Over 200 patients at NYU Langone Medical Center were transfered to other facilities after back up generators failed to restore power.

New York University Langone Medical Center was evacuated between Monday evening and Tuesday, after back-up generators failed to restore power to the lower Manhattan hospital; more than 200 patients, including 20 intensive care unit newborns were moved to nearby facilities.

The hospital reports that it began experiencing “large-scale power failures in critical areas,” Monday evening due to the storm, and the “emergency systems did not kick in,” the New York Times reported.

Patients were moved to area hospitals such as Sloan Kettering, Mount Sinai and New York-Presbyterian in private ambulances with the assistance of the New York Fire Department, city officials said.

"It's a challenging situation," NYU Medical Dean Robert Grossman told WCBS-TV. "We drill all the time for this kind of thing. But this isn't a drill. This is the real thing."

Two-dozen ambulances collected patients and transferred them to local hospitals. The youngest and sickest were taken first. Some patients were being sustained on battery-operated respirators. While others, such as cancer patients, were carried down stairs; with the power out, there was no access to elevators, Grossman said.

"Every four minutes a patient comes out and an empty ambulance pulls up. The lobby of the Medical Center is full of hospital personnel, family members, and patients," ABC's Josh Haskell reported at the scene.

Normally, the hospital houses 800 patients, but many were discharged prior to the storm, according to ABC News. With the remaining patients transferred to other hospitals, emergency management officials are considering it “a total evacuation.”

Con Edison reported that most of the Lower Manhattan power outages, including at NYU Langone, were due to an explosion at an electrical substation. John Miksad, senior vice-president for electric operations, said it remains unclear whether flooding or flying debris caused the explosion.