ECG analysis for sleep apnea detection.

OBJECTIVES: The objective of our study was to find out whether obstructive sleep apnea (OSA) may be detected on ECGs recorded during sleep. METHODS: We have analyzed 70 eight-hour single-channel ECG recordings taken at polysomnographia. The 70 data sets were annotated for definition of regular sleep and phases with sleep apnea. From the 70 data sets, 35 have been used as a learning set. Our analysis is based on spectral components of heart rate variability. Frequency analysis was performed using Fourier and wavelet transformation with appropriate application of the Hilbert transform. Classification is based on four frequency bands: ULF band (0-0.013 Hz), VLF band (0.013-0.0375 Hz), LF band (0.0375-0.06 Hz) and the HF band (0.17-0.28 Hz). Linear discriminant functions were applied using mainly spectral components derived from the records. Classification of cases was based on three variables. RESULTS: For the Testing Set, a sensitivity (Se) for apnea of 92.3% at a specificity (Sp) of 94.6% was achieved. For the minutes allocation on the Learning Set Se was 90.8% at Sp 92.7%. CONCLUSION: ECG analysis is useful for the detection of sleep apnea and may help to differentiate causes of cardiac arrhythmias.

SCP-ECG and Vital Signs Information Representation--two examples of successful transcontinental cooperation in medical informatics standardization.

During the past 2-3 decades, development work on Medical Devices focused on improving their functionality (device control, signal analysis, pattern recognition and classification). At present, the dominant user requirement is information integration. This requires interconnectivity and interoperability of devices and device systems. Today, due to all major manufacturers operating globally, this requirement can only be met if standards with global relevance are developed. The first standard discussed is the Standard Communications Protocol for Computerized Electrocardiography (SCP ECG). The rapid finalization of the standards document itself was possible due to some of the experts being involved on basic problems for greater than 10 years. Their knowledge and long standing cooperation contributed significantly to a fast consensus finding process as well as quick implementation. The second standard discussed in this paper deals with Vital Signs Information Representation. Before starting to write the document, an International inventory workshop was held to document the state of the technology and of user requirements. This common base of problem understanding makes consensus finding less difficult. Continuous cooperation between the IEEE P1073 group and the CEN/TC251 WG5 project team also ensures early harmonization of the related standards. Finally, early implementation projects in Europe and the US allow verification and modifications based on feed-back from practical experience.

Integration and communication for the continuity of cardiac care (I4C).

The project I4C (Integration and Communication for the Continuity of Cardiac Care) is carried out for the advancement of cardiac care, from prevention to follow-up. The goals of I4C are: (1) integrated access to patient data, wherever they are stored; (2) support of evidence-based care; (3) consistent recording of patient data (eg, patient history, electrocardiograms IECGs] or cine-angios) in a multimedia patient record; and (4) a documented reference data set for research. In several clinics, workstations are being installed to serve the four goals. Integration with other information systems in clinical care is realized by encapsulation. A computer-based patient record (ORCA) has been developed to support the collection, consultation, and sharing of patient data. In I4C, ORCA is intended for use in a research setting as well as routine patient care. The functionality of ORCA covers the collection of patient history data in a highly structured manner, the recording of drug prescriptions, an overview of laboratory test results, and viewers for ECGs and angiographic images. At present, structured data entry and consultation is supported in six European languages.

Analysis of interval measurements on CSE multilead reference ECGs.

As part of the International Electrotechnical Commission's (IEC) efforts to develop standards for interpretive electrocardiographs, electrocardiograms (ECGs) of the Common Standards of Electrocardiography (CSE) multilead measurement database were processed to (1) determine whether original ECG data would offer any advantage over cycle-repeated (artificial) ECGs and (2) compare the "clean" ECGs selected for interval measurement compliance testing (n = 100) with the remaining 25 "noisy" or nonsinus rhythm ECGs. Two sets of CSE measurement ECGs, namely, 125 original ECGs of the MO1 series and 125 artificial ECGs of the MA1 series were divided into 100 IEC-selected clean ECGs with good P and T waves and 25 noisy or nonsinus ECGs and processed for global wave duration and interval measurements (P duration, PR interval, QRS duration, and QT interval). The measured duration and interval values were compared against the CSE reference values (medians of referee values for 25 physician overread ECGs and medians of interpretive programs for the rest) to compute the measurement "differences". Also, the data of the median cardiac complex were evaluated for noise content. The original ECG data gave consistently smaller differences for all of the four measurements than the differences with the artificial ECG data. The noise levels in the median complexes formed from the original ECG data were significantly lower than the noise levels in the median complexes from the artificial ECG data. The noise levels in the medians of the 100 clean ECGs were lower than the noise levels in the medians of the 25 noisy ECGs for both the original and artificial ECG data.

