Joan Rivers is resting comfortably in critical but stable condition at a New York City hospital after she suddenly stopped breathing Thursday during routine throat surgery. The update was provided late last night by Rivers' daughter, Melissa.

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On Fox and Friends this morning, we heard from anesthesiologist Dr. Nina Radcliff on what may have caused the complications. She said it seems like Rivers was undergoing an upper endoscopy, where a small camera is placed down the patient's throat.

Radcliff said the device must pass behind the vocal cords, which function as a "gate and a guard" against foreign substances entering the lungs.

"It's a normal response for us to cough or for our vocal cords to spasm. ... When they close, oxygen cannot get into your lungs," she explained.

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NEW YORK –  Joan Rivers' daughter said late Thursday her mother was "resting comfortably" after being admitted to Mount Sinai hospital in New York earlier in the day, reportedly after she stopped breathing during throat surgery.

A report on E! News, where Rivers works, stated that the comedian was in the Intensive Care Unit.

"I want to thank everyone for the overwhelming love and support for my mother," her daughter Melissa said in her statement. "She is resting comfortably and is with our family. We ask that you continue to keep her in your thoughts and prayers."

A source with knowledge of the situation told FOX411 that Rivers was in critical condition when she was taken to the hospital.

Police would not confirm Rivers' identity, but told FOX411: "An 81-year-old female was transported Mount Sinai in critical condition... at 9:39 a.m."Law enforcement said the woman was "unconscious and unresponsive."

The New York Fire Department added the person transported was in cardiac arrest.

TMZ reports Rivers stopped breathing during a throat procedure at a local clinic, and someone at the clinic called 911. E! News said Rivers was undergoing a minor endoscopic procedure.

Mount Sinai did not release any information about Rivers' condition.

"Her family wants to thank everybody for their outpouring of love and support," a rep for the hospital said. "We will provide an update on her condition as it becomes available.”

E! News reported Melissa Rivers rushed to New York to be with her mother. Rivers is the lead host on E!'s hit series "Fashion Police," which features a panels of celebrity commentators dissecting the outfits stars are spotted wearing.

Rivers' rep was not available to comment.