Online Cardiac Monitoring (OCM) emerges as a compelling enhancement for the next-generation video streaming platforms. It enables various applications including remote health, affective computing, and deepfake detection. Yet the physiological information encapsulated in the video streams has been long neglected. In this paper, we present the design and implementation of CardioLive, the first online cardiac monitoring system in video streaming platforms. We leverage the naturally co-existed video and audio streams and devise CardioNet, the first audio-visual network to learn the cardiac series. It incorporates multiple unique designs to extract temporal and spectral features, ensuring robust performance under realistic streaming conditions. To enable the Service-On-Demand OCM, we implement CardioLive as a plug-and-play middleware service and develop systematic solutions to practical issues including changing FPS and unsynchronized streams. Extensive evaluations demonstrate the effectiveness of our system. We achieve a Mean Squared Error of 1.79 BPM error, outperforming the video-only and audio-only solutions by 69.2% and 81.2%, respectively. CardioLive achieves average throughput of 115.97 and 98.16 FPS in Zoom and YouTube. We believe our work opens up new applications for video stream systems. Code is available at https://github.com/aiot-lab/CardioLive.