- You will have a cardiopulmonary exercise test (CPET) to check your lung function. The CPET checks how well your lungs are working by measuring how much oxygen and carbon dioxide your lungs take in and release when you breathe. To perform this test, you will be asked to walk on a treadmill until you reach a point that you can no longer exercise. You will be breathing into the machine while you walk. This test will take about 30 minutes to complete. The study doctor will be present for each CPET and available to respond to any complication.