With this study the investigators intend to examine the long-term outcome on cardiac output after heart surgery in VSD-patients. It presents an opportunity to also evaluate the correlation between cardiac output determined by gas-exchange and by MRI. The overall objectives of this study are to 1) examine whether VSD-operated patients have reduced cardiac output during exercise in comparison with matched controls, and furthermore 2) to evaluate a correlation between cardiac output measured by MRI and cardiac output determined by gas-exchange.