NEWARK — The calls for patients in cardiac arrest came in one after another.

A 39-year-old man, followed by a 65-year-old, whose neighbor called 911 after getting no response when he rang the doorbell. Then a 52-year-old woman’s heart stopped, as did that of a 90-year-old, who had collapsed on her bedroom floor.

The ambulances turned on their sirens and screamed through red lights. But what the paramedics did after rushing to the victims — or more precisely, what they did not do — is a window into how a deadly virus has reshaped emergency medicine. After confirming that the patients’ hearts had flatlined, they declared each of them dead at the scene, without attempting CPR.

Before coronavirus cases hit hard a few weeks ago, John McAleer, a paramedic who responded to the call for the 90-year-old woman, would have begun chest compressions. His partner would have started an IV to administer epinephrine, which acts as a stimulant. They might have used the defibrillator to try to shock her heart back to life.

He would have done this even though studies have found that only about 1 to 3 percent of people found in her condition can be resuscitated. For that is what emergency workers have been trained to do: make every possible effort to save every life.

“It’s unsettling because it does go against everything we’ve been taught,” said Mr. McAleer, 51.

Around the country, in cities and counties in the grip of the pandemic, emergency medical technicians have had to do something they’re not used to: think of their own well-being before that of their patients. With so many paramedics falling ill, emergency units have changed their practices to limit exposure to the virus.

The most unsettling change, according to interviews with paramedics in a half-dozen of the most affected states, is the decision to suspend, or limit, resuscitation in cases when the odds of survival are near zero.

“This is medicine that we have never done before. It’s scary. There are ethical dilemmas that come with it,” said Terry Hoben, the coordinator of emergency medical services at University Hospital in Newark, whose ambulances responded to the calls. “We do not take this lightly.”

The decision has caused so much concern that Mr. Hoben’s department is discussing whether to allow paramedics to resume CPR this coming week while limiting the amount of time it is performed.

But last month, after the virus sickened dozens of employees, Mr. Hoben said he had to take more drastic measures.

“To resuscitate and save one life, and risk five?” he asked. “It’s not balanced.”

Sick and Overwhelmed

Nationwide, emergency departments have varied in their response to the pandemic, mirroring the intensity of the crisis locally.

At one end is Washington State, where the virus first took hold but few paramedics have contracted it. The only statewide change is the use of extra protective gear, said Catie Holstein, the emergency medical service manager for Washington’s Health Department.

In the middle are states like Texas and Louisiana. There, agencies have reduced the amount of time they do CPR from as much as 40 minutes to as little as 10, according to medical directors in San Antonio and New Orleans.

At the other extreme are New York and New Jersey. Last month, New York, with the nation’s highest caseload, was the first to announce a policy on withholding CPR, before pulling back less than a week later. In New Jersey, with the second highest, a majority of emergency units have adopted the guidelines, at least in part.

The situation got particularly bad at University Hospital, which treats the highest number of uninsured patients in the state. It has buried seven employees who died of complications from Covid-19. Close to one-third of its roughly 600 nurses have taken leave in recent weeks, according to a hospital spokesman.

The shortage left the emergency room overwhelmed. Dr. Shereef Elnahal, the hospital’s chief executive, instructed his colleagues to call every staffing agency it could for backup, but that was not enough. So the hospital asked Mr. Hoben if he could recruit E.M.T.s. They were normally licensed to work only outside the hospital’s walls, but the state had issued a waiver.

Mr. Hoben posted an S.O.S. message on a private Facebook group for emergency responders: “I repeat this is an emergency,” he wrote.

Within hours, more than a dozen off-duty and retired health workers rushed to the E.R., caring for patients lining the hallways in gurneys. Nurses, who had thought the hospital was about to buckle, burst into tears.

The hospital asked for federal help, and eventually received 85 medical workers sent by the Defense Department, including doctors, pharmacists and respiratory therapists. The Federal Emergency Management Agency sent dozens of ambulance crews from as far afield as Arizona, Mr. Hoben said.

But by the time the help arrived, one-fifth of Mr. Hoben’s 270 responders were out sick. One of them, a 49-year-old veteran paramedic named Lisa Kahle, was so short of breath that in order to get out of bed, she had to grab herself by her sweatpants and swing each leg to the floor.

Colleagues were alarmed that she might have fallen ill on the job. Ms. Kahle had been sent to care for a wheezing patient, and she arrived in an N95 mask but no goggles or face shield, because full protective gear was not yet required. In an enclosed doctor’s office, she and her partner gave the man a nebulizer, which creates a mist infused with medication and is suspected of aerosolizing the virus. Ms. Kahle got sick soon after, believing she contracted the virus through her unshielded eyes. (She has since recovered.)

Over numerous conference calls with the unit’s medical and legal team, Mr. Hoben hashed out a new policy. They consulted new guidance from the Centers for Disease Control and Prevention. It made clear that emergency workers should protect their eyes, and the team equipped ambulances with goggles and face shields. When Covid-19 seemed likely, paramedics were told to put a surgical mask on the patient before beginning treatment.

As for what care to withhold, the guidelines were less clear. The C.D.C. advises emergency workers to “exercise caution” with treatments that may aerosolize the virus: manual resuscitators, used to “bag” the patient; nebulizers; intubation; and CPR.

For several techniques, Mr. Hoben’s team found a workaround to limit the risk of exposure. But they could not find an alternative for CPR.

Chest compressions, by their nature, are a close-up act. With every thrust of the rescuer’s hands, air is forced from the victim’s lungs, and along with it potentially infectious particles.

