An implantable medical system for detecting incipient edema has an implantable medical lead including an optical sensor having a light source and a light detector. The medical system further has an edema detection circuit that activates the light source to emit light, the light being directed into lung tissue of a patient and that obtains a light intensity value corresponding to an intensity of light received by the light detector, and that evaluates the light intensity value to detect a consistency with incipient edema.