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As part of the polysomnographic study, continuous electrocardiographic (ECG) signals were recorded at the following sample frequencies: 50 Hz (2), 200 Hz (594), 250 Hz (15), 256 HZ (176), 512 Hz (86). A Notch filter of 60Hz was applied.
QRS complexes (R-points) were detected using Compumedics (Abbotsford, VIC, Australia) Somte software Version 2.10 (Builds 99 to 101). The R-points were classified as normal sinus, supraventricular premature complex or ventricular premature complex. The automated annotations were reviewed by a trained technician, who made appropriate corrections.
In this analysis, we follow the Task Force of The European Society of Cardiology and The North American Society of Pacing and Electrophysiology, Heart rate variability (HRV) standards of measurement, physiological interpretation, and clinical use (European Heart Journal, 1996;17:354–81).
Traditional heart rate variability (HRV) measures are commonly divided into two broad categories: time domain measures and frequency domain measures.
The time domain HRV statistics include the following measures:
The frequency domain measures include:
Traditionally frequency domain measures are calculated by resampling the original NN interval series and then applying the fast Fourier transform. This resampling, however, can cause an attenuation in the high frequency components. To eliminate the need for evenly sampled data required by the standard Fourier Transform, frequency domain spectra can be calculated using the Lomb periodogram for unevenly sampled data.
Although the long term (24-hour) statistics of SDANN, SDNNIDX and ULF power can be calculated for shorter data lengths they will become increasingly unreliable. For short term data (less than 15 minutes in length) only the time domain measures of AVNN, SDNN, rMSSD and pNN10, pNN20, pNN30, pNN40 and pNN50, and the frequency domain measures of total power, VLF power, LF power, HF power and LF/HF ratio are computed.
HRV of the entire night - Analysis is performed from sleep onset to sleep termination. This analysis requires the following information: the time of occurrence of each R-wave, and the sleep onset and termination times. These times were extracted from the file containing the sleep stage annotations as the first and last 30 second episode of stage 1, 2, 3 or 5 (REM), respectively. RR intervals larger than 2.5 seconds were excluded from the analysis.
HRV of consecutive 5-minute segments with no overlap - The results of this analysis were used to quantify HRV by sleep stage with and without respiratory events. In addition to restricting the analysis to RR intervals <2.5 seconds, only 5-minute windows with at least 150 normal sinus beats were analyzed.
Result datasets have been posted for HRV analysis. Datasets are keyed on
The open source code used for this analysis along with a tutorial on its use is available at the NIH-sponsored Research Resource for Complex Physiologic Signals: http://physionet.org/tutorials/hrv-toolkit/