Design of piezoelectric oscillometry, accuracy in tracking time-varying impedance and implications on the frequency dependence of resistance
Abstract
Oscillometry (OS) is commonly performed by actuators that are heavy and bulky.
In this thesis I designed and simulated a new single frequency OS system. This novel
design used a piezoelectric bimorph actuator on resonance. To predict the performance, a
dynamic model was simulated that included realistic respiratory impedance loads, and
realistic recorded breathing noise. Model performance was also validated in a scaled
prototype device. We found that while breathing noise substantially lowered SNR, the
model could produce sufficient pressure and flows for acceptable SNR and accuracy.
Together the results of the simulations and the scaled prototype indicated that this design
is a feasible approach to develop an accurate lightweight, portable, single-frequency OS
device.
Tracking the within-breath impedance of the lung has gained attention among
researchers because it is believed that it contains important information regarding the
health of the lung. However, to date, there are very few studies that address the accuracy
of estimating time-varying impedance. In this thesis we analytically and computationally
developed a time-frequency transfer function that demonstrated increasing error with
increasing breathing rate. Then we evaluated the accuracy of using short-time Fourier
transform (STFT) methods in tracking time-varying impedance in simulations of a
sample population (children). As expected however, these errors were much higher than
noise-free simulations. Results indicated that current methods can track the impedance
versus time reasonably accurately, but at high breathing rates, errors may be
unacceptable.
It is established that resistance exhibits a pronounced frequency dependence that
is increased in obstructive disease. Similarly, and thought to be unrelated, variations in
reactance are also increased in COPD. By analyzing the equation of motion in the timedomain
with time-varying resistance and elastance we found and proved that variations in
elastance surprisingly influence the frequency dependence of resistance. This relationship
was demonstrated again by the same time-frequency transfer function presented
previously. Although this effect was small in the OS frequency range it was substantial at
breathing frequencies. This is important as it provides a novel mechanism for frequency
dependence of resistance, indistinguishable from OS methods alone, in addition to tissue
viscoelasticity and heterogeneity of the lung.