| Summary: | 博士 === 國立臺灣大學 === 電機工程學系 === 85 === Heart rate variability (HRV) is mainly due to the modulation
of theautonomic inputs to the heart, and the analysis of HRV
also provides aprobe to understand the autonomic function.
Recently, severalphysiological and clinical studies have
demonstrated that the spectralcomponents of HRV at low
frequency and high frequency can reveal thefunction of two
branches of the autonomic nervous system - sympatheticand
parasympathetic nervous systems, respectively. The dysfunction
ofthe cardiovascular system will also deteriorate the normal
function ofthe autonomic nervous system and reduce HRV.Most
studies dealing with the spectral analysis of HRV is based on
theFourier spectral analysis. However, the characteristic
of HRV ischanged quickly in some situations, e.g.
postural change,pharmacological influence, anesthetic
events, etc. So, the Fourierspectral analysis is unable to
catch the transient characteristic dueto its underlying
assumption of stationarity of data. Although theadvanced
spectral analysis method, time-frequency distribution
canreveal the time-related spectral components of the
nonstationarysignals, the special characteristics of HRV
(like 1/f spectralcharacteristic, very low frequency trend,
transient change, etc.) mayaffect the time-frequency
representations of HRV. In thisdissertation, a new
time-frequency distribution is proposed. Theconcept of
the proposed method is to change the
spectralcharacteristic of HRV by f^alpha pre-filtering before
time-frequencyanalysis, and the change of spectral energy
is recovered bymultiplying a frequency compensation
function 1/f^alpha at finalstage. Based on the tests by
simulation and real HRV signals, theproposed method is
superior than other time-frequency distributions inrevealing the
time-related frequency components of HRV and estimatingthe
spectral energy qualitatively and quantitatively.Based on the
proposed time-frequency distribution, the
spectralcharacteristics of HRV under general anesthesia (awake
-> thiopentoneinduction -> tracheal intubation -> isoflurane
anesthesia maintenance)is investigated. Our result shows a
reduction of low-frequency andhigh- frequency heart rate
fluctuations from awake to thiopentoneanesthesia, and low-
frequency fluctuation is further reduced inisoflurane
maintenance. In addition, instead of the elevation oflow-
frequency spectral power, the sympathetic activation due
totracheal intubation reflects on the increase of the
ratio oflow-frequency to high-frequency spectral power.The
autonomic imbalance due to the sympathetic over-activation is
oneof the syndromes of severe congestive heart failure.
The HRV ofcongestive heart failure usually has two special
characteristics: (1)a very low heart rate fluctuation, (2) a
very low frequency heart rateoscillation, named as Cheyne-
Stokes respiration (CSR). The former ismainly owing to the
suppression of the autonomic nervous system, andthe latter
is probably due to that the interaction between
theautonomic nervous system and cardiovascular system is
controlled by aspecific mechanism. Clinically, the low-dose
beta-blocker therapy canimprove the autonomic imbalance and
the cardiovascular function.Nevertheless, it is not clear that
the change of HRV, its relatedautonomic function and cardiac
performance after low-dose beta-blockertherapy. In this
dissertation, the change of the characteristic of HRVis
investigated after low-dose beta-blocker therapy by 24-hour
HRVstudy. Our result demonstrates that the low-
frequency andhigh-frequency heart rate fluctuations are
increased as the elevationof cardiac function, which implies
the recovery of the autonomicfunction. The augmentation of
low-frequency spectral power is largerthan that of high
frequency, and it might be due to the recovery ofbaroreceptor
sensitivity. In addition, we employ time-
frequencydistribution to interpret the time-related spectral
characteristics ofCSR, and develop a new method to discriminate
CSR based on the theoryof pattern recognition. Our result
reveals that the occurrence of CSRhas been reduced as the
recovery of cardiac and autonomic functionsafter low-dose
beta-blocker therapy.
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