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sqrs attempts to locate QRS complexes in an ECG signal in the specified record. The detector algorithm is based on example 10 in the WFDB Programmer’s Guide, which in turn is based on a Pascal program written by W.A.H. Engelse and C. Zeelenberg, ‘‘A single scan algorithm for QRS-detection and feature extraction’’, Computers in Cardiology 6:37-42 (1979). sqrs does not include the feature extraction capability of the Pascal program. The output of sqrs is an annotation file (with annotator name qrs) in which all detected beats are labelled normal; the annotation file may also contain ‘artifact’ annotations at locations that sqrs believes are noise-corrupted.
sqrs can process records containing any number of signals, but it uses only one signal for QRS detection (signal 0 by default; this can be changed using the -s option, see below). sqrs is optimized for use with adult human ECGs. For other ECGs, it may be necessary to experiment with the sampling frequency as recorded in the input record’s header file (see header(5) ) and the time constants indicated in the source file.
sqrs uses the WFDB library’s setifreq function to resample the input signal at 250 Hz if a significantly different sampling frequency is indicated in the header file. sqrs125 is identical to sqrs except that its filter and time constants have been designed for 125 Hz input, so that its speed is roughly twice that of sqrs. If the input signal has been sampled at a frequency near 125 Hz, the quality of the outputs of sqrs and sqrs125 will be nearly identical. (Note that older versions of these programs did not resample their inputs; rather, they warned if the sampling frequency was significantly different than the ideal frequency, and suggested using xform(1) to resample the input.)
This program is provided as an example only, and is not intended for any clinical application. At the time the algorithm was originally published, its performance was typical of state-of-the-art QRS detectors. Recent designs, particularly those that can analyze two or more input signals, may exhibit significantly better performance.
It may be necessary to set and export the shell variable WFDB (see setwfdb(1) ).
To mark QRS complexes in record 100 beginning
5 minutes from the start, ending 10 minutes and 35 seconds from the start,
and using signal 1, use the command:
sqrs -r 100 -f 5:0 -t 10:35 -s 1
The output annotations may be read using (for example):
rdann -a qrs -r 100
To evaluate the performance of this program, run it on the entire record,
sqrs -r 100
and then compare its output with the reference annotations by:
bxb -r 100 -a atr qrs
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Updated 28 May 2015