Chicago Police Supt. Garry McCarthy has been battling high cholesterol and high stress, eating salads and working out to stay healthy while meeting the demands of his intense boss, Mayor Rahm Emanuel.

McCarthy often received the mayor’s first telephone call of the morning and the last one of the day.

Thursday morning, the stress from McCarthy’s pressure-cooker job might have taken a toll on his heart, experts say.

He awoke about 5 a.m. to pain and nausea. Worried about his heart, he took an aspirin and jumped in a cab. Tests at Northwestern University Memorial Hospital showed an irregularity and he underwent a procedure to remove severe blockages in his arteries.

He’d suffered a heart attack, a City Hall source said.

Thursday afternoon, McCarthy was in good spirits, cracking jokes in the hospital. The mayor visited him for about 15 minutes, avoiding talk of work, sources said. McCarthy will continue resting at Northwestern for the next few days, according to the police department.

“The superintendent was in good spirits and we were talking just fooling around with his daughters and family around,” Emanuel said at an event Thursday night, adding that McCarthy was watching a replay of the previous night’s White Sox victory over the Dodgers.

“He’s in very good health and good spirits. Very talkative, up, actually getting briefed by members of the police department about things that are going on. So, he’s all there.”

Was Emanuel worried his police superintendent wouldn’t be able to return to duty?

“No, not at all,” the mayor said.

“The superintendent appreciates everyone’s well wishes and is looking forward to being back on patrol with his fellow police officers very soon. The first deputy is always in command during the superintendent’s absence,” Chicago Police spokesman Adam Collins said in a statement.

McCarthy, 55, had a 100-percent blockage in one artery and 80-percent blockage in another, a source said. A third artery was clear. 

He underwent an angioplasty, in which a thin wire is inserted in the artery allowing a balloon to pass through. The balloon is expanded to push aside the plaque or clot causing the blockage. Then a stent — a tube — is placed inside to keep the artery open.

Patients who have an angioplasty are often sent home the next day or maybe even the same day, and usually resume normal activities in three to five days, said Dr. Atman P. Shah of the University of Chicago Department of Medicine.   

Elevated cholesterol levels, high blood pressure and smoking can lead to accumulation of plaque in the arteries, according to the National Institutes of Health. 

Stress may have played a role in McCarthy’s heart problems, experts said.

McCarthy has been watching what he eats — gravitating to salads and turkey burgers instead of steaks — to control his cholesterol, sources said. He exercises regularly and appears fit.

But he undoubtedly falls into the “chronic stress” category, said Dr. Amir Darki, an interventional cardiologist at Loyola University Medical Center.

He can return to his job as police superintendent, but must learn to process his stress in a dramatically different way, Darki said.

“When people don’t change their habits, it frequently leads to recurrent problems,” he said. “I would encourage him to change his relationship to his job and to take more time for himself.”

Dean Angelo, newly elected president of the Fraternal Order of Police, said he expects McCarthy to return to the job he has held since Emanuel took office more than three years ago.

“When someone has a health scare, some people re-evaluate. That’s why sometimes, when someone has an incident occur, they figure it’s time to go golfing and fishing,” Angelo said.

“But I think he’s too young for that. He’s got too much fire for that. I don’t see the superintendent hanging it up any time soon. He seems to me to be a heavy lifter. Maybe his diet will change.”

Angelo acknowledged griping among the rank-and-file about McCarthy’s leadership. But he argued a lot of it is ricochet anger misdirected at the superintendent.

“Our members lost 1,500 spots. Manpower is a big issue. I don’t think the superintendent controls manpower. People might want to blame the guy for that, but he doesn’t do the hiring. His responsibilities are day-to-day operations — not staffing,” Angelo said.

“The man I’ve met seems to be a police officer with police officer concerns. He went through a lot in New York during 9/11. You don’t come away from that without realizing the risk and how fleeting and dangerous this job is.”

None of the four previous superintendents remained more than five years. McCarthy’s predecessor, former FBI official Jody Weis, was top cop less than four years. Phil Cline served about four years; Terry Hillard, about five years; and Matt Rodriguez, about five years.

Emanuel hired McCarthy in 2011 after being elected mayor. McCarthy also was a finalist for the job in 2003.

Two years ago, his frontline leadership during the NATO Summit turned him into a folk hero. He helped defuse a potentially volatile confrontation with Black Bloc provocateurs at Michigan and Cermak.

McCarthy, who regularly hits the streets on patrol with his commanders, has helped make several arrests, including one in early 2013 when he spotted handguns on the floor of a car that officers curbed. A passenger was arrested on gun charges.

McCarthy’s hands-on approach was featured on the CNN documentary “Chicagoland.”

The native New Yorker ran the Newark Police Department from 2006 to 2011. He had been a top police official in the New York Police Department. He was in charge of the CompStat crime analysis system, which he has replicated in Chicago.

Under McCarthy, specialized citywide police units have been disbanded in favor of boosting street patrols. The department has flooded high-crime pockets of the city with cops, trying to reduce shootings. “Gang audits” help police determine where to deploy officers.

Emanuel has credited McCarthy’s strategies with an overall drop in the city’s crime rate, although critics have said the city doesn’t seem safer.