A cardiovascular monitoring system for measuring at least cardiac output comprises an implantable unit (2) configured for placement in a section of artery of a patient and a portable external unit (1) configured for mounting against or close to a patient's skin for bidirectional communication with the implantable unit. A cross- sectional area measurement sensor (8) includes at least one elastic conductive loop (15, 15a, 15b) and an electronic circuit (16) connected to the conductive loop configured to measure an inductance value of the conductive loop. At least one ultrasound transducer (7,11) is further configured to measure a velocity of blood flow based on a Doppler effect.