President Donald J. Trump’s blood oxygen level sank to a precariously low level after he announced that he had tested positive for the coronavirus last year, according to a new book by Mark Meadows, his former chief of staff.

The new details contradict Mr. Trump’s denials this year that his Covid bout was more dire than White House medical officials had acknowledged at the time.

Mr. Meadows’s book, titled “The Chief’s Chief,” goes on sale on Tuesday. He describes his tenure in the White House, alternately promoting Mr. Trump’s false claims that the 2020 election was stolen from him and attacking the news media. Mr. Meadows also revealed previously undisclosed details about the former president’s medical condition in October 2020.

Mr. Trump, who has long been fearful of appearing weak, has tried to camouflage those details. The White House staff and members of his medical team aided that effort, publicly downplaying how sick he was at the time. The former president denied a detailed New York Times report this year that he was more ill than his aides had revealed, with depressed oxygen levels and lung infiltrates, which occur when they are inflamed and filled with fluid or bacteria.

Mr. Meadows recounts in extraordinary detail how severe Mr. Trump’s illness was.

On Friday, Oct. 2, 2020, hours after the president announced on Twitter that he had tested positive for the virus, he recorded a blood oxygen level of about 86 percent, Mr. Meadows wrote. That is roughly 10 points below what would be considered normal. Most healthy people have a blood oxygen level of about 95 to 98 percent, although some people may have lower normal readings.

Mr. Trump’s health had deteriorated so much that day that members of his medical team feared they would not be able to treat him adequately without immediate attention from hospital staff, Mr. Meadows wrote.

“That morning, Dr. Conley pulled me aside and delivered some bad news,” Mr. Meadows wrote, referring to Dr. Sean P. Conley, the head of Mr. Trump’s White House medical team. “Although the president’s condition had improved slightly overnight, his oxygen levels had now dipped down to about 86 percent and could be trending lower, a dangerously low level for someone his age.”

Mr. Meadows wrote that Mr. Trump’s doctors “had decided to put the president on oxygen in the residence and hope for the best.”

When it was clear he would need outside care, Mr. Trump’s medical team made the case to Mr. Meadows to intervene.

“We don’t have the resources to do it here,” Dr. Conley told Mr. Meadows about the president’s condition.

“I worried that the notion of him going to the hospital, in his mind, would seem like an act of capitulation,” Mr. Meadows wrote. “I was right.”

He described walking into Mr. Trump’s private residence to see him in a T-shirt, sitting up in bed. “It was the first time I had seen him in anything other than a golf shirt or a suit jacket,” Mr. Meadows wrote. The president, he said, was making phone calls and trying to work.

“If it hadn’t been for the oxygen tank by his side, I might have forgotten he was sick at all,” Mr. Meadows wrote. But his attention snapped back to Mr. Trump’s appearance: “I remembered what Dr. Conley had said. We were in trouble, and the president needed to get to the hospital.”

Mr. Trump had red streaks in his eyes, Mr. Meadows recounted, and “his hair was a mess from the hours he’d spent getting Regeneron in bed,” referring to an antibody infusion the president had received.

The president was initially resistant to leaving the White House and told Mr. Meadows that he was going to be fine. Mr. Meadows intensified his pleas. “It’s better that you walk out of here today under your own strength, your own power, than for me to have to carry you out on a gurney in two days,” he recounted.

Mr. Trump relented. On the walk out to his helicopter, he had lost so much strength that he dropped a briefcase he had planned to carry outside, where reporters were lined up to observe him, Mr. Meadows recalled.

A spokeswoman for Mr. Trump did not respond to an email seeking comment.

Last week, The Guardian obtained a copy of Mr. Meadows’s book and revealed that Mr. Trump had first tested positive for the coronavirus on Sept. 26, 2020, three days before his first presidential debate with Joseph R. Biden Jr.

Mr. Meadows has criticized reporters for focusing on that test and not the negative result from a different test a short time later. Mr. Trump has tried to make the same case, insisting that Mr. Meadows had affirmed that he was not sick before or “during” the debate.

Yet in his own book, Mr. Meadows writes that “we’ll probably never know whether President Trump was positive that evening.” Mr. Trump, according to an adviser, has grown angrier at Mr. Meadows in recent days, describing him as “dumb” for discussing the test results.

On Oct. 1, 2020, Mr. Trump revealed he was taking a coronavirus test after news reports that his aide Hope Hicks had tested positive. Although the president implied that it was Ms. Hicks who had infected him, Mr. Meadows wrote that it was clear to him that Mr. Trump was already significantly ill that night.

“I’ve lost so much strength,” Mr. Meadows quoted Mr. Trump as telling him, adding, “The muscles are just not responding.”

By then, Mr. Meadows had arranged for a monoclonal antibody therapy made by Regeneron to be secretly delivered to the White House. He said he asked the company’s chief executive, Leonard S. Schleifer, for four doses, mentioning that Ms. Hicks and Melania Trump, the first lady, had tested positive. Mr. Meadows did not reveal who else might have needed it.

He wrote that a senior aide had orchestrated a “special authorization” to land a small jet at Ronald Reagan Washington National Airport, where the doses would arrive and be ferried to the president. Food and Drug Administration officials cleared the treatment for Mr. Trump, even though it had not yet been authorized by the agency.

“We’d rigged the four-poster bed in the president’s room so that he could recline and take the drug while he was still alert and giving orders,” Mr. Meadows wrote.

Mr. Trump’s condition after he was hospitalized was still grave. After he was moved to Walter Reed National Military Medical Center, in Bethesda, Md., Mr. Meadows asked for an update from one of the president’s doctors about his condition.

“The Regeneron treatment we’d given him was working, they said, but his oxygen levels were still dangerously low,” Mr. Meadows wrote. “Judging by the details he gave me, it was clear that the staff at Walter Reed was prepared for a long stay — weeks, maybe longer.”

He wrote that “every movement was difficult for” Mr. Trump. “Every few minutes, he would need to sit back down and rest.”

At one point during the president’s stay, Mr. Meadows tried to surreptitiously tell reporters that the situation was more dire and was caught on camera. Mr. Trump erupted in anger, according to people who spoke with him.

Within days, and with the help of a cocktail of other drugs, Mr. Trump was back at the White House, with aides fearful that he might still have been contagious.

Rebecca Robbins contributed reporting.