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Techniques to characterize variability |
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| Variability analysis |
Description |
Advantages |
Limitations |
Output variables |
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| Time domain |
Statistical calculations of consecutive intervals |
Simple, easy to calculate; proven clinically useful; gross distinction of high and low frequency variations |
Sensitive to artifact; requires stationarity; fails to discriminate distinct signals |
SD, RMSDD Specific to HRV: SDANN, pNNx |
| Frequency distribution (plot number of observations falling in selected ranges or bins) |
Visual representation of data; can fit to normal or log-normal distribution |
Lacks widespread clinical application; arbitrary number of bins |
Skewness (measures symmetry): positive (right tail) versus negative (left) Kurtosis (measures peakedness): flatter top (<0) versus peaked (>0) |
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| Frequency domain |
Frequency spectrum representation (spectral analysis) |
Visual and quantitative representation of frequency contribution to waveform; useful to evaluate relationship to mechanisms; widespread HRV evaluation |
Requires stationarity and periodicity for validity; sensitive to artifact; altered by posture, sleep, activity |
Total power (area under curve) Specific to HRV: ULF (<0.003 Hz), VLF (0.003–0.04 Hz), LF (0.04–0.15 Hz), HF (0.15–0.4 Hz) Time spectrum analysis |
| Scale invariant (fractal) analysis |
Power law: log power versus log frequency |
Ubiquitous biologic application; characterization of signal with single linear relationship; enables prognostication |
Requires stationarity and periodicity; requires large datasets |
Slope of power law Intercept of power law |
| DFA |
Identifies intrinsic variations 2°system (versus external stimuli), does not require stationarity |
Requires large datasets (>8000 patients) |
Scaling exponent a1 (n < 11) Scaling exponent a2 (n > 11) a–ß filter |
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| Entropy |
Measures the degree of disorder (information or complexity) |
Unique representation of data; requires fewest data points (100–900 patients) |
Needs to be complemented by other techniques |
ApEN SampEN Multi-scale entropy |
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ApEn, approximate entropy; DFA, detrended fluctuation analysis; HF, high frequency; HRV, heart rate variability; LF, low frequency; pNNx, proportion greater than x ms; RMSDD, root mean square of standard deviation; SampEn, sample entropy; SD, standard deviation; SDANN, standard deviation of 5 min averages; ULF, ultralow frequency; VLF, very low frequency. | ||||
Seely and Macklem Critical Care 2004 8:R367 doi:10.1186/cc2948 |
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