ORLANDO — In a scene more like a battlefield than an emergency room in a large American city, dozens of people hit by gunfire poured into the Orlando Regional Medical Center in the dark predawn hours of Sunday morning, lining the hallways and filling the operating rooms.

The largest mass shooting in US history happened just a few blocks from the region’s only major trauma care hospital — an event that illuminates the new challenges facing emergency medicine. The gunman fired on his victims in a packed gay nightclub with an assault rifle that caused deep, gaping wounds. He also shot at them with a handgun whose smaller-caliber rounds, in some cases, bounced around inside their bodies, inflicting internal injuries.

“If they had not been three blocks from the hospital, they might not have made it to the hospital,’’ said Dr. William S. Havron, a trauma surgeon at the center.

Specialists in emergency medicine say the escalating severity of mass shootings in the United States calls for a reevaluation of the medical response. In the past, disaster drills have focused on crises like bus accidents or plane crashes, which involve blunt trauma injuries, not gunshots from high-powered weapons capable of mowing down dozens of people at a time.

Recognizing that more mass killings are likely, Dr. Jay A. Kaplan, the president of the American College of Emergency Physicians, said the group created a task force in January to determine how to improve the responses. Using the military as a model, the group wants to study “patterns of wounding’’ in civilian shootings to help find the best ways to save lives.

“The battlefield has been brought to our communities, in terms of the kind of injuries we’re seeing,’’ said Kaplan, who is also vice chairman of emergency services at Ochsner Health System in New Orleans. He added, “We need to learn and we need to be better prepared.’’

On Sunday, the casualties came to the medical center in two waves, the first around 2 a.m., shortly after the shooting started. Victims arrived several to an ambulance or in cars or trucks driven by people on the scene. The second wave, victims who had been trapped in the club with the gunman, arrived about 5 a.m., after police blasted through a wall in the club and killed the assailant.

Of the 44 victims brought to the hospital, nine died within minutes of arriving, Dr. Michael L. Cheatham, a trauma surgeon at the center, said. Of the remaining 35, eight have gone home and 27 were still hospitalized on Tuesday. Six of those 27 are in critical condition, in the intensive care unit. Five others are in guarded condition, and 16 are stable.

At a news conference at the hospital on Tuesday, trauma surgeons, emergency medicine specialists, nurses, and one of the victims recounted the horrific events that began with the attack at the gay nightclub that left 50 dead, including the gunman. In interviews, doctors gave more details.

“This is not a drill,’’ Dr. Chadwick P. Smith, a surgeon at the center, said he told the trauma surgeons he woke up at home with phone calls summoning them to the emergency room.

The surgeons went from one operating room to the next, performing 28 operations on Sunday, eight on Monday, and eight on Tuesday.

One victim, rushed into surgery on Sunday, had a severe abdominal wound and was also close to bleeding to death from shots to the arm and leg, said Dr. Matthew W. Lube, another trauma surgeon at the center.

Cheatham said that he would not be surprised if one or two of the six victims who are in critical condition do not survive and that he had concerns about whether others would recover fully. At least one had been shot in the head.

Eleven victims were taken to other hospitals in Orlando.

When it came to rescuing victims at the site, the usual rules did not apply, Dr. George Ralls, the medical director for Orange County, where Orlando is located, said in an interview.

At most sites with multiple casualties, emergency medical technicians assess the victims and color-tag them according to how urgently they need to be evacuated. Yellow can wait, red means hurry up, black means it’s too late.

But that night, unless someone obviously needed urgent help for hemorrhaging or breathing trouble, technicians skipped the assessments and loaded people into ambulances and rushed them to the medical center, Ralls said.

When it came to rescuing victims at the site, the usual rules did not apply, Dr. George Ralls, the medical director for Orange County, where Orlando is located, said in an interview. At most sites with multiple casualties, emergency medical technicians assess the victims and color-tag them according to how urgently they need to be evacuated. Yellow can wait, red means hurry up, black means it’s too late.

But that night, unless someone obviously needed urgent help for hemorrhaging or breathing trouble, technicians skipped the assessments and loaded people into ambulances and rushed them to the medical center, Ralls said. Because it was so close, the technicians realized they could drive patients there, several at a time, faster than they could assess them. And for critically injured people, the thing most likely to save their lives is getting to the hospital as fast as possible.

A doctor waited in the driveway outside the emergency room performing triage on patients as they arrived and told the technicians where to take them.

“At one point we had 90-plus patients in the emergency department,’’ said Dr. Timothy B. Bullard, an emergency physician at the center and a member of the command structure that is activated to manage mass casualties. (Some patients were already at the hospital for reasons unrelated to the attack at the nightclub.)

At the peak of the crisis, there were eight emergency doctors, six senior trauma surgeons and several residents, a few orthopedic surgeons, a vascular surgeon, a neurosurgeon, at least two specialists in critical care, respiratory therapists, chaplains, counselors, X-ray personnel, and countless nurses working, Bullard said. Police officers and security guards were also on duty.

With so many patients, critical medical decisions had to be made quickly and doctors had to act more aggressively than they normally would, Bullard said. For example, if a patient showed any signs of breathing trouble, instead of waiting to see if it would resolve, doctors would put in a breathing tube so they could move on to the next patient.

“I’ve been here 31 years at the trauma center, and I’ve not seen anything like this, nothing on this scale, nothing of this nature,’’ Bullard said. “I guess, being it’s the largest mass shooting in the history of the United States, nobody else has, either.’’