Multi-Class ECG Feature Importance Rankings: Cardiologists vs. Algorithms

Philip Aston1, Temesgen Mehari2, Alen Bosnjakovic3, Peter Harris4, Ashish Sundar4, Steven Williams5, Olaf Doessel6, Axel Loewe7, Claudia Nagel8, Nils Strodthoff9
1University of Surrey / National Physical Laboratory, 2Heinrich Hertz Institute, 3Institute of Metrology of Bosnia and Herzegovina, 4National Physical Laboratory, 5University of Edinburgh, 6Institute of Biomedical Engineering, Karlsruhe Institute of Technology, 7Karlsruhe Institute of Technology (KIT), 8Institute of Biomedical Engineering, Karlsruhe Institute of Technology (KIT), 9University of Oldenburg

Abstract

Introduction Cardiologists diagnose over 150 conditions from an electrocardiogram (ECG) based on interval, amplitude and timing features. For each pathology, conditions on specific features are well documented.

Conversely, there are numerous algorithms that rank feature importance for a particular classification task. However, different algorithms often give quite different rankings. Therefore, we compared the feature importance ranking obtained by various algorithms with the features that cardiologists use for diagnosis.

Methods In previous work, we considered feature importance rankings for binary classifications of Normal vs. a single pathology. Here we consider a multi-class classification with classes of Normal, first degree atrioventricular (AV) block, right bundle branch block (RBBB) and left branch bundle block (LBBB). Cardiologists use only the PR interval to diagnose first degree AV block, and 7 or 14 features to diagnosis RBBB or LBBB respectively. For Normal class diagnosis, we took the union of features used for diagnosis of the three pathologies, which gave a total of 18 features.

We obtained four feature importance rankings for binary classifications of one class vs. all the other classes. A scoring algorithm was devised to compare rankings with the features used by cardiologists.

Results As with the single pathology rankings, most methods ranked the PR interval first for AV block. For the other classes, our scoring algorithm generally gave high scores for RBBB, middling scores for Normal, and low scores for LBBB. Including features that correlate with the features used for diagnosis increased the scores, in some cases significantly. Methods based on Random Forests generally gave the best scores while ReliefF performed poorly.

Conclusions In this multi-class classification problem, the feature rankings are quite similar to those for the corresponding binary classifications with scores for RBBB being higher than for LBBB. Scores generally decrease for conditions that involve many features for diagnosis.