< PREVIOUS SLIDE SLIDE 1 of 3 NEXT SLIDE > An FDNY paramedic disinfects the ambulance equipment after bringing a patient to Wyckoff Hospital in the Bushwick section of Brooklyn, April 5, 2020, in New York. © Bryan R. Smith/AFP via Getty Images

As more than 1,300 New York City first responders return to work after recovering from the novel coronavirus or calling out sick with symptoms of the virus, they're responding to a rapid increase in 911 calls for cardiac arrest, the FDNY said on Tuesday.

The city's firefighters, paramedics and emergency medical technicians (EMT) are responding to "a record numbers of calls, and they continue to meet this unprecedented challenge head on,” said Fire Commissioner Dan Nigro. "I am incredibly proud of the men and women of this department who are demonstrating every single day throughout this pandemic why they are known as the best and the bravest."

Nearly 500 members of the FDNY have tested positive for COVID-19 since the outbreak was detected in New York State on March 1.

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As of Monday, more than 138,000 New Yorkers have tested positive for the virus, according to the state's Department of Health. More than half of the positive COVID-19 patients in New York State are located within the five boroughs, deemed the epicenter of the health crisis.

The city's density has contributed to the spread of the virus, according to health experts. With 27,000 people per square mile, the city is the densest metropolitan area in the U.S.

The FDNY has experienced a 50% increase in daily calls as well as a huge increase in cardiac-related calls, the department said.

A year ago -- during the same time frame of March 20 to April 5 -- the FDNY responded to an average of 54 to 74 cardiac arrest calls per day, with 22 to 32 deaths.

Now on average it’s 300 cardiac calls a day, with well over 200 deaths.

While it's not always clear if those who die from cardiac arrest have the coronavirus, the CDC has issued updated guidance for certifying deaths due to COVID-19 -- protocols similar to those in place for pneumonia and influenza.

According to the new directives, if a patient has died from pneumonia, for example, but also tested positive for COVID-19, someone is required to specify whether COVID-19 played a role in the death and whether it was actually the underlying, primary cause.

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The U.S. has more confirmed COVID-19 cases than any other country, with almost 380,000 people diagnosed with the virus, according to data compiled by the Center for Systems Science and Engineering at Johns Hopkins University.

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ABC News' Dr. Mark Abdelmalek, Josh Margolin, Eden David and Ella Torres contributed to this report