UC Research Repository Collection:
http://hdl.handle.net/10092/10
2015-10-10T00:05:57ZPressure predictions of Darcian flow in non-uniform media
http://hdl.handle.net/10092/11132
Title: Pressure predictions of Darcian flow in non-uniform media
Authors: Becker, S.; Lehmann, S.
Abstract: The work presented concerns laminar flow through a porous media whose pore length scales vary in the direction of bulk fluid flow. The goal is to be able to predict local pressure and flow data without having to explicitly solve the creeping flow problem. In this study, an empirical model is developed to estimate the local permeability as a function of local pore length scale. The constants empirical relationship can be developed from the data of as few as 4 numerical simulations (each using a different set of length scales). The result is that the local pressure and bulk flow velocity at any point in the media are able to be predicted with only knowledge of: the media geometry, the fluid viscosity, and the total pressure drop over the media. The results of the predicted pressure field agree with those of the numerical simulations within 2.5%.2015-01-01T00:00:00ZA better way to determine sample size to detect changes in length of mechanical ventilation?
http://hdl.handle.net/10092/11131
Title: A better way to determine sample size to detect changes in length of mechanical ventilation?
Authors: Chiew, Y.S.; Pretty, C.G.; Redmond, D.; Shaw, G.M.; Desaive, T.; Chase, J.G.
Abstract: Introduction:
Estimation of effective sample size (N/arm) is important to ensure power to detect significant treatment effects.
However, traditional parametric sample size estimations depend upon restrictive assumptions that often do not
hold in real data. This study estimates N to detect changes in length of mechanical ventilation (LoMV) using
Monte-Carlo Simulation (MCS) and mechanical ventilation (MV) data to better simulate the cohort.
Methods:
Data from 2534 MV patients admitted to Christchurch Hospital ICU from 2011-13 were used. N was estimated
using MCS to determine a sample size with power of 80%, and compared to the Altman’s nomogram for two
patients groups, 1)all patients and 2)targeted patients with 1<LoMV≤15 days. MCS allows any range of
intervention effect to be simulated, where this study tested a 10 and 25% difference in LoMV (0.5–1.25 days for
mean LoMV of 5 days). The simulated LoMV for the intervention group is compared to the LoMV in a control
group using the one-sided Wilcoxon Ranksum Test, Student T-Test, and Kolmogorov-Smirnov test to assess
central tendency and variation.
Results:
The distribution of LoMV is heavily skewed. Altman’s nomogram assumes a normal distribution and found
N>1000 to detect a 25% LoMV change. Panels (1-2) show N for 80% power if all patients were included, and
Panels (3-4) when for the targeted patient group. Panels (1) and (3) show that it is impossible to achieve 80%
power for a 10% intervention effect. For 25% effect, MSC found N=400/arm (all patients) and N=150/arm
(targeted cohort).
Conclusions:
Traditional parametric sample size estimation may over-estimate the required patients. MCS can estimate
effective N/arm and evaluate specific patient groups objectively, capturing local clinical practice and its impact
on LoMV. It is important to consider targeting specific patient groups by applying patient selection criteria that
can be easily translated into trial design.2015-01-01T00:00:00ZA pressure reconstruction method for spontaneous breathing effort monitoring
http://hdl.handle.net/10092/11130
Title: A pressure reconstruction method for spontaneous breathing effort monitoring
Authors: Damanhuri, N.S.; Chiew, Y.S.; Othman, N.A.; Docherty, P.D.; Shaw, G.M.; Chase, J.G.
Abstract: Introduction:
Estimating respiratory mechanics of mechanically ventilated (MV) patients is unreliable when patients exhibit
spontaneous breathing (SB) efforts on top of ventilator support. This reverse triggering effect [1] results in an
M-wave shaped pressure wave. A model-based method to reconstruct the affected airway pressure curve is
presented to enable estimation of the true underlying respiratory mechanics of these patients.
Methods:
Airway pressure and flow data from 72 breaths of a pneumonia patient were used for proof of concept. A
pressure wave reconstruction method ‘fills’ parts of the missing area caused by SB efforts and reverse triggering
by connecting the peak pressure and end inspiration slope (Figure 1). A time-varying elastance model [2] is then
used to identify underlying respiratory elastance (AUCEdrs). The area of the unreconstructed M-wave has less
pressure, resulting in a lower overall AUCEdrs without reconstruction. The missing area of the airway pressure or
AUCEdrs is hypothesized to be a surrogate of patient-specific inspiratory to assess the strength of SB efforts.
AUCEdrs and missing area A2 are compared with/without reconstruction.
Results:
Median AUCEdrs and breath-specific effort using reconstruction were 24.99[IQR:22.90-25.98] cmH2O/l and 3.64
[IQR:0.00-3.87] % versus AUCEdrs of 20.87[IQR:15.24-27.48] cmH2O/l for unreconstructed M-wave data,
indicating significant patient and breath specific SB effort, and the expected higher elastance (p < 0.05).
Conclusions:
A simple reconstruction method enables the real-time measurements respiratory system properties of a SB
patient and measure the surrogate of the SB effort, that latter of which has clinical useful in deciding whether to
extubate or re-sedate the patient.2015-01-01T00:00:00ZDevelopment of a cost-efficient method for micro-scale heat transfer and temperature studies
http://hdl.handle.net/10092/11121
Title: Development of a cost-efficient method for micro-scale heat transfer and temperature studies
Authors: Collinson, D.; Becker, S.
Abstract: Thermo-chromatic liquid crystals (TLCs) are chiral molecules that reflect specific wavelengths of visible light. The wavelength reflected is proportional to the temperature of the TLCs. Electroporation used as an active enhancement for transdermal drug delivery produces micro scale thermal patterns around local transport regions (LTRs) on the skin surface. A previous study conducted used thermo-chromatic liquid crystals to produce isotherms around the LTRs. Another method involving TLCs are also used to map convection coefficients on surfaces exposed to fluid flow. The goal of this study was to develop experimental methods involving TLCs to be used on the micro scale at low financial cost. A microscope was built utilising a cell phone camera and a lens from a laser pointer to provide magnification shown to be 100x-200x. The capabilities of TLC was conducted at the University of Canterbury by collecting image data of TLC coated surfaces with a cellphone and then post processing the images in MATLAB to produce temperature maps based on collected colour data. Studies conducted show thermal gradients >104 Km-1 can be captured and that the entire colour range could be utilised for studying electroporation. Future research developments use the method to study microfluidics along with existing applications. Beyond research studies, it is also applicable as an educational tool due to its accessibility and the low financial capital required.2015-01-01T00:00:00Z