Know Your Hear Better: Multimodal Cardiac Output Monitoring using Earbuds

Cardiac Output (CO) is a critical indicator of health, offering insights to cardiac dysfunction, acute stress responses, and cognitive decline. Traditional CO monitoring methods, like Doppler ultrasound and impedance cardiography (ICG), are invasive and impractical for daily use, leading to a gap in continuous, non-invasive monitoring. Although recent advancements explored wearables on heart rate monitoring, these approaches face challenges in accurately estimating CO due to the indirect nature of the signals. To address these challenges, we introduce {\name}, a non-invasive multimodal CO monitoring system with Photoplethysmography (PPG) and Ballistocardiogram (BCG) signals on commodity earbuds. A novel feature fusion method is proposed to integrate raw signals and prior knowledge from both modalities, improving the systems interpretability and accuracy. {\name} achieves a lower error of 1.080~L/min in the leave-one-subject-out settings with 62 subjects, making cardiovascular health monitoring accessible and practical for daily use.