A Multilayer Monte Carlo Model for the Investigation of Optical Path and Penetration Depth at Different Perfusion States of the Colon

There is a great interest in monitoring the oxygen supply delivered to the colon. Insufficient oxygen delivery may lead to hypoxia, sepsis, multiorgan dysfunction and death. For assessing colonic perfusion, more information and understanding is required relating to the light-interaction within the colonic tissue. A multilayer Monte Carlo model of a healthy human colon has been developed to investigate the light-tissue behavior during different perfusion states within the mucosal layer of the colon. Results from a static multilayer model of optical path and reflectance at two wavelengths, 660 nm and 880 nm, through colon tissue, containing different volume fractions of blood with a fixed oxygen saturation are presented. The effect on the optical path and penetration depth with varying blood volumes within the mucosa for each wavelength has been demonstrated. The simulation indicated both wavelengths of photons penetrated similar depths, entering the muscularis layer

When sorry is not an option: CSR reporting and 'face work' in a stigmatized industry - A case study of Barrick (Acacia) gold mine in Tanzania

This paper investigates how a stigmatised company mobilised accounting, particularly CSR reporting, to manage a crisis of legitimacy and spoiled identity following a series of social and environmental crises. Specifically, it uses Goffman’s (1959, 1963) writings on stigma and presentation of self and Benoit’s (1995) image restoration theory to explore how a large mining company in Tanzania used various strategic responses in striving to distance and dissociate itself from the attached ‘stigma’. The evidence shows that, in response to attacks from pressure-group organisations, rather than apologising for the social and environmental crises, the company was preocupied with ‘defensive stigma management’ strategies, including denial and refocusing attention, evading responsibility, image bolstering, excuses and dissociation. Drawing on our findings, we argue that, rather than making the effects of stigma more visible, accounting and CSR disclosures were mobilised to conceal the threats of stigma, manage the legitimacy crisis and repair spoiled identity. This study moves beyond the current focus on legitimacy in the extant CSR reporting literature, by bringing in the concept of organisational stigma to examine stigma management strategies implemented by a company operating in a stigmatised industry to avoid, reduce or minimise its audiences’ disapproval

Monte Carlo model for reflectance Pulse Oximetry using pulsatile monolayer perfused skin tissue

Pulse Oximetry (PO) is a non-invasive method for estimating arterial oxygen saturation from Photoplethysmography signals recorded from peripheral tissue sites. Despite a very wide range of clinical applications, the fundamental working principles of PO is still not very well understood. In this work, a Monte Carlo model is executed for a monolayer volume of human skin dermis in a reflectance detection modality to investigate the basic nature of light-tissue interaction in the tissue. Differences in systolic and diastolic blood volume explained the pulsatility. The distribution of light-tissue interaction was illustrated, systolic and diastolic reflectance at red and infrared light were simulated and the 'calibration curve' was produced

Estimating the Dependence of Differential Pathlength Factor on Blood Volume and Oxygen Saturation using Monte Carlo method

Differential Pathlength Factor (DPF) is a vital parameter for the Beer-Lambert law based calculations in estimating tissue perfusion using non-invasive optical techniques. A significant error in the measured concentration of oxyhemoglobin and deoxyhemoglobin has been reported due to the usage of wrong DPF values. The dependence of DPF on blood oxygen saturation and blood volume has never been studied earlier. In this work, a Monte Carlo model of perfused skin tissue was developed and executed at 660 nm and 940 nm optical wavelengths at a reflectance geometry. DPFs were simulated through 1-10 mm source detector separations at different blood volumes and oxygen saturations. Results showed higher DPFs at lower wavelengths and considerable variation with blood volume and oxygen saturation

Evaluation of oesophageal pulse oximetry in patients undergoing cardiothoracic surgery

Pulse oximetry probes placed peripherally may fail to give accurate values of blood oxygen saturation when the peripheral circulation is poor. Because central blood flow may be preferentially preserved, we investigated the oesophagus as an alternative monitoring site. A reflectance blood oxygen saturation probe was developed and evaluated in 49 patients undergoing cardiothoracic surgery. The oesophageal pulse oximeter results were in good agreement with oxygen saturation measurements obtained by a blood gas analyser, a CO-oximeter and a commercial finger pulse oximeter. The median (IQR [range]) difference between the oesophageal oxygen saturation results and those from blood gas analysis were 0.00 (-0.30 to 0.30 [-4.47 to 2.60]), and between the oesophageal oxygen saturation results and those from CO-oximetry were 0.75 (0.30 to 1.20 [-1.80 to 1.80]). Bland-Altman analysis showed that the bias and the limits of agreement between the oesophageal and finger pulse oximeters were -0.3% and -3.3 to 2.7%, respectively. In five (10.2%) patients, the finger pulse oximeter failed for at least 10 min, whereas the oesophageal readings remained reliable. The results suggest that the oesophagus may be used as an alternative monitoring site for pulse oximetry even in patients with compromised peripheral perfusion

