This paper presents a novel method for the measure of the cross-sectional area (CSA) of an artery to be used for the direct and continuous (24/7) monitoring of cardiac output (CO). The method is based on the inductance change of an anchoring loop mounted on a miniaturized system implanted in a section of the pulmonary artery. The inductive sensor comprises an inductive readout circuit and a conductive anchoring loop, which changes its inductance according to the deformation of the artery and can be correlated to the diameter and CSA of the artery section. Direct and periodic measurement of the CSA can improve the accuracy of CO monitoring in heart failure patients. An oscillator-based inductive readout IC was realized in a 180-nm CMOS process and the anchoring loop was implemented by a nitinol wire. The readout IC achieves 0.42 nH inductance resolution in a range from 181 nH to 681 nH and draws 39.7 μA to 51.2 μA current from a 1.2 V supply. The correlation between the artery diameter and loop inductance is demonstrated and the sensor achieves 0.24 mm resolution in a diameter range from 20 mm to 30 mm, a factor of four higher than the lateral resolution of echocardiography.