For seven and a half hours, the two New York City paramedics had worked a surprisingly normal shift: a few people with difficulty breathing, one trip to the hospital — and, miraculously it seemed, no clear cases of coronavirus.

They were about to head back to their station when an urgent call flashed over their dispatch screen: cardiac arrest, with a patient who had been feeling sick for a week.

“Heads up,” a radio barked. “Possible Covid.”

The paramedics, Sean Mahoney and Kenny Cheng, rushed to the apartment building, donned gowns, goggles and face masks and restarted the patient’s heart. But then the person crashed. The ambulance doors closed as Mr. Cheng frantically began chest compressions. Moments later, the patient was dead.

Such is the disturbing new normal for the city’s paramedics, whose days can be mundane until — suddenly — they are not.