Heart Rate Variability analysis
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The heart rate variability analysis is
a powerful tool in assessment of the autonomic function. It is
accurate, reliable, reproducible, yet simple to measure and process.
The source information for HRV is a continuous beat-by-beat measurement
of interbeat intervals. The electrocardiograph (ECG or EKG) is
considered as the best way to measure interbeat intervals. ECG is an
electrical signal measured with special conductive electrodes placed on
chest around heart area or limbs. It reflects minute changes in
electrical field generated by heart muscle cells originating from its
SA node. ECG signal has a very specific and robust waveform simple to
detect and analyze. Because of that cardiac rhythm derived from ECG is
the best way to detect not only true sinus rhythm but all types of
ectopic heartbeats, which must be excluded from consideration of
HRV
analysis.
Schema showing the baroreflex functionality
Pulse wave measurement.
The other approach to measure cardiac intervals is a measurement of pulse wave. It is less
invasive and simple method of measurement based on photoplethysmograph.
PPG is a signal reflecting changes in a blood flow detected when
infrared light is emitted towards microcirculatory blood vessels.
Depending on blood flow volume certain portion of that light is
absorbed letting other part to pass or be reflected. An optical sensor
detects a quantity of light passed (or reflected from) the blood flow
producing a waveform identifying pulse wave. Such waveform can also be
processed to derive beat-by-beat interbeat intervals. Although PPG
gives the summary information reflecting both cardiac and
blood vessel components of HRV, some research studies showed a
significantly high correlation between interbeat interval data measured
by both ECG and PPG in short-term steady-state recordings.
One of the important issues when
measuring either ECG or PPG is the absence of abnormal heartbeat used
in interval detection. Only heartbeats originated in SA node can be
processed to obtain HRV data. Whether there are ectopic heartbeats
(PVCs or other types of extrasystolic heartbeats) or various movement
artifacts on ECG (or PPG) considered as heartbeats, they must be
excluded from consideration. There are various statistically-based
algorithms of detection of such abnormal heartbeats that minimize
chances to get contaminated HR recordings. Nevertheless, for the sake
of accuracy in
HRV analysis it is important to be able to visually
verify all heartbeats automatically found, remove abnormal ones and
include missing. The Heart Rhythm Scanner has an automatic detection of such movement artifacts and also gives the possibility to manually correct it.
Example of an abnormal heartbeats
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