LYNN – A Lynn firefighter who collapsed while battling a house fire Tuesday morning remains hospitalized and is undergoing a series of medical tests.Firefighter Marc D. Ducharme, 52, who lives at 200 Locust St. with his wife, Debbie, was about to ascend Ladder 1 when he fell to the ground. Paramedics and emergency medical technicians (EMT) treated him at the scene of the 29 Oakwood Ave. blaze. Rescuers performed CPR on Ducharme before he was taken by ambulance to Salem Hospital and later transferred to Union Hospital.On Thursday, the firefighter remained a patient in the intensive care unit at Union Hospital and was scheduled for a battery of tests at Salem Hospital.An EMT, Ducharme recalled responding to the fire and searching the burning building for possible trapped occupants.”I came out of the house, but I don’t remember anything after that,” he said. “When I woke up in the hospital, I thought it was (the same day),” but it was already Wednesday.Ducharme, who joined the Fire Department 23 years ago, said he has undergone a series of medical tests, including some aimed at determining whether he suffers from a blocked artery.”That could be what caused this,” he said Thursday during a brief interview from his hospital bed. “They put a stent in and we’ll see how it works out. I’m extremely lucky.”With a passion for physical workouts, laying in a hospital bed has left Ducharme bored. Doctors are weighing various treatment options, he said.Ducharme is the son of Donald and Pauline Ducharme of Pleasant Street.