Assessing Feature Importance in Cardiovascular Variability for the Classification of Physiological Stress

Explainable artificial intelligence improves the interpretability of machine learning models, which is essential for reliable decision-making in healthcare. Integrating information-theoretic (IT) approaches into feature selection (FS) facilitates a more rational assessment of feature importance (FI) by capturing redundant and synergistic contributions. This study proposes a novel combined FI-FS method that extends the High-order Interactions Feature Importance (Hi-Fi) framework by replacing variance-based with IT metrics for improved quantification of high-order feature interactions. The method adapts the Leave One Covariate Out metric to identify feature subsets that maximise or minimise Conditional Mutual Information (CMI), prioritising features that increase synergy and reduce redundancy. Feature interpretation is further supported by analysing how selected variables interact, revealing both individual and joint predictive relevance. The proposed framework is applied to cardiovascular time series from 127 young, healthy individuals recorded at rest and under stress conditions (postural and mental stress). The analysed features are extracted from beat-to-beat electrocardiographic RR intervals, pulse-pulse intervals (PP), and systolic and diastolic blood pressure (SBP, DBP) time series and are computed in the time, frequency, and information domains. FI results show that features reflecting variability, spectral content, and entropy, especially from RR, DBP, and PP, are more informative than static measures. RR features consistently show the highest unique information, while PP contributes mainly through synergy. The FS process reduces feature count by ∼20%, and a Support Vector Machine trained on the selected features achieves ∼80% accuracy in multi-class stress classification. Overall, this IT-based Hi-Fi framework captures individual informativeness and complex signal interactions, improving dependency detection, interpretability, and physiological insight.