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I'm looking for a book that I think I read about forty years ago. About a number of patients in a coma ward that lived in a subconscious world run by another powerful patient also in a coma... somehow they are all linked together.

While the title wasn't QB VII, it was something similar.

Taking into consideration the title similar to "QB VII" my guess would be Ubik by Philip K. Dick.

The novel starts with a group of organization employees goes on to a base for a meeting that appears to be an ambush. A bomb explodes putting one of the participants into coma, but rest of the trip seems unaffected. They rush back Earth to allow a better treatment for the injured. Over time others start experiencing strange things.

Eventually it turns out that they are all in coma state and the one supposedly injured person is the only who was actually unaffected. Ubik is a substance allowing to keep connection with the real world (to some level).

I'm writing it from memory and I read it some 25 years ago or so. The novel was written in 1969.

Edit as suggested by DavidW

The ward you're mentioning is probably Moratorium, where half-lifers are kept. This theme is in general throughout the book. Even at the very beginning where Runcinter wants to consult his wife:

As owner of the Beloved Brethren Moratorium, Herbert
Schoenheit von Vogelsang, of course, perpetually came to
work before his employees. At this moment, with the chilly,
echoing building just beginning to stir, a worried-looking
clerical individual with nearly opaque glasses and wearing a
tabby-fur blazer and pointed yellow shoes waited at the
reception counter, a claim-check stub in his hand. Obviously,
he had shown up to holiday-greet a relative. Resurrection
Day - the holiday on which the half-lifers were publicly
honored - lay just around the corner; the rush would soon
be beginning.

That's where those people in coma-like state (half-lifers) are put, allowing their close ones to keep contact with them.

Also apparently a powerful half-lifer can impact others:

“After prolonged proximity,” von Vogelsang explained,
“there is occasionally a mutual osmosis, a suffusion between
the mentalities of half-lifers. Jory Miller’s cephalic activity is
particularly good; your wife’s is not. That makes for an
unfortunately one-way passage of protophasons.”

As we learn later that impact is really significant:

"I did what I do," Jory said. "It’s hard to explain, but I’ve been doing it a long time to lots of half-life people. I eat their life, what remains of it."

and indeed allows to control the reality of other half-lifers:

Pausing in his task of reading Joe’s blood pressure, Dr. Taylor twisted his head to see. Both he and Joe watched as the vapor now condensed; puddles of it glistened on the carpet, and down the wall behind Denny it drizzled in bright streaks.

The cloud concealing Denny evaporated.

The person standing there, in the center of the vaporizing stain of Ubik that had saturated the worn and dingy carpet, was not Don Denny.

Later he reveals more:

Joe stared at him, then said, “Didn’t he see you change, Jory? Hasn’t he heard what you’ve been saying?”

“Dr. Taylor is a product of my mind,” Jory said. “Like every other fixture in this pseudo world.”

“I don’t believe it,” Joe said. To the doctor he said, “You heard what he’s been saying, didn’t you?”

With a hollow whistling pop the doctor disappeared.

“See?” Jory said, pleased.

This sounds like "Disappearing Act" by Alfred Bester.

As I recall the world isn't controlled by one of the patients, but aside from this matches. People are escaping from a seemingly perpetual war by going into a coma in which they inhabit a fantasy world. The people in the coma are kept in Ward T.

However this is a short story not a full length novel. I was wondering if QB VII, or whatever it was, could be the title of an anthology the short story was in but I can't see anything like that in the anthologies listed for the story on ISDB.

This was previously identified as the answer to Lone wolf or beserker behaviour of young men in an a very authoritarian society

Outside chance you could be thinking of "Ubik" by Philip K. Dick.

Doesn't have a coma ward, but features a group of people who may or may not be in a coma-like state of "half-life" finding themselves facing reality shifts, possibly controlled by another individual in the same state.

Aside from a slightly similar plot, primarily mentioning it since the title is also a little similar to QB VII. Plus, published in 1969 so the timing works.

This sounds similar to the Outer Limits episode "The Refuge."

I've looked around a little, and I can't find any mention that this story is based on an earlier written work.