A medical system comprising: an implantable medical lead (6a, 26a, 106a) including at least one optical sensor comprising at least one light source (4, 104) adapted to emit light with at least one predetermined wavelength at at least one predetermined intensity and a light detector (9, 109), an implantable medical device (2, 20) being connectable to said implantable medical lead (6a, 26a, 106a) and comprising: a pulse generator adapted to produce cardiac stimulating pacing pulses, and an edema detection circuit adapted to activate said at least one light source (4, 104) to emit light, during at least one measurement session, at said at least one predetermined wavelength and at said at least one predetermined intensity, wherein said edema detection circuit is adapted to obtain at least one light intensity value corresponding to at least one intensity of light received by said light detector (9, 109) and resulting from said light emitted during a measurement session, characterized in that said medical system is a medical system for detecting incipient edema in a lung of a patient, said light emitted by said at least one light source (4, 104) is directed into lung tissue of said patient, said edema detection circuit is adapted to evaluate said at least one light intensity value by comparing said at least one light intensity value with at least one reference intensity value, and said edema detection circuit is adapted to detect a consistency with incipient edema in said lung tissue when at least one of said at least one light intensity value is below said at least one reference intensity value. an implantable medical lead (6a, 26a, 106a) including at least one optical sensor comprising at least one light source (4, 104) adapted to emit light with at least one predetermined wavelength at at least one predetermined intensity and a light detector (9, 109), an implantable medical device (2, 20) being connectable to said implantable medical lead (6a, 26a, 106a) and comprising: a pulse generator adapted to produce cardiac stimulating pacing pulses, and an edema detection circuit adapted to activate said at least one light source (4, 104) to emit light, during at least one measurement session, at said at least one predetermined wavelength and at said at least one predetermined intensity, wherein a pulse generator adapted to produce cardiac stimulating pacing pulses, and an edema detection circuit adapted to activate said at least one light source (4, 104) to emit light, during at least one measurement session, at said at least one predetermined wavelength and at said at least one predetermined intensity, wherein said edema detection circuit is adapted to obtain at least one light intensity value corresponding to at least one intensity of light received by said light detector (9, 109) and resulting from said light emitted during a measurement session, characterized in that said medical system is a medical system for detecting incipient edema in a lung of a patient, said light emitted by said at least one light source (4, 104) is directed into lung tissue of said patient, said edema detection circuit is adapted to evaluate said at least one light intensity value by comparing said at least one light intensity value with at least one reference intensity value, and said edema detection circuit is adapted to detect a consistency with incipient edema in said lung tissue when at least one of said at least one light intensity value is below said at least one reference intensity value.