Prof. Hager is a pediatric cardiologist heading the exercise laboratory at the Department of Pediatric Cardiology and Congenital Heart Disease performing annually more than 1000 CPET in adolescents and adults with congenital heart disease. His CPET database includes more than 10000 CPET in that patient group and is analyzed continually. He published more than 150 papers predominantly on the long-term follow-up of patients with complex congenital heart defects. He is head of the outpatient section with about 7000 consultations per year, which is a referal center for rare congenital heart diseases or complex defects.

As a member of the Guideline Committee of the German Pediatric Cardiology Society he authored several guidelines and reviewed all other guidelines about congenital heart diseases and pediatric cardiologic issues.

Prof. Hager is board member of CPX International and hosted the Practicum on Cardiopulmonary Exercise Testing in 2015.