Cardiac output (CO), as an evaluation index of cardiac pulsation capacity, is crucially important to be actively and accurately monitored in clinical settings. Recent studies have demonstrated the promising results of electrical impedance tomography (EIT) in CO monitoring, while also facing certain challenges, such as low accuracy of CO calculation based on the simplified 2D imaging and significant interference from the lungs during cardiac measurement. In this regard, this article proposes a method for CO monitoring based on 3D imaging with a dedicated EIT planar sensor array, and accelerates the reconstruction speed by introducing the region of interest (ROI) sensing and imaging strategy. Through the optimization of sensor layout, most of the injected current can focus on the cardiac region, thereby reducing the interference from the lungs. Numerical simulations were carried out to validate the proposed method. Results show that the proposed method can realize real-time and accurate CO monitoring.