njit-etd2003-081 - New Jersey Institute of Technology

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series is available until the next heartbeat occurs. Therefore, the amplitude of all of the
interpolated values between a beat at time T m-1 , and the beat at Tm, were set equal to the
time difference between Tm and Tm-1 . The interpolated interbeat interval (IIBI) is then
sampled to produce an IIBI with evenly spaced samples. For example, in Figure 3.6(c)
if a beat occurs at a time equal to 2 seconds and the next beat occurs at a time equal to
2.9 seconds, then the interpolated values between time 2 seconds and 2.9 seconds are all
0.9 as shown in Figure 3.6(d). After the IIBI signal is obtained, it is detrended using a
locally weighted robust regression algorithm (available in LabVIEW), which acts like a
filter. Essentially, this removes low frequency components below .05 Hz. If these low
frequency components are not removed, they can dominate the power spectrum and
decrease the detail of the components in the frequencies above 0.05 Hz. Another
example of an IBI signal and an IIBI signal is shown in Figure 3.7.
Figure 3.7 1131 and IIBI signals of heart rate.