Postural Response of Low-Frequency Component of Heart Rate Variability: Statistical Analysis

Postural Response of Low-Frequency Component of Heart Rate Variability: Statistical AnalysisPower spectral density was computed by a 256-point fast-Fourier transformation, was corrected for loss of variance resulting from the sampling and filtering processes described earlier, and was integrated by > 0.04 to 0.15 Hz, 0.20 to 0.30 Hz, and 0.00 to 0.50 Hz, respectively, for obtaining the LF, the HF, and the total power. The powers of these frequency components were expressed as the natural logarithm of the absolute value, and the power of the LF also was expressed as a normalized unit (LFnu), which was calculated by dividing the power by the total power minus power below 0.03 Hz. Heart rate was calculated from the mean of normal-to-normal R-R intervals in each position. The postural response of each measure was evaluated as the difference between measurements made in the supine and the tilt positions (value during tilt — value during supine). Figure 1 shows the time series and power spectra of the R-R interval during the HUTT in representative patients.
Statistical Analysis
We used a computer program (SAS; SAS Institute; Cary, NC) for all statistical analysis. One-way analysis of variance and the x2 tests with Yates correction were used for between-group comparisons of quantitative and categoric variables, respectively. The Bonferroni method was used for multiple comparisons to guard against an increase in the type-I error level. A Cox proportional hazards regression model was used for survival analysis. The associations of the baseline demographic and clinical variables with a risk for death were evaluated by the univariate Cox model. Prognostic associations of HRV measures were determined by both univariate models and multivariate models adjusted for the other clinical variables. For the graphic display of survival probabilities, Kaplan-Meier survival curves were used. Quantitative data were presented as the mean ± SD, and risks for death were presented as risk ratio (RR) with the 95% confidence interval (CI). For all statistical analyses, p < 0.05 was considered to be significance. canadian neighborhood pharmacy

Fig1
Figure 1. Trendgrams (top left, A, and top right, B) and amplitude spectra (lower left, C, lower middle left, D, lower middle right, E, and lower right, F) showing the responses of R-R interval variability to 70° HUTT (supine position, 5 min; tilting, 5 min) in representative patients with CAD. Left side: data from a 52-year-old male patient whose coronary angiogram showed a 90% stenosis in the circumflex branch of the left coronary artery. He was alive 97 months after undergoing the test. The spectra (lower left, C, and lower middle left, D,) demonstrate that not only the HF but also the LF showed a decrease with tilting (LF drop). Right side: data from a 49-year-old male patient whose coronary angiogram showed a 75% stenosis in the anterior descending branch of the left coronary artery. He died suddenly 25 months after undergoing the test. The spectra (lower middle right, E, and lower right, F) demonstrate that the HF showed a decrease during tilt, while the LF showed an increase during tilt (LF rise).

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