Poggiali was taken into custody over the weekend in Lugo, northern Italy, after a 78-year-old patient died while being treated for a routine illness, the Daily Mail reported at its website.
Tests showed the woman died with a high amount of potassium in her system, which can provoke cardiac arrest, according to Central European News.
Police said Tuesday they had expanded their investigation to include 38 patient deaths.
Chief prosecutor Alessandro Mancini told a press conference covered by the newspaper Corriere Della Sera that the nurse seemed "unperturbed" when she was arrested.
Other meda reports cited a co-worker as describing Poggiali as a "cold person but always eager to work."
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