Multiple Instance Learning Framework can Facilitate Explainability in Murmur Detection

Objective: Cardiovascular diseases (CVDs) account for a high fatality rate worldwide. Heart murmurs can be detected from phonocardiograms (PCGs) and may indicate CVDs. Still they are often overlooked as their detection and correct clinical interpretation requires expert skills. In this work, we aim to predict the presence of murmurs and clinical outcome from multiple PCG recordings employing an explainable multitask model. Approach: Our approach consists of a two-stage multitask model. In the first stage, we predict the murmur presence in single PCGs using a multiple instance learning (MIL) framework. MIL also allows us to derive sample-wise classifications (i.e. murmur locations) while only needing one annotation per recording ("weak label") during training. In the second stage, we fuse explainable hand-crafted features with features from a pooling-based artificial neural network (PANN) derived from the MIL framework. Finally, we predict the presence of murmurs as well as the clinical outcome for a single patient based on multiple recordings using a simple feed-forward neural network. Main results: We show qualitatively and quantitatively that the MIL approach yields useful features and can be used to detect murmurs on multiple time instances and may thus guide a practitioner through PCGs. We analyze the second stage of the model in terms of murmur classification and clinical outcome. We achieved a weighted accuracy of 0.714 and an outcome cost of 13612 when using PANN model and demographic features on the CirCor dataset (hidden testset of the George B. Moody PhysioNet challenge 2022, team "Heart2Beat", rank 12 / 40). Significance: To the best of our knowledge, we are the first to demonstrate the usefulness of MIL in PCG classification. Also, we showcase how the explainability of the model can be analyzed quantitatively, thus avoiding confirmation bias inherent to many post-hoc methods. Finally, our overall results demonstrate the merit of employing MIL combined with handcrafted features for the generation of explainable features as well as for a competitive classification performance.