The effects of thermistor linearization techniques on the T-history characterization of phase change materials

Phase Change Materials (PCMs) are increasingly being used in the area of energy sustainability. Thermal characterization is a prerequisite for any reliable utilization of these materials. Current characterization methods including the well-known T-history method depend on accurate temperature measurements. This paper investigates the impact of different thermistor linearization techniques on the temperature uncertainty in the T-history characterization of PCMs. Thermistor sensors and two linearization techniques were evaluated in terms of achievable temperature accuracy through consideration of both, non-linearity and self-heating errors. T-history measurements of RT21 (RUBITHERM ® GmbH) PCM were performed. Temperature measurement results on the RT21 sample suggest that the Serial-Parallel Resistor (SPR) 1 linearization technique gives better uncertainty (less than ±0.1°C) in comparison with the Wheatstone Bridge (WB) 1 technique (up to ±1.5°C). These results may considerably influence the usability of latent heat storage density of PCMs in the certain temperature range. They could also provide a solid base for the development of a T-history measuring device. © 2012 Elsevier Ltd. All rights reserved

An oesophageal pulse oximetry system utilising a fibre-optic probe

A dual-wavelength fibre-optic pulse oximetry system is described for the purposes of estimating oxygen saturation (SpO2) from the oesophagus. A probe containing miniature right-angled glass prisms was used to record photoplethysmographic (PPG) signals from the oesophageal wall. Signals were recorded successfully in 19 of 20 patients, demonstrating that PPG signals could be reliably obtained from an internal vascularised tissue site such as the oesophageal epithelium. The value of the mean oxygen saturation recorded from the oesophagus was 94.0 ± 4.0%. These results demonstrate that SpO2 may be estimated in the oesophagus using a fibre-optic probe

Free flap pulse oximetry utilizing reflectance photoplethysmography

The successful salvage of a free flap is dependent on the continuous monitoring of perfusion. To date there is no widely accepted and readily available post-operative monitoring technique to reliably assess the viability of free flaps by continuously monitoring free flap blood oxygen saturation. In an attempt to overcome the limitations of the current techniques a reflectance photoplethysmographic (PPG) processing system has been developed with the capability of real-time estimation of arterial oxygen saturation (SpO2) levels implemented in LabVIEW. This system was evaluated in clinical measurements on five patients undergoing breast reconstruction using Deep Inferior Epigastric Perforator (DIEP) flap. Good quality PPG signals were obtained from the flaps and fingers simultaneously. The estimated free flap SpO2 values were in broad agreement with the oxygen saturation readings from the commercial pulse oximeter. The results suggest that reflectance free flap photoplethysmography can be used as a continuous monitoring technique to non-invasively monitor the perfusion of free flaps

Empirical mode decomposition analysis of HRV data from patients undergoing local anaesthesia (brachial plexus block)

Spectral analysis of heart rate variability (HRV) is used for the assessment of cardiovascular autonomic control. In this study, a data-driven adaptive technique called empirical mode decomposition (EMD) and the associated Hilbert spectrum has been used to evaluate the effect of local anaesthesia on HRV parameters in a group of 14 patients undergoing axillary brachial plexus block. The normalized amplitude Hilbert spectrum was used to calculate the error index associated with the instantaneous frequency. The amplitude and the frequency values were corrected in the region where the error was higher than twice standard deviation. The intrinsic mode function (IMF) components were assigned to the LF and the HF part of the signal by making use of the centre frequency and the standard deviation spectral extension estimated from the marginal spectrum of the IMF components. The optimal range of the stopping criterion was found to be between 4 and 9 for the HRV data. The statistical analysis showed that the LF/HF ratio decreased within an hour of the application of the brachial plexus block compared to the values at the start of the procedure. These changes were observed in 13 of the 14 patients included in this study

Time-Frequency analysis of HRV data from locally anesthetized patients

Spectral analysis of Heart Rate Variability (HRV) can be used for the assessment of cardiovascular autonomic control. In this study Smoothed-Pseudo Wigner-Ville Distribution (SPWVD) has been used to evaluate the effect of local anesthesia on HRV parameters in a group of fourteen patients undergoing brachial plexus block (local anesthesia) using the transarterial technique. Instead of using the fixed boundaries of the LF (0.04-0.15 Hz) and the HF (0.15-0.4 Hz) components, the center frequency and the standard deviation spectral extension was used to estimate the boundaries related to the two components of the HRV signal. The boundaries related to the HF component of the signal were estimated using the cross-spectrum between the HRV signal and the respiration signal. The LF component boundaries were estimated directly from the time-frequency representation of the HRV signal. The statistical analysis showed that the LF/HF amplitude ratio decreased within an hour of the application of the brachial plexus block compared to the values at the start of the procedure. These changes were observed in eleven of the fourteen patients included in this study