USER'S GUIDE - Biosignal Analysis and Medical Imaging Group

60How to select the interpolation rate of the RR series?The interpolation rate is related to the cubic spline interpolation that is used forconverting the RR interval series to equidistantly sampled series. The defaultvalue for the interpolation rate is 4 Hz which works well for normal human HRVdata. It should be noted that the interpolation rate should be at least twice ashigh as the highest expected frequency in the RR interval series. When changingthe interpolation rate, it should be remembered that it affects on the smoothnesspriors based detrending method, i.e. when decreasing the interpolation rate alsothe λ value of the smoothness priors method should be decreased.How to select the window width and overlap for the FFT spectrum estimate?The FFT spectrum in the software is calculated using the Welch’s periodogrammethod where one or more overlapped segments are extracted from the data.Then FFT spectrum is calculated for each segment and as a result the averageof the segment spectra is calculated. The selection of the window width andoverlap in this method is simply a trade-off between the frequency resolution andvariance of the spectrum estimate. The frequency resolution of FFT spectrum isroughly the reciprocal of the sample length (i.e. the frequency resolution of theFFT spectrum of a 100 second sample is ∼ 0.1 Hz). The variance of the FFTspectrum estimate, on the other hand, does not depend on the sample length, butcan be decreased by averaging several shorter samples (which leads to decreasedfrequency resolution).How to select the AR model order for the AR spectrum estimate?The AR method is a parametric method which can be used also for spectrumestimation. In this method the RR interval series is modeled with an autoregressivemodel of specific order. The roots of the AR polynomial (which areactually complex conjugate pairs) correspond to the spectral peaks in the ARspectrum. Thus, the order of the AR model has to be at least twice the numberof expected spectral peaks in the spectrum. In practice, the order is, however,always higher than this minimum and the few extra roots do not disturb thespectrum estimate. Even though, an exaggerated model order can induce spuriouspeaks into the AR spectrum estimate and distort the results. The AR modelorder naturally depends on the interpolation rate of the RR interval series, butin many cases the default order of 16 is reasonable. Why do I get a warning “Negative component power in AR spectrum estimate”?When factorization is used (see the Analysis options) for the AR spectrum estimate,the AR spectrum is divided into separate components (VLF, LF, andHF components). The power of each component is estimated using a methodpresented in [19]. This method works for well-separated AR roots, but for rootsclose to each other power estimates can yield even negative values which areobviously highly erroneous and a warning message is displayed. Because of this,and some other drawbacks, the AR spectrum factorization should be used judiciously.Instead, by disabling the factorization, more robust and in that sensemore reliable results are obtained.Kubios HRV Analysisversion 2.0 betaBiosignal Analysis and Medical Imaging GroupDepartment of PhysicsUniversity of Kuopio, FINLAND