| Summary: | Thesis (Ph.D.)--University of the Witwatersrand, Faculty of Health Sciences, 2000. === Oesophageal pressure (Res) is often measured to estimate pleural pressure
(Ppl) for the calculation of respiratory system elastance and resistance. High-fidelity
dynamic Ppl estimation requires that Ppl waveforms be transmitted unchanged both
across the Ppl-Pes tissue barrier and across Pes catheter-manometers.
In this study the frequency responses of liquid- and gas-filled catheter
manometers used in clinical practice were examined in detail using the in vitro sinewave
technique. The assumption that fluid-filled catheter-manometer frequency
responses fit a second order system was tested by comparing second order curvefits
to measured curves, An acute lung injury (ALI) model of human respiratory
disease was developed in monkeys. In health and A L direct Ppl and Pes were
measured simultaneously to determine the Ppl-Pes tissue barrier amplitude
frequency response. The relevant bandwidth of dynamic Pes waveforms was
determined.
It is found that liquid-filled feeding catheters measure dynamic Pes within a
5% error up to a maximum respiratory rate (FRR) of 82 breaths/min and are suitable
for use only in subjects with low-frequency respiratory mechanics. FRR differences
exist between French Gauge sizes and differing catheter brands: French Gauge size
is a poor predictor of Pes measurement suitability.
Infant air-balloon catheters' FRR is up to 148 breaths/min. They possess
superior frequency response characteristics compared to matching liquid-filled
catheters, have lower frequency response variability within catheter samples, and
are suited to dynamic Pes measurements during high-frequency respiratory
mechanics.
ii
Frequency responses of fluid-filled feeding catheters employed in Pes
catheter-manometers do not adequately fit second order systems, casting doubt on
the validity of applying second order mathematical models to predict cathetermanometer
behaviour from stap-responses.
Decreased dynamic lung compliance, reduced alveolar gas exchange and
diffuse alveolar capillary leak similar to that of comparable humans evolves following
oleic-acid administration in monkeys; the model is suitable for evaluation of
pulmonary mechanics and gas exchange during ALI.
The Ppl-Pes tissue barrier has a uniform frequency response within the
bandwidth of conventional Pes waveforms in healthy or diseased lungs, and does
not attenuate Ppl-Pes waveform transmission between 1 - 40 Hz. At Pes
frequencies higher than conventional regions of clinical interest the Ppl-Pes barrier
resonates, is pressure amplitude dependent at low pressure offsets, and altered by
ALI. During conventional ventilation for ALI, Pes-manometers require a uniform
frequency response up to 8.5 Hz to achieve a s 5% in vivo Pes waveform
measurement error.
These findings advance the accuracy of pulmonary function studies in high
frequency respiratory mechanics, such as conventional infant ventilation or high
frequency ventilation.
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