The physionet QT database: Study on the reliability of P-wave manual annotations under noisy recordings

Thanks to its manual annotations, the PhysioNet QT database (QTDB) has been widely used as the reference of ECG delineators. However, a significant percentage of its annotations have been reported as inaccurate. Thus, any precise ECG delineator will never be able to meet, without error, all its annotations. The present work analyzes these inaccuracies and also how noise altered the final timing of annotations. As this effect is higher for low amplitude waveforms, P-waves were studied through a robust P-wave delineator. Its delineation results were compared with manual annotations under two scenarios. Firstly, a direct comparison without ECG denoising was performed. Secondly, the P-waves were delineated after efficient Wavelet-based denoising. Results showed that automatic annotations were closer to manual annotations for noisy ECGs and farther in the case of denoised ECGs, thus proving that noise altered the timing of manual annotations. An unreal improvement in delineation performance for noisy ECGs was obtained for P-wave onset, peak and offset in 45.83%, 57.29% and 56.25% of the recordings, respectively. Thus, to improve delineators reliability, either the need to review the QTDB annotations or its replacement by a better annotated database are suggested.