s had stretch marks and “three rolls of flesh” on his stomach since he was a child. “For years even with lovers I refused to take off my shirt,” he says, “and I’m not sure I can remember ever being shirtless outside; it’s ridiculous how much the thought horrifies me.”

He’s a poet and a teacher who lives in Iowa City with his lover, L, who’s also a poet (and a more successful teacher). They’re renovating an old house. L is from Spain; they alternate days speaking in English and Spanish. They are bohemian lovebirds in mellow early-middle age, stranded almost happily in the Midwest.

One day the narrator experiences, out of the blue, annihilating physical pain, as if “someone had plunged a hand into my gut and grabbed hold and yanked.” He’s taken to the E.R. and then the I.C.U. This is the Covid era, and there are persistent delays and fears. He waits and waits for a diagnosis. His case is so unusual that specialists clamor to meet him, as if he were a candidate for a medical mystery column.

What he has, it turns out, is an infrarenal aortic dissection — a tear in the inner wall of his aorta. These can snuff you and are difficult to treat. They are usually seen in older people (the narrator is in his 40s) and those with comorbidities.

Most of “Small Rain” takes place in the hospital, where the narrator, with pills down his throat and tubes up his nose, wonders what will happen next, and if he will make it out alive. There is the usual humbling slapstick of bathroom visits and I.V.s and exploratory needle jabs. There is little talk about who will pay for these treatments. The narrator is surely in line for an invasive procedure Tony Soprano might call a walletdectomy.

You would think that illness, and the world that surrounds it, would be primal territory for Greenwell. His two previous novels, “What Belongs to You” (2016) and “Cleanness” (2020), were frank explorations of the body’s workings and urges. You felt the force of other people fully met in his pulverizing gay sex scenes. Both novels were so gleaming with erotic phosphorescence that they might have been retitled, for their paperback editions, “Best American Sex Writing 2016” and “Best American Sex Writing 2020.”

There is little lovemaking in “Small Rain,” though awareness of desire filters in at the margins. For example, the sentence I quoted above, about disliking being shirtless, ends this way: “I’ve learned there are men turned on by it, in back rooms and bathrooms, when I let men press their faces into my stomach and chest, sucking on my nipples or burrowing under my arms.”

The good news about “Small Rain” is that medical sagas, like heist movies, have layers of built-in drama. The novel’s abiding theme is isolation, intensified by Covid, by social media and especially by illness, which one must go through alone even when surrounded by friends and family.

The bad news is that, in its slow piling on of medical minutiae over more than 300 pages, “Small Rain” listlessly drifts. The narrator is trapped in a hospital bed, and we are strapped in beside him. Reading this interminable novel is like watching a competitive bicyclist, before a race, depilate his legs hair by hair.

Greenwell’s narrator does not get much distance on his condition; his reflections on his experience are mostly so mundane that they hardly count as reflections at all. There is little humor, and this lack gives the book a passive quality. (Gallows humor is frequently the best part of illness stories.) The reader gets the sense that Greenwell is not on top of this material so much as the material is on top of him.

His sentences are long and strung with clauses and subclauses; the paragraphs and chapters are long as well. Each page is a tall palisade one must climb slowly down, with little hope of a place for eyes and wits to rest.

I have left things out. There are sections on the narrator’s difficult childhood and his estrangement from members of his family. There is some observant talk about music and about poetry. A construction fiasco is memorably described. He is touchy about being from Kentucky.

The narrator hangs on, like Harold Lloyd on a clock’s hand. He learns to bear more fully his own mortality. His weight does not seem to be a cause of his torn artery lining. One possibility is late-stage syphilis. He contracted the disease many years earlier but had no symptoms.

This book’s title is taken from a 16th-century song called “Westron Wynde” that has a line about the falling of “the smalle rayne.” The song trails a literary lineage behind it.

Madeleine L’Engle titled her semi-autobiographical 1945 novel “The Small Rain,” and Thomas Pynchon placed the same title, in 1959, atop his first published story. “Westron Wynde” has been mentioned or quoted in novels by Ernest Hemingway, George Orwell, Virginia Woolf and others.

It gives me no pleasure to find so little pulse in “Small Rain.” I’m a Greenwell fan. Can a misfire be a blessing in disguise? As the great Max Beerbohm put it, a man whose career is always great “does sorely try our patience.”