The Cardiopulmonary Exercise Test is a highly sensitive, non-invasive stress test. It is considered a stress test because the exercise stresses your body’s systems by making them work faster and harder. A disease or condition that affects the heart, lungs or muscles will limit how much faster and harder these systems can work. A CPET assesses how well the heart, lungs, and muscles are working individually, and how these systems are working in unison. Your heart and lungs work together to deliver oxygen to your muscles, where it is used to make energy, and to remove carbon dioxide from your body.