Influence of ECG measurement accuracy on ECG diagnostic statements.

Computer analysis of electrocardiograms (ECGs) provides a large amount of ECG measurement data, which may be used for diagnostic classification and storage in ECG databases. Until now, neither error limits for ECG measurements have been specified nor has their influence on diagnostic statements been systematically investigated. An analytical method is presented to estimate the influence of measurement errors on the accuracy of diagnostic ECG statements. Systematic (offset) errors will usually result in an increase of false positive or false negative statements since they cause a shift of the working point on the receiver operating characteristics curve. Measurement error dispersion broadens the distribution function of discriminative measurement parameters and, therefore, usually increases the overlap between discriminative parameters. This results in a flattening of the receiver operating characteristics curve and an increase of false positive and false negative classifications. The method developed has been applied to ECG conduction defect diagnoses by using the proposed International Electrotechnical Commission's interval measurement tolerance limits. These limits appear too large because more than 30% of false positive atrial conduction defect statements and 10-18% of false intraventricular conduction defect statements could be expected due to tolerated measurement errors. To assure long-term usability of ECG measurement databases, it is recommended that systems provide its error tolerance limits obtained on a defined test set.

Comparison of 12-lead repolarization parameters during follow-up study in middle-aged sportsmen and in patients during acute myocardial infarction.

Consecutive electrocardiographic (ECG) analysis is very useful in acute coronary ischemia, but it is known that ECG patterns can be misleading in subjects with left ventricular hypertrophy, mainly during the repolarization phase. An automated system was developed to collect, store, and follow-up all heterogeneous data concerning a cohort of 1,898 subjects (1,039 men), 45-65 years old, 50% of whom were physically active. The reliability of several ECG markers of ischemia was tested during chronic follow-up study (1993-1995) in 23 healthy sedentary men without hypertension (group 1) recorded in our database, as well as in 9 subjects performing regular sporting activity (SA) (group 2). The same parameters were evaluated in the intensive care unit in nine patients affected by coronary artery disease, during either successful or unsuccessful thrombolytic therapy of acute myocardial infarction (AMI) (group 3). Twelve-lead ECGs were recorded, analyzed by the Hannover ECG system program, compressed, and stored according to the Standard Communication Protocol in each of the three groups. The changes in ST amplitude 20, 60, and 80 ms alter the J point were very small in each subject of groups 1 and 2, while upsloping from 1 to 10 mm in several leads was observed slowly, rapidly, or intermittently in group 3 patients during ischemia. The ST slope and the concordance of the T wave and ST amplitude were helpful in differentiating normal and SA subjects from AMI patients. These results, obtained in resting conditions, underline that the difference among ST-T abnormalities in subjects with left ventricular hypertrophy due to SA are consistently different from those observed in patients with AMI. The serial digital ECG can be helpful to underline these differences.

A system for integrated ECG analysis.

A system for combined analysis of resting, rhythm and exercise electrocardiograms has been developed. Also an "expert" module for rule-based processing with backward chaining for reasoning has been implemented. The system can be used for routine ECG analysis as well as for research and teaching in electrocardiography.

New trends in serial ECG analysis.

