With respect to many tasks performed by an ECG analyzer, dealing with noise is the major problem faced by system designers. Although measurements such as ST deviation may be obtained reliably in clean signals, the presence of noise may render them inaccurate. In some instances, it is sufficient to recognize the presence of noise and either to mark measurements as unreliable or to avoid making measurements altogether. In other cases, excluding noisy data is inappropriate (for example, given the multiple correlations among physical activity, noise, and transient ischemia, excluding noisy signals is likely to introduce sampling bias in an ischemia detector).
It is difficult to measure the effects of noise on an ECG analyzer using ordinary recordings. Even if existing databases include an adequate variety of both ECG signals and noise, the sample size is certainly too small to include all combinations of noise and ECG signals that may be encountered in clinical use. In ordinary recordings, it is difficult or impossible to separate the effects of noise from the intrinsic problems of analyzing clean signals of the same type.
The noise stress test circumvents these problems. By adding noise in calibrated amounts to clean signals, any combination of noise and signal types is possible. Since both the noise-corrupted signal and the clean signal can be analyzed (in separate experiments) by the same analyzer, the effects of noise on the analysis are readily separable from any other problems that may arise while analyzing the clean signals. Finally, since the test can be repeated using different amounts of noise, it is possible to characterize analyzer performance as a function of signal-to-noise ratio.
The major criticisms of the noise stress test are that not all noise is additive, and that the characteristics of the added noise may not perfectly match those of noise observed in clinical practice. These points, though formally irrefutable, do not negate the value of the test. In practice, most of the troublesome noise is additive; thus (given appropriate inputs) the noise stress test can simulate most of the noisy signals of interest. The NST DB includes noise recordings made using standard ambulatory ECG electrodes and recorders, but with electrodes placed on the limbs of active volunteers in configurations in which the subject's ECG is not apparent in the recorded signals. Given the recording technique used, it is not surprising that the characteristics of the recorded noise closely match those of noise in standard ambulatory ECG recordings. Although it may be argued that the particular muscles responsible for the recorded noise might produce different signals than those that generate the EMG present in noisy ECGs, no such differences are apparent from comparisons of either the signals or their power spectra.
The NST DB includes a small set of ECG records with calibrated amounts of added noise. EC38 specifies that performance on these records must be reported, although no specific performance levels are required. Program nst can be used to generate additional records for noise stress testing. To do so, choose an ECG record and a noise record (the latter may be bw, em, or ma from the NST DB, or any other available noise recording). Run nst and answer its questions to generate a noisy ECG record that may then be used in the same way as any other WFDB record. By default, nst adds no noise during the first five minutes of the record, then adds noise for the next two minutes, none for the following two minutes, and repeats this pattern of two minutes of noise followed by two minutes of clean signals for the remainder of the record. The scale factors for the noise, if determined by nst, are adjusted such that the signal-to-noise ratios are equal for each signal. The durations of the noisy periods, and the scale factors for each signal, are recorded in a protocol annotation file, which is generated by nst unless an existing protocol annotation file is supplied as input. To change these parameters, simply edit the protocol annotation file (using, for example, rdann to convert it to text form, any text editor to make the modifications, and wrann to convert it back to annotation file format), then rerun nst using the protocol file to generate a new record.
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Updated 26 January 2018