Joan Rivers -- Critical Drug Missing During Fatal Procedure

Joan Rivers would be alive today if the clinic where she stopped breathing had a proper crash cart ... sources connected with her family's legal team tell TMZ.

TMZ broke the story ... there were a number of screw ups, including doctor's notes that showed an alarming amount of Propofol in her body. We're told the notes were not accurate and there was nothing in the blood work to suggest Propofol killed Joan.

But the legal team believes the Yorkville Endoscopy Center did not have critical lifesaving equipment on the crash cart ... specifically a drug called Succinylcholine, used to relax a patient's trachea. The legal team believes Joan's vocal chords spasmed and closed shut. The drug would have instantly opened them.

To make matters worse ... the legal team says the doctors were asleep at the wheel, failing to identify "deteriorating vital signs."