Real-Time, High-Frequency QRS Electrocardiograph
- Tuesday, 01 July 2003
Signals indicative of ischemia or infarction can be identified in real time.
An electronic system that performs real-time analysis of the low-amplitude, high-frequency, ordinarily invisible components of the QRS portion of an electrocardiographic signal in real time has been developed. Whereas the signals readily visible on a conventional electrocardiogram (ECG) have amplitudes of the order of a millivolt and are characterized by frequencies <100 Hz, the ordinarily invisible components have amplitudes in the microvolt range and are characterized by frequencies from about 150 to about 250 Hz. Deviations of these high-frequency components from a normal pattern can be indicative of myocardial ischemia or myocardial infarction (see figure).
Prior to the development of this system, analyses of the high-frequency components of QRS signals entailed laborious, time-consuming post-measurement calculations and thus had research value only: they did not provide data quickly enough to provide guidance for clinical decisions for treating cardiac patients at immediate risk. The ability to provide clinically relevant information in real time makes the present system a prototype of a superior QRS electrocardiograph that could find its way into nearly every emergency room, ambulance, intensive-care unit, surgical operating room, cardiac hospital ward, cardiac-exercise/ECG testing facility, and cardiac-catheterization laboratory.
The system includes standard electrocardiograph electrodes, which are connected to high-input-impedance field-effect-transistor (FET) leads. The outputs of the FET leads are fed to a preamplifier, the output of which is fed to a data-acquisition circuit card in the Personal Computer Memory Card International Association (PCMCIA) slot in a personal computer. The card digitizes the electrocardiographic signal at a sampling rate of 1,000 Hz. The data thus acquired by the card are analyzed by special-purpose software running in a Windows operating system. Optionally, the data can be stored in a file for subsequent playback and analysis.
Whether processing real-time or previously recorded data, the special-purpose software performs several functions. The software detects R waves and QRS complexes and analyzes them from several perspectives. A conventional, unfiltered, beat-to-beat limb-lead ECG signal with an amplitude of the order of a millivolt is shown running across a window at the top of the computer screen. The software also computes and displays a signal that is similar except that it has been averaged over a number (selectable by the user) of consecutive beats in order to increase the signal-to-noise ratio.
The software includes a provision for special-purpose non-recursive digital filters with varying low- and high-frequency cutoffs. These filters, applied to the averaged signal, effect a band-pass operation in the frequency range from 150 to 250 Hz. The output of the band-pass filter is the desired high-frequency QRS signal. Further processing is then performed to obtain the power spectrum of the filtered signal, the beat-to-beat root mean square (rms) voltage amplitude of the filtered signal, certain variations of the rms voltage, and such standard measures as the heart rate and R-R interval at any given time.
This work was done by Todd T. Schlegel of Johnson Space Center, Jude L. DePalma of Colorado State University-Pueblo, and Saeed Moradi of Eix, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Bio-Medical category.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Johnson Space Center, (281) 483-0837. Refer to MSC-23154.
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Real-Time, High-Frequency QRS Electrocardiograph (reference MSC-23154) is currently available for download from the TSP library.
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