One data point stood out to Mr. Hoben as he devised the new safeguards. Before the pandemic, his ambulance team typically saw three to five cardiac arrests in a 24-hour period. In April, they averaged 14.

While a heart attack involves a blockage in the heart, a cardiac arrest occurs when the heart stops functioning altogether. Patients are found without a pulse and no longer breathing. Mr. Hoben suspects the spike is linked to Covid-19, with cardiac arrest marking the end stage of the disease.

If that is the case, then his paramedics are especially at risk, because chest compressions would disperse the virus when its load may be highest.

Mr. Hoben issued the policy against CPR on April 11. It includes exceptions — for those under 18, for pregnant women, for survivors of near drownings. But otherwise, once they confirm a patient’s heart has flatlined — in what is known as an “asystole” or “agonal” rhythm on an EKG — paramedics are to pronounce the victim dead, with no attempt to resuscitate.

At least 10 of the 14 cardiac arrests they now see daily fall under the policy, according to data from Mr. Hoben’s dispatchers. That means 10 patients a day who would have received CPR as recently as last month no longer do.

Not ‘Grey’s Anatomy’

On a recent Sunday morning, nearly two dozen health workers brought by FEMA stood in the parking lot outside Mr. Hoben’s office. They had come from as far away as California and Florida to help the overwhelmed Newark unit.

They were handed Motorola radios and told to listen for Code 88, the internal code for Covid-19.

“It’s important that you know our protocols on cardiac arrests,” Mr. Hoben stressed.

One of the new arrivals was Mark Radice, a paramedic from Prescott, Ariz., a part of the country that has seen few Covid cases. Within days, his ambulance was called to the home of a man who had turned blue. When he and another rescue worker hooked up the heart monitor and saw that the man had flatlined, Mr. Radice did nothing more than call time of death.

The family became hysterical, he recalled, and the paramedics slipped out as the police arrived.

“Normally, we would have tried to fix that,” he said in an interview, referring to administering CPR. “But I understand why that policy exists.”

The data could not be clearer: Study after study has shown that when a patient flatlines outside a hospital, with no bystander or medical professional there to witness it, the chance of survival after subsequent CPR is between 1 and 3 percent. For 2018, the most recent year for which nationwide data exists, a registry created by the C.D.C. found that only 2.4 percent of those who were resuscitated survived.

Long before Covid-19, doctors were debating whether it made sense to attempt reviving patients who were very likely to die or else end up with neurological deficits, said Dr. Mark Merlin, who chairs New Jersey’s Emergency Medical Services Council. He pointed to an article in The New England Journal of Medicine, published in 1994, concluding that once a patient’s heart has gone into asystole, the chance that it will respond to any intervention is “vanishingly small.”

Still, some of the paramedics are conflicted. They argue that saving even one out of 100 lives is worth the effort.

“Our mentality is: We need to try. We have a duty to act,” said Ms. Kahle, the paramedic, describing the ethos of emergency workers.

In mid-April, New York became the first jurisdiction to issue a policy against CPR, only to cancel it later. Officials said the reversal was because of a drop in call volume, but some emergency workers, speaking on the condition of anonymity, said that public perception also played a role.

While similar guidelines have been adopted throughout New Jersey — a majority of the 22 agencies that provide advanced life support in the state have pulled back on CPR, Dr. Merlin said — the state remains an outlier.

An internal survey of medical directors nationwide found that only 10 percent had changed their resuscitation policy, according to Dr. Craig Manifold, the medical director of the National Association of Emergency Medical Technicians.

Part of the calculus may be a gap in perception, with the public believing that CPR is as effective as it is portrayed on medical dramas, on which some 70 percent of make-believe victims survive.

“I can tell you that the majority of saves are nothing like what you see on ‘Grey’s Anatomy,’” says Dr. Richard Kamin, the medical director of the Connecticut Office of Emergency Medical Services.

Code 88

On a recent shift in Newark, the first cardiac arrest of the day was inside a brick condominium where a 65-year-old man was not answering his neighbor’s increasingly frantic knocks.

When the emergency workers arrived, the door had been forced in and the TV was on. The man lay on the floor, his head in the crook of his arm. They attached electrodes to his chest, but it was more a formality than anything else — his body was already in rigor mortis. No amount of CPR could have saved him. He was pronounced dead, just as he would have been before the pandemic.

But on a call a few hours later, the new policy came into action. “Unconscious, unresponsive female,” the dispatcher’s voice crackled over the radio. “Code 88.”

The ambulance pulled up to a two-story house, and Mr. McAleer and his partner rushed upstairs. A 90-year-old woman with Covid-19 had been found by her adult children on her bedroom floor, her body still warm.

When the EKG was connected to the electrodes on her chest, a horizontal line squiggled out, followed by a few tiny waves marking the last electrical activity in her heart. Mr. McAleer dialed the unit’s physician-on-call and described the shape of the EKG: the “asystole” rhythm from which almost no one comes back.

Chest compressions would almost certainly not revive her, and the pumping could spread infectious particles into the small room. Although the paramedics were wearing goggles, face shields and masks, the gear is not foolproof.

It fell to Mr. McAleer’s 30-year-old partner, Angel Garcia, to speak with the woman’s family. They had questions, he said, and he did his best to explain why more wasn’t being done to save her.

Mr. McAleer carried the EKG outside and tore off the slip of paper containing the last pulses of the woman’s heart. He handed it to a police officer, who radioed the woman’s death to his headquarters.