In a device and method for a medical implant for monitoring progression of heart failure in a human heart, an activity sensor provides information related to the activity level of a patient and an oxygen sensor provides information related to the level of oxygen content in venous blood. A determined level of venous oxygen content at a determined activity level is obtained, and that level of venous oxygen content is compared to stored values at a corresponding activity level. The result of the comparison is used as a basis for determining a degree of heart failure.