Rosemary Vogel's 66-year-old husband was already in critical condition as he recovered from heart surgery at the Chandler Regional Medical Center.
Rosemary is a registered nurse and was a hospital employee at one time, according to police.
She somehow got poop into a syringe and appears to have inserted it into her husband's IV, according to KTVK-TV.
Emergency alarms went off on Thursday afternoon and responding nurses found the woman holding the IV bag. The nurses saw a brown liquid in the bag and immediately pulled it from her arm and went to get help.
When the nurse returned, Vogel was trying to empty the poop-soaked bag into a trash can but was stopped.
Later tests on the brown substance returned positive for faecal matter - but it is unclear whose it was or how Vogel did it.
Police also do not have a motive yet, the New York Daily News reported.
"This is the first that I've heard where faecal matter has been injected into an IV system," Sgt. Joe Favazzo of the Chandler police told KTVK.
Cops also found three syringes in the woman's purse, including two filled with fluid and a third filled with faeces, cops said.
The critically ill man is expected to survive the poop attack and his wife is behind bars in lieu of a USD 100,000 bond.
She also faces a vulnerable adult abuse charge, the report said.
"He's in his recovery process not only from his medical procedure, but from this attempt on his life," Favazzo told the TV station.