The pair were involved in a relationship and the nurse's husband is among the suspected victims, Italian media reported.
According to investigators, anaesthetist Leonardo Cazzaniga, 60, is accused of causing the deaths of at least four elderly patients. The suspected victims, who died between February 2012 and April 2013, suffered from different illnesses including cancer and Parkinson's disease.
Preliminary investigating judge Luca Labianca said a cocktail of medicines, including extremely high doses of morphine and Propofol, was administered to the victims "in overdoses and in rapid succession".
According to one of Cazzaniga's colleagues the anaesthetist frequently referred to himself as an "angel of death".
The 45-year-old husband of Laura Taroni, a 40-year-old nurse who worked with Cazzaniga at Saronno Hospital in the Varese region, also died under suspicious circumstances.
Both Cazzaniga and Taroni are suspected of involvement in his death; the pair reportedly convinced the man he was diabetic, with Cazzaniga falsifying a medical report and blood test results.
Taroni's husband died in summer 2013, after regularly taking medicine which the police report said was "absolutely incongruous with his actual health conditions, weakening and eventually killing him".
Police said that the number of deaths could be higher than five, but that in other cases the possibility of death by natural causes could not be ruled out, due to the poor health of the deceased. Prosecutors are also investigating the deaths of some of Taroni's husband's relatives, according to Italian daily La Repubblica.
Phonecalls intercepted by police recorded the couple as they discussed their crimes.
In one exchange published by Il Corriere, Cazzaniga asked Taroni if he could be convicted of euthanasia (which is illegal in Italy), to which she explained "euthanasia is a different thing; you sign an agreement and then make a cocktail of drugs." The anaesthetist's response was: "So it's murder that I could be accused of."
In one of the most disturbing calls, Taroni told Cazzaniga she was prepared to kill her own children, aged eight and 11, to which the anaesthetist replied: "No, not the children".
Investigators also recorded Taroni discussing the "perfect murder" with her elder son.
The investigation into the suspicious deaths began in June 2014 after another nurse filed a report with police. The couple were arrested on Tuesday and Taroni's two children have been put into care.
A further 12 workers at the Saronno Hospital are under investigation in connection with the deaths, including one head physician and the head of the emergency department, suspected of aiding and abetting and failing to report breaches in hospital protocol.