Accuracy of intra-oesophageal dynamic pressure probes, and their validity in determining dynamic intra-pleural pressure
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Date
2014-03-18
Authors
Hartford, Craig Gordon Francis
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Abstract
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.
Description
Thesis (Ph.D.)--University of the Witwatersrand, Faculty of Health Sciences, 2000.