Since the beginning of the coronavirus outbreak, thousands of New York City residents with symptoms of the disease it causes have called for ambulances, only to die before or just after the EMTs arrived.

Why? The experts and the data suggest the reasons are linked to the patients' home addresses — but also to effects of the virus on the body that have come into better focus only after six weeks of deaths. Many patients probably didn't know how sick they really were.

NBC News reviewed data from Emergency Medical Services, the division of the city's fire department that handles 911 calls, showing that the number of cardiac calls — calls for patients whose hearts have stopped or are near death — has spiked since the beginning of the pandemic, as has the number of those calls that end in death. The surge is particularly noticeable in the city's poorest neighborhoods.

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On March 23, EMS call volume started rising to record levels, from 4,000 on an average day to more than 6,500 calls a day.

The fire department's chief of emergency medical services, Lillian Bonsignore, said at the time, "I've been in this profession for about 30 years, and I've never seen anything like this in my whole career — or in my life, for that matter."

From March 1 to April 13, poor neighborhoods in the Bronx, Brooklyn and Queens recorded two to three times the number of cardiac calls compared to the same period in 2019, with three to five times as many patients dying.

From March 1 to April 13, 2019, almost 60 percent of cardiac call patients in the borough of Queens survived. This year during the same period, 70 percent died.

Experts say that patients in those poor neighborhoods are more likely to have underlying conditions like heart disease and diabetes that can make coronavirus infection lethal and that they may be wary of seeking health care until an emergency arises because of the cost. The neighborhoods also have higher numbers of people per residence and many multigenerational households, aiding the spread of infectious diseases.

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A look at the city's poverty rates by ZIP code and data from the fire department's 911 dispatch system shows the link. In the Bronx neighborhoods encompassing ZIP codes 10456, 10467 and 10468, 28 percent to 38 percent of residents live below the poverty line, compared to a citywide rate of 18 percent. In those ZIP codes, there were 114 cardiac calls and 46 deaths a year ago. This year, over the same March 1-to-April 13 period, there were 346 cardiac calls and 223 deaths.

In the Rockaways, an area of Queens, the poverty rate is close to 20 percent, and 60 percent of the population identifies as black or Hispanic, both groups hit hard by COVID-19, the disease associated with the coronavirus. There were 76 cardiac calls and 35 deaths a year ago. This year, the totals were 204 cardiac calls and 151 deaths.

In Brooklyn's East New York, the poverty rate approaches 25 percent, and more than 90 percent of the population identifies as black or Hispanic. There were 79 cardiac calls with 34 deaths last year, compared to 168 cardiac calls and 114 deaths this year.

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The data sharply contrast with the numbers from affluent lower Manhattan. In ZIP codes that cover the financial district, Tribeca and parts of SoHo, the poverty rate averages 8.8 percent and more than two-thirds of residents are white. Last year, there were 30 cardiac calls with 12 deaths, and this year there were 42 cardiac calls with 23 deaths. There were more calls and more deaths, but nothing like the uptick in poorer, nonwhite neighborhoods.

Dr. Ashwin Vasan, an assistant professor at Columbia University's Mailman School of Public Health, called the data dramatic, saying, "This virus has come into our society and simply exposed the structural weaknesses and the inequities and the disparities that were already there."

Lung capacity

But Vasan and Dr. Eili Klein, an assistant professor at the Johns Hopkins School of Medicine's Department of Emergency Medicine, both said other, non-demographic factors also fed the spike in cardiac calls.

They said messaging to members of the public early in the outbreak about avoiding hospitals unless they were very ill probably led people to stay home with lesser symptoms.

As doctors' understanding of the disease improved, they discovered that many patients had greatly diminished lung capacity long before they displayed those serious symptoms that were supposed to make them call 911.

Klein said COVID-19 can cut lung capacity by as much as 50 percent before a patient even knows it, like a form of "cryptic" pneumonia. People can feel sick but not short of breath.

"It's attacking the lungs in a way that's not fully giving all the signals to your immune system that it's attacking the lungs, so the lungs are sort of acting regularly," Klein said.

Vasan said this hidden phenomenon can lead to a sudden decline in a COVID-19 patient's health, the kind that elicits a 911 call when it's too late. "People are at home with these really low oxygen levels," he said, "and then they hit that tipping point, and it just gets beyond the point of return — in the case of this data, the point of cardiac arrest."

A doctor who has been treating patients at one of New York City's busiest hospitals, Richard Levitan, wrote in a New York Times opinion piece about patients he treated who had symptoms for long periods before they went to the hospital.

"To my amazement," Levitan wrote, "most patients I saw said they had been sick for a week or so with fever, cough, upset stomach and fatigue, but they only became short of breath the day they came to the hospital."

"We are just beginning to recognize that COVID pneumonia initially causes a form of oxygen deprivation we call 'silent hypoxia' — 'silent' because of its insidious, hard-to-detect nature," he wrote.

Vasan and Klein both said EMS did the best it could in March and April given the hand it was dealt.

"I think our EMS operators are heroes, our FDNY are heroes," Vasan said, "and they did the best and they've been doing the best with a very difficult set of circumstances."