In the present study, using noise-free simulated signals, we performed a comparative examination of several preprocessing techniques that are used to transform the cardiac event series in a regularly sampled time series, appropriate for spectral analysis of heart rhythm variability (HRV). First, a group of noise-free simulated point event series, which represents a time series of heartbeats, was generated by an integral pulse frequency modulation model. In order to evaluate the performance of the preprocessing methods, the differences between the spectra of the preprocessed simulated signals and the true spectrum (spectrum of the model input modulating signals) were surveyed by visual analysis and by contrasting merit indices. It is desired that estimated spectra match the true spectrum as close as possible, showing a minimum of harmonic components and other artifacts. The merit indices proposed to quantify these mismatches were the leakage rate, defined as a measure of leakage components (located outside some narrow windows centered at frequencies of model input modulating signals) with respect to the whole spectral components, and the numbers of leakage components with amplitudes greater than 1%, 5% and 10% of the total spectral components. Our data, obtained from a noise-free simulation, indicate that the utilization of heart rate values instead of heart period values in the derivation of signals representative of heart rhythm results in more accurate spectra. Furthermore, our data support the efficiency of the widely used preprocessing technique based on the convolution of inverse interval function values with a rectangular window, and suggest the preprocessing technique based on a cubic polynomial interpolation of inverse interval function values and succeeding spectral analysis as another efficient and fast method for the analysis of HRV signals.
Treadmill testing (TT) is the standard method to evaluate patients with intermittent claudication (IC), but cycling test (CT) has been demonstrated as an alternative to minimise the impact of body mass and altered gait on exercise performance. To investigate the differences in cardiopulmonary responses and tissue saturation of calf measured by near-infrared spectroscopy (NIRS) between TT and CT, we evaluated 9 men with IC. For TT was used with a fixed speed of 3.2 km/h and 0% grade for 5 min with increases of 3.5% every 3 min, as described by Hiatt (1988). CT was performed as described by Askew (2002), pedalling at 20W for 5 min and 20W increases every 3 min. Patients underwent both tests, with an interval between 3 and 14 days. Gas exchange was assessed by a breath-by-breath system (CPX Ultima, Medgraphics). Tissue saturation was evaluated by a portable wireless system of NIRS (Portamon, Artinis) attached to the calf. As a physiological calibration of NIRS, an arterial occlusion manoeuvre (AO) was done before each test maintaining a cuff inflated on the thigh, at a minimum of 250 mmHg for 5 min. To examine the normal distribution of data, Shapiro-Wilk test was done. Data comparisons were done by using paired t-tests. Significance level was previously set at p < 0.05. No significant differences were found for peak oxygen uptake in mL/kg.min (TT: 17 ± 4; CT: 16 ± 5; p = 0,608), peak heart rate in bpm (TT: 124 ± 17; CT: 125 ± 14; p = 0,82), peak minute ventilation in L/min (TT: 49 ± 11; CT: 53 ± 9; p = 0,466) and peak respiratory exchange ratio (TT: 1.08 ± 0.10; CT: 1.17 ± 0.07; p = 0,055). Peak exercise saturation at calf was more reduced during TT (TT: 48.7 ± 6.3; CT: 58.0 ± 7.3; p = 0,005) and both were higher than minimum saturation obtained during AO (TT: 48.7 ± 6.3; AO: 45.3, 0 ± 6.2; p = 0,004 / CT: 58.0 ± 7.3; AO: 45.1 ± 5.3; p = 0,002). Calf demand for blood in incremental exercise is higher in treadmill than in cycle ergometer, despite similar cardiopulmonary responses.
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