Beat-to-beat cardiovascular variability assessment can include a number of measurements, including blood pressure variability,
10,11 sympathetic-nerve activity variability,
12,13 and vascular variability, which is normally measured either in the skin
14 or in specific blood vessels, such as the middle cerebral artery.
15 However, the simplest and most widely adoptable measurement is HRV, because the investigator does not need to use indwelling catheters and expensive Doppler (ultrasound or laser) or photoplethysmography systems. Heart rate variability is based on the theory that the inherent variation in the R-R intervals noted during standard electrocardiography (ECG) are primarily based on changes in cardiac sympathetic and vagal nerve activity at the sinoatrial node.
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Heart rate variability assessment involves ECG (performed using 1 channel, with the precise lead used unimportant as long as a distinguishable R wave is present) and measurement of respiratory excursions (eg, using a piezoelectric chest transducer) with analog outputs. For data collection, an analog-to-digital converter (eg, from ADI Instruments, BioPac Systems Inc, iWorx Systems Inc, or Dataq Instruments) with a sampling frequency of 200 Hz or greater should be used to ensure appropriate spectral resolution of the signal interfaced with a personal computer. Digital records can be analyzed either post hoc, by using an R-wave peak detection algorithm to detect R-R intervals, or in nearly real time, by using a calculation channel located in the converter software. Alternately, the R-R interval can be streamed by routing the ECG signal through a cardiotachometer and then from the cardiotachometer to the analog-to-digital converter. Time-series data can then be converted to frequency data by use of a fast Fourier transform algorithm available as custom software (eg, from the Biomedical Signal Analysis Group in the Department of Physics at the University of Kuopio in Finland) or commercially available software (eg, AutoSignal [Systat Software], DADiSP [DSP Development Corp], or Matlab [MathWorks]). The duration of data recording can vary depending on the type of analysis, but it most often is within a 5- to 6-minute window. This time frame is long enough for low-frequency (LF) resolution to be obtained while still allowing for economy of measurement.
Spectral power analysis of HRV has been used to study balance of the ANS in humans. It is generally accepted that the high-frequency (HF) power spectra (0.15-0.4 Hz) in this type of analysis are a marker for cardiac vagal modulation, whereas the LF power spectra (0.04-0.15 Hz) are a marker for cardiac sympathetic modulation.
8 Because intrasubject variation in power spectra is considerable, especially in the LF range, data are often normalized to total power and thus are expressed in normalized units. In most circumstances, the LF:HF ratio provides a reasonable index of sympathovagal balance.
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