The prerequisite for future development and assessment of serial electrocardiographic analysis programs is the availability of easy to use database management systems for storage and retrieval of electrocardiographic data and results and efficient means for electronic interchange of digital electrocardiographic data. The basic aim of project 2026 of the European program for advanced informatics in medicine is to encourage "open European data interchange and processing for electrocardiography" (OEDIPE) and to provide objective decision support techniques in the field of quantitative electrocardiology. The final objective is to develop demonstration systems for the follow-up evaluation of (1) selected populations with heart disease, (2) integrating serial analysis, (3) decision support, (4) open databases, and (5) communication protocols, which will foreshadow large-scale monitoring of ambulatory or critically ill patients at home, during their transportation in an emergency vehicle, or during their stay in a hospital. The first-year objectives and achievements of the OEDIPE project are briefly described. Emphasis is put on the management of serial electrocardiographic records and data and on the control strategies for the management of the serial analysis process.

European Conformance Testing Services for computerized electrocardiography. New procedures and standards.

Performance requirements and national, as well as international, standards for computer-assisted electrocardiography are behind today's technology. Upgrading is urgently needed from a legal point of view, as well as from a technologic point of view. In Europe a new directive on medical devices requires that instruments with a measurement function have to provide the appropriate measurement accuracy and that manufacturers must specify its measurement accuracy. This study addresses the problem. A description for a new testing system for computer-assisted electrocardiography is given. Also, new tools, for example, the Conformance Testing Services database, and minimum performance requirements for testing computer-assisted electrocardiographs are described and discussed.

Standard interchange for computerized electrocardiography.

In the SCP-ECG project (AIM #1015) of the pilot AIM Programme a standard communications protocol was developed for computerized electrocardiography, consisting of standards for the interchange, encoding and storage of digital ECG data. Part of this work is continued under the main AIM Programme (January 1992- December 1994) as AIM Project #2026, entitled "Open European Data Interchange and Processing for Computerized Electrocardiography (OEDIPE)" and was taken up by the CEN/TC 251 Committee as a priority item, for implementation into an official European Standard in Health Care Informatics. The objectives and implementation of this Standard are described in the present paper.

Data compression for storage of resting ECGs digitized at 500 samples/second.

Data compression of resting electrocardiograms (ECGs) digitized at 500 samples per second (sps) is presented. Tradeoffs between the fidelity of reconstructed data and the overall compression are examined. Data of the median (average) complexes are retained at 500 sps and full resolution and encoded only to reduce redundancy. The raw data for rhythm analysis are evaluated for lowpass filtering and down-sampling (decimation) and requantization. After subtracting the medians from the raw data, the resulting residue signal is examined in detail for data reduction and encoding. Various options for compression of the residue signal are presented. Specific issues examined are the acceptable decimation and requantization of the residue signal. Another important aspect evaluated is the bimodal decimation of the QRS and the rest of the cardiac complex. Here, the QRS complexes are kept at 500 sps and the rest of the data decimated to lower sampling rates. This novel approach reduces data distortion while achieving significant compression. Details of the compression scheme and its evaluation on uncompressed 500-sps ECGs from the European Common Standards for Electrocardiography (CSE) database (128 ECGs with normal sinus rhythm, atrioventricular blocks, atrial fibrillation and flutter, ventricular arrhythmias, and excessive noise) are elucidated. Performance indexes [root mean square (RMS) error, percent RMS difference, normalized RMS difference, maximum peak error, and compression ratio] are computed. To validate the compression method, qualitative evaluation was performed by two physicians overreading the ECGs by comparing the reconstructed waveforms with the original uncompressed data. The median data are retained at 500 sps and full precision. For rhythm data, the bimodal decimation of the residue signal to 125 sps at 10 microvolts resolution preserved the fidelity of the ECG signals well, while giving good data compression. Abnormal atrial activity was well preserved and the QRS was retained without distortion. The average size of a 10-sec compressed ECG with the medians is around 4.5 kilobytes.

A standard communications protocol for computerized electrocardiography.

In an international collaborative project aimed at producing common standards for quantitative electrocardiography, a standard communications protocol (SCP-ECG) was developed for computerized electrocardiography. The protocol consisted of standards for the interchange, encoding, and storage of digital ECG data. The work was performed in three distinct, but closely related, work-packages and in close collaboration with representatives from 13 manufacturers of computerized electrocardiographs from all over the world. The objectives and results of SCP-ECG are briefly described in this paper.

Common standards for quantitative electrocardiography: goals and main results. CSE Working Party.

