Joan Rivers -- Doctors Reportedly Kept Operating As Vital Signs Plummeted

Doctors performing Joan Rivers' fatal procedure were rooting around in her throat with a surgical instrument even as her vital signs deteriorated ... this according to a new report.

And the doctors delayed calling 911 for a critical 10 minutes and instead tried reviving her themselves ... this according to the NY Post.

The Post report says even before Joan went into cardiac arrest ... her pulse and blood pressure were dropping quickly -- a sign she was in the danger zone -- but the doctors continued scoping her larynx and trachea.

Joan's pulse had reportedly sunk to 47 prior to the anethesiologist administering Propofol.

TMZ broke the story ... a federal report showed there were major problems with the amount of Propofol used on Joan.

Dr. Lawrence Cohen from Yorkville Endoscopy and Joan's own ENT, Dr. Gwen Korovin attempted to resuscitate Rivers, but failed during the critical 10 minutes.

Dr. Korovin and Dr. Cohen have not responded to reports detailing alleged errors during the procedure.