The Taxonomies, Training, and Applications of Event Stream Modelling for Electronic Health Records

Abstract

The widespread adoption of electronic health records (EHRs) enables the acquisition of heterogeneous clinical data, spanning lab tests, vital signs, medications, and procedures, which offer transformative potential for artificial intelligence in healthcare. Although traditional modelling approaches have typically relied on multivariate time series, they often struggle to accommodate the inherent sparsity and irregularity of real-world clinical workflows. Consequently, research has shifted toward event stream representation, which treats patient records as continuous sequences, thereby preserving the precise temporal structure of the patient journey. However, the existing literature remains fragmented, characterised by inconsistent definitions, disparate modelling architectures, and varying training protocols. To address these gaps, this review establishes a unified definition of EHR event streams and introduces a novel taxonomy that categorises models based on their handling of event time, type, and value. We systematically review training strategies, ranging from supervised learning to self-supervised methods, and provide a comprehensive discussion of applications across clinical scenarios. Finally, we identify open critical challenges and future directions, with the aim of clarifying the current landscape and guiding the development of next-generation healthcare models.

Figures (4)

• Figure 1: The flow diagram of the literature searching strategy.
• Figure 2: Illustration of the event stream and multivariate time series representations of EHR. (a) Raw EHR Data: Clinical events spanning multiple heterogeneous types (e.g., vital signs, lab tests, medications, procedures, and diagnoses) are recorded at irregular, continuous timestamps. (b) Event Stream: All extracted clinical events are arranged chronologically into a sequence, with co-occurring events assigned an arbitrary relative order. (c) Multivariate Time Series: The data is converted into a structured grid by aligning variables to fixed time intervals (e.g., hourly windows). To reduce sparsity, features with a high missing rate are filtered out, and missing data for the retained variables are imputed, resulting in a dense, regularly sampled matrix.
• Figure 3: Overview of event stream modelling. This overview categorises the field into three primary components, each highlighted using a distinct colour. Event stream modelling details the modelling methods for different components of an event. Training strategy outlines different learning paradigms for event stream models, including supervised and self-supervised methods. The application focuses on the different healthcare scenarios, including critical care, emergency department, hospital and secondary care, and primary care.
• Figure 4: Illustration of self-supervised learning methods for EHR event stream modelling. The four main approaches are: (a) Next-token prediction, where the model learns to predict the subsequent event in the sequence given the preceding events. (b) Masked reconstruction, where events in the input sequence are deliberately hidden (masked), and the model is trained to reconstruct the complete sequence. (c) Skip-gram ($n=1$), where the model uses a single target event (e.g., $t_2$) to predict its surrounding context events (e.g., $t_1$ and $t_3$). (d) Contrastive learning, which trains the model to maximise the embedding similarity between an original sequence and a positive pair that are generated via event swapping of co-occurring events, while minimising the similarity against a negative pair that is generated via event replacement.