Computer processing of electrocardiograms (ECGs) has over the last 15 years increased rapidly. Still, there are at present no standards for computer ECG interpretation. Different techniques are used not only for measurement and interpretation, but also for transmission and storage of data. In order to fill these gaps, a large international project, sponsored by the European Commission, was launched in 1980 to develop "Common Standards for Quantitative Electrocardiography (CSE)". The main objective of the first CSE study was to reduce the wide variation in wave measurements currently obtained by ECG computer programs. The second study was started in 1985 and aimed at the assessment and improvement of diagnostic classification of ECG interpretation programs. To this end reference libraries of well documented ECGs have been developed and comprehensive reviewing schemes devised for the visual and computer analysis of ECGs. This task was performed by a board of cardiologists in a Delphi review process, and by 9 VCG and 10 standard 12-lead programs developed by university research groups and by industry. A third action was started in June 1989 to harmonize acquisition, encoding, interchange and storing of digital ECG data. The action thus performed have become internationally recognized milestones for the standardization of quantitative electrocardiography.

Evaluation of ECG interpretation systems: signal analysis.

Performance analysis of biosignal processing systems which provide diagnostic statements requires particular care. Besides general accuracy requirements, psychological and legal implications for patient and physician have to be considered on both the development and the user sites. Cybernetics and control engineering have provided the basic methodology for performance analysis of systems: in technical systems often mathematically defined functions and signals can be fed into the system to be tested and its response and output provide the necessary performance characteristics after adequate mathematical analysis. For systems which process biosignals, as for example ECG analysis systems, instead of analytically given signals learning and test sets of data derived from patients have to be applied. The performance analysis is done on a statistical basis. In this paper construction and composition of learning and test data sets as well as methods for performance evaluation of the signal processing part of ECG programs are described. Specific reference is made to the European project Common Standards for Quantitative Electrocardiography (CSE) where ten ECG- and nine VCG-programs have been tested. The results of these tests provide reference data and standards for further program development as well as for independent system performance evaluation.

Signal analysis for ECG interpretation.

In ECG interpretation usually two main areas are discerned: the signal analysis and the diagnostic classification. This article reviews the major developments in the first area. ECG signal analysis itself is subdivided into the stages data acquisition, data transformation, feature selection, and data reduction. These stages are consecutively reviewed, while in the data transformation stage digital filtering, detection, wave typing, beat selection, and boundary recognition are discussed.

Methodology of ECG interpretation in the Hannover program.

The Hannover ECG program HES has been designed for measurement and interpretation of resting and (moderate) exercise electrocardiograms. In the signal analysis part the program follows an averaging strategy. For diagnostic classification a hybrid model with decision trees and scoring algorithms, and with multivariate probabilistic tests for derivation of category A statements is applied. The multivariate classification technique allows to adjust sensitivity and specificity for specific application areas without changing the diagnostic criteria.

Stability of computer ECG amplitude measurements in the presence of noise. The CSE Working Party.

An important feature of an ECG analysis program is its ability to provide reliable measurements under various operating conditions, e.g., on noise-free and noisy ECGs. Therefore, within the European cooperative project "Common Standards For Quantitative Electrocardiography" (CSE), the accuracy and stability of ECG measurements obtained by several computer programs has been compared. To investigate the stability of measurements two sets of 10 ECGs with and without seven different high- and low-frequency types of noise--altogether 160 electrocardiograms and 160 vectorcardiograms--have been analyzed by eight electrocardiographic and five vectorcardiographic computer programs. The stability of measurement was tested with respect to results obtained for the noise-free recordings. In a previous paper, the influence of noise on wave boundary recognition has been reported. In the present paper, the effect of noise on amplitude measurements and on problems of waveform definitions within the QRS complex are described. The results indicate that programs analyzing an averaged beat exhibit less variability than programs which measure every complex or a selected beat. Comparability and stability of measurements could be improved if a standardized procedure for amplitude references were to be introduced. In addition, the stability of QRS waveform labelling could be improved if waveforms' minimum amplitude and duration were to be validated against the noise level which itself should be determined by a standardized procedure.