Introduction
The importance of recordings or sketches has never been neglected in the medical sciences, and even the oldest surgical manuals contain descriptions of reference cases. For many years collections of ECG recordings were only on paper, except for long-term recordings stored on magnetic tapes. Nowadays, digital storage remains the only practical data carrier, and we often wonder how we could ever manage without it.
Since the first issue of the American Heart Association Standard for the ECG (AHA, 1967), databases played several roles in electrocardiology, including these most important ones:
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as references for interpreting (manual or automatic) medical signals,
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to impose standards on data storage and transmission formats, and
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as starting points for new challenges in the of signal content exploration.
Databases have two principal aspects in cardiology: to provide specific raw records and an exemplary knowledge explaining the interpretation. Considering that knowledge is on a continuum, increasing the number of examples leads to better knowledge representation.
The data format is an inherent part of the database, although it is usually specified by a separate document. The description of physiological parameters contains data of various origin (time sequences, text, still images, movies, and audio) and the format must support such diverse data. Despite the need for a general format, the recording-specific formats are in common use (e.g., SCP ECG Protocol for the 12-lead ECG and VCG; Willems, 1991). The existence of well-established common formats is of paramount importance to patients because examinations are no longer limited to health centers or by manufacturers of medical equipment.
Joint representation of signals along with the accompanying medical information was the principal area of applications for early databases. The examples recognized worldwide are the MIT-BIH Arrhythmia Database (Moody & Mark, 1993) and the CSE Multilead Database (Willems, 1990). Many other databases were a result of clinical trials performed in leading research centers, and some of them are freely available from Physionet (www.physionet.org). At the moment, these databases are used for training cardiology students as well as for tuning and validating software. Apart from problem-oriented databases, few recent data collections (e.g., ICU-Database) describe the simultaneous vital signs that provide researchers with an opportunity to study correlations between the activities of different human organs.
Clinical practice is under continuous development, and the data format must provide support for currently unknown vital signs and annotations. The most frequent disadvantage of the current formats is their poor flexibility as new parameters emerge and old ones die out. The format extensibility may partially be achieved in a combination of DICOM Waveform Interchange (or HL7 level 3.0) standards and XML-structured reporting forms.