Every Beat Counts: Beat-Anchored Probing of Frozen ECG Encoders for Cross-Site Diagnosis

Electrocardiogram (ECG) diagnosis models that match cardiologists within one hospital degrade markedly at another, which hinders their clinical adoption. While existing efforts scale up encoder pretraining, we revisit this challenge from the readout side by asking where frozen encoders keep the information that cross-site diagnosis requires. Through phase-resolved probing of frozen tokens around each heartbeat, we find that such information is present and locked to the beat, yet mean pooling dilutes it for diagnoses confined to part of the beat. A multi-query attentive probe recovers part of it, but under diagnostic supervision alone its queries remain unassigned, covering only part of the beats and keeping to no phase of their own. Motivated by this, we propose the Beat-Anchored Probe (BAP), which expands one R-peak per beat into attention targets for the P, QRS, ST and T phases. Its training-only alignment loss allocates each query over every beat and registers it to one phase, leaving what the query reads to the diagnostic loss. With the encoder frozen, BAP achieves the highest mean transfer among the compared probes and improves on the linear probe on all four encoders, with the largest gains on the diagnoses that mean pooling dilutes, and its phase roles stay fixed across training seeds and decodable on an external benchmark. We further release CPSC2018-R, cardiologist-annotated R-peaks for CPSC2018, which provides the manual reference for phase-resolved probing and for verifying such probes. Code and data are at https://anonymous.4open.science/r/BAP-1490/.