So sedation’s biggest safety issue isn’t the drugs themselves, several experts told us. It’s recognizing trouble immediately — and knowing how to respond.

Indeed, some people are sensitive to lower doses than Junior received, just as some get tipsy from one glass of wine on an empty stomach. “Even standard and acceptable doses of drugs can cause significant morbidity and mortality,” the Pediatrics researchers stressed in another 2000 paper.

“This works,” Mashni told us, “until you get a patient that lies outside the normal expected response.”

Dr. Michael Mashni, who has such a license in California, said some dentists resort to pushing the bounds of conscious sedation. They hope to make children groggier without quite knocking them out.

Deep sedation costs more money, for training and support staff and equipment. It also requires a special license that most dentists, including Riehs, don’t have.

“Practitioners should recognize that ‘conscious sedation’ is an oxymoron for many children” under 6, researchers warned in a report that the prominent journal Pediatrics published in 2000. Deep sedation — in which patients aren’t easily aroused and may need breathing assistance — “is usually required to gain the cooperation of this age group.”

He played in the waiting room, had his blood pressure checked — and then drank a three-drug cocktail. (Family photos)

The monitoring

An aide carried Junior into a treatment room, where dental records say he “awoke and whined.” She laid him on the restraint board. “Cuffs and straps were engaged.”

He tolerated numbing injections and having a mask put over his nose for nitrous oxide — a fourth sedative, commonly called “laughing gas.”

But Junior did not, in the words of Riehs’ website, “become completely at ease.”

Treatment records call the sedation “semieffective.” Ten minutes into the procedure, Junior’s heart was beating twice as fast as at the beginning. He cried much of the time. Mucus had to be suctioned from the back of his throat.

“Treatment may have to be discontinued if the sedation proves completely or partially ineffective,” says a consent form that Riehs and the boy’s father signed.

He was telling me that he should be fine, that it was just sort of a bad reaction to the medicine. The doctor told me, ‘I don’t know why they’re taking him.’ Junior’s father, Salomon Barhona, after his son was taken to Children’s Medical Center Dallas by ambulance.

Riehs did not stop. About 35 minutes after drilling began, he decided that six teeth were ready for crowns.

Then he learned that a sensor had come off Junior’s right index finger, one that measured oxygen in his blood. And when the lone assistant in the room reattached it, dental records say, “no oxygen saturation or pulse registered.”

The boy was limp. His lower lip was turning blue.

Riehs and the assistant repeatedly tried CPR, without success. The office manager dialed 911.

Dental records estimate the call time as 2:14 p.m. City records, however, show that the call came in at 2:26.

Riehs kept trying CPR. After he injected the boy with a drug to reverse the narcotic’s effects, a pulse returned.

Meanwhile, an office staffer escorted everyone in the waiting room outside — everyone except Junior’s parents. Then a police officer appeared, and then a Dallas Fire-Rescue crew.

Daniela and Salomon still didn’t know what was going on.

“I tried to open the door to go to the back,” she said. “But the door was locked.”

Finally they were let in. There lay Junior, unconscious, on a gurney.

Daniela rode with him in the ambulance to Children’s Medical Center Dallas. Her husband spoke briefly with Riehs before heading there, too.

“He was telling me that he should be fine, that it was just sort of a bad reaction to the medicine,” Salomon said. “The doctor told me, ‘I don’t know why they’re taking him.’”

Daniela Barahona stayed by her 4-year-old’s hospital bed to the end. Junior “tried to wake up” on the first day, she said, and “I could see the tears coming out of his eyes.” (Family photo)

The hospital

In the intensive care unit, the question wasn’t whether Junior had suffered brain damage. It was how much. By some estimates, loss of oxygen can harm a small child in 90 to 120 seconds.

“At one point he tried to wake up,” Daniela said, recalling the first day at Children’s. “They had to tie him down,” because of the breathing tube in his throat.

“I could see the tears coming out of his eyes,” she said. “He was squeezing my hand because he was afraid.”

On Day 2, the parents learned that Junior would never walk again. On Day 3 — New Year’s Day of 2014 — they watched helplessly as he suffered seizures. And on Day 4 he died.

At one point he tried to wake up. They had to tie him down. I could see tears coming out of his eyes. He was squeezing my hand because he was afraid. Junior’s mother, Daniela Barahona, on Junior’s condition at the hospital.

Somewhere in there, Riehs came to the hospital and spoke to Junior’s father in a hallway.

“He broke down,” Salomon recalled, “and said he was so sorry.”

The parents stayed with the body for two days after death, until organs were harvested for transplant.

In the coming weeks, the Barahonas could only wonder what, exactly, had gone wrong. Behind the scenes, big-name dentists feared that they knew, according to state emails we obtained.

In one message, Dr. Tammy Gough reported hearing that Riehs used “WAY above recommended dosages.”

Gough, a pediatric specialist in Collin County, served on the Texas State Board of Dental Examiners then. Her email went to the executive director of the board, which was investigating Junior’s death.

She also passed along an email from a dentist on the Children’s hospital board, Dr. Robert E. Morgan. It said his medical colleagues were “frustrated with the number [of] dental patients they are seeing in the ICU” and “surprised that in dentistry we are allowed to be our own ‘anesthetists.’ ”

No law specifically prevents a dentist or physician from performing a procedure while monitoring major sedation. But hospitals generally require a separate professional for each task.

In a dental office, the only person working with the dentist may be someone with a high school education, no license and only on-the-job training.

Morgan, now president of the medical and dental staff at Children’s Medical Center Plano, has long chosen to treat children who need significant sedation at a hospital.

“There is a multidisciplinary team of specialists in the operating room, including a pediatric anesthesiologist,” he told us. “There is safety in numbers.”