Autonomous Heart Rate Tracking Methodology Using Kalman Filter and the EM Algorithm

Accurate heart rate monitoring during intense physical activity is a challenging problem due to high levels of motion artifacts (MA) in sensors that rely on stable physical proximity/contact for accurate measurement extraction. Photoplethysmography (PPG) sensor is a non-invasive optical sensor that is widely used in wearable devices, such as smartwatches, to measure blood volume changes using the property of light reflection and absorption; these measurements can be used to extract the heart rate (HR) of an individual wearing that device. The PPG sensor is susceptible to the motion artifact which increases with physical activity. Since the frequency of the motion artifact is very close to the range of HR, estimation of HR information becomes very challenging. As a result, MA removal remains an active research topic over the last few years. Several approaches have been developed in the recent past for MA removal and accurate HR estimation. Among these recent works, a Kalman Filter (KF) based approach showed promising results for accurate estimation and tracking of HR based on PPG measurements. However, the previous KF based HR tracker was demonstrated for a particular dataset with manually tuned filter parameters. Such a custom tuned approach might not perform accurately in practical scenarios where the amount of motion artifact and the heart-rate variability depend on numerous, unpredictable factors. In this paper, we develop an approach to automatically tune the KF based HR tracker based on the expectation maximization (EM) algorithm. The applicability of the proposed approach is demonstrated using an open-source PPG database that was recorded during varying pre-determined physical activities.