The Netherlands international, who only joined Sevilla from Valencia this summer, is set to have further examinations to find out more about the condition in the coming days.
Club medics did not detect the problem when conducting his standard medical as part of his free transfer to Andalucia, but were able to upon carrying out extra tests after he had signed.
The heart abnormality was not detected during the Dutchman's routine medical checks earlier this summer, but came to light during extra tests.
Maduro, 27, has 18 caps for Netherlands.
Sevilla suffered great tragedy in 2007 when their midfielder Antonio Puerta suffered a cardiac arrest on the field of play. He died three days later in hospital.