The setting was different, but for Jim Harbaugh the feeling was hauntingly familiar.

It started the night before the Chargers' game against the Denver Broncos on Sunday, when an irregular heartbeat – a problem he struggled with in 1999 and 2012 – began causing him discomfort. He informed at least some of his players and told them that he was just excited about the game.

But the arrhythmia – diagnosed as atrial flutter – recurred in a more dramatic way during pregame warmups, prompting him to briefly duck into the blue medical tent on the sideline and then head to the locker room, flanked by medical team personnel.

There is so much going on on the sidelines of the NFL that many players had no idea that defensive coordinator Jesse Minter had taken over head coaching duties in Harbaugh's absence.

Meanwhile, in the visitors' locker room, paramedics measured the electrical activity of Harbaugh's heart by performing an electrocardiogram. Marco Zucconi, the Chargers' director of player health, wellness and performance, gave the coach an IV and magnesium to treat his condition.

“It’s the heart,” Harbaugh said after the 23-16 win, “so you take it seriously.”

Within minutes, his heart rate had returned to a normal rhythm and he returned to the game, walking down the sideline and accepting the red challenge flag from Minter, who had been defensive coordinator at Harbaugh's University of Michigan.

After his two previous episodes of atrial flutter, Harbaugh underwent ablations, which are minimally invasive procedures used to treat irregular heartbeats by destroying heart tissue that causes abnormal electrical signals.

Thirteen years passed between his first and second ablations, the second was twelve years ago.

“I thought I was approaching the 13-year mark and would need another one,” he said.

Chargers coach Jim Harbaugh (left) is congratulated by Broncos coach Sean Payton after Los Angeles' road win.

(Dustin Bradford/Getty Images)

Harbaugh appeared tired after the win but otherwise normal. He is fit and training with his players, looking younger than his 60 years, but leaned forward and put his hands on his knees before walking to the team bus, ostensibly to catch his breath.

He said he didn't know if the altitude in the Mile High City made his suffering worse.

CBS showed a video showing Harbaugh leaving and re-entering the field. However, the reason for this was not made public until the coach announced it in his post-match press conference. However, his family knew because his older brother John, coach of the Baltimore Ravens, cut short his press conference after four questions so he could get information about his brother.

Chargers general manager Joe Hortiz, who came over from the Ravens, called Eric DeCosta, his Baltimore counterpart, and informed him of the situation. Chad Steele, who handles media relations for the Ravens, then interrupted John Harbaugh's comments from the lectern.

John, in turn, called his parents, Jack and Jackie Harbaugh, in Ann Arbor, Michigan. That's when he learned his younger brother had been given permission to return to the sidelines.

“From a worrying perspective, heart rate increases every time people have such rhythms,” said Dr. Nikhil Warrier, medical director of electrophysiology at MemorialCare Heart & Vascular Institute at Orange Coast Medical Center in Fountain Valley.

“They can be sitting around doing nothing and all of a sudden their heart rate goes up to 130, 140, 150 beats per minute, and that's unusual for anyone.”

According to Warrier, there are a number of factors that can lead to this condition, including excessive stress and caffeine consumption, as well as structural problems with the heart.

“Often we see that patients have no risk factors other than the fact that they showed up and have this arrhythmia,” he said. “In this case, it is curable through ablation. So that ends up being the most common strategy we use.”

Harbaugh was playing quarterback for the Chargers when he first experienced atrial flutter. The second episode occurred while he was coaching the San Francisco 49ers during a Monday night game against the Chicago Bears.

The first time he missed a day or two of training. The second time, in addition to an ablation, he underwent electrical cardioversion, which uses paddles that deliver an electrical current to return the heart to a regular rhythm.

He said after Sunday's game that he planned to see a cardiologist on Monday.

“Trust the doctors,” he said. “If you don’t trust your doctors, who will you trust?”