http://hdl.handle.net/2328/2990 2013-05-20T05:49:38Z 2013-05-20T05:49:38Z McEwen, Mark Bull, Geoff Reynolds, Karen Jane http://hdl.handle.net/2328/7295 2013-05-13T01:29:25Z 2009-01-01T00:00:00Z

Title: Vessel calibre and haemoglobin effects on pulse oximetry Authors: McEwen, Mark; Bull, Geoff; Reynolds, Karen Jane Abstract: Despite its success as a clinical monitoring tool, pulse oximetry may be improved with respect to the need for empirical calibration and the reports of biases in readings associated with peripheral vasoconstriction and haemoglobin concentration. To effect this improvement, this work aims to improve the understanding of the photoplethysmography signal - as used by pulse oximeters, and investigates the effect of vessel calibre and haemoglobin concentration on pulse oximetry. The digital temperature and the transmission of a wide spectrum of light through the fingers of 57 people with known haemoglobin concentrations were measured, and simulations of the transmission of that spectrum of light through finger models were performed. Ratios of pulsatile attenuations of light as used in pulse oximetry were dependent upon peripheral temperature and on blood haemoglobin concentration. In addition, both the simulation and in vivo results showed that the pulsatile attenuation of light through fingers was approximately proportional to the absorption coefficients of blood, only when the absorption coefficients were small. These findings were explained in terms of discrete blood vessels acting as barriers to light transmission through tissue. Due to the influence of discrete blood vessels on light transmission, pulse oximeter outputs tend to be dependent upon haemoglobin concentration and on the calibre of pulsing blood vessels - which are affected by vasoconstriction/vasodilation. The effects of discrete blood vessels may account for part of the difference between the Beer–Lambert pulse oximetry model and empirical calibration.

2009-01-01T00:00:00Z McEwen, Mark Reynolds, Karen Jane http://hdl.handle.net/2328/2995 2013-05-13T01:36:31Z 2006-03-01T00:00:00Z

Title: Noninvasive detection of bilirubin using pulsatile absorption. Authors: McEwen, Mark; Reynolds, Karen Jane Abstract: Bilirubin, the yellow substance usually responsible for neonatal jaundice, is currently monitored invasively or by observing/measuring skin colour. This paper investigates the feasibility of monitoring serum bilirubin concentration using light absorbance in a similar fashion to pulse oximetry. The light absorbance of bilirubin is shown to be sufficiently different to haemoglobin to in theory allow direct noninvasive serum bilirubin monitoring using light absorbance around 480nm.

2006-03-01T00:00:00Z McEwen, Mark Roxby, David John http://hdl.handle.net/2328/2992 2013-05-13T01:36:23Z 2007-08-01T00:00:00Z

Title: Can latent heat safely warm blood? – in vitro testing of a portable prototype blood warmer Authors: McEwen, Mark; Roxby, David John Abstract: Background Trauma/retrieval patients are often in shock and hypothermic. Treatment of such patients usually involves restoring their blood volume with transfusion of blood (stored at 2°C – 6°C) and/or crystalloids or colloids (stored at ambient temperature). Rapid infusion of these cold fluids can worsen or even induce hypothermia in these patients. Warming of intravenous fluids at accident sites has traditionally been difficult due to a lack of suitable portable fluid warmers that are not dependent on mains electrical or battery power. If latent heat, the heat released when a liquid solidifies (an inherently temperature limiting process) can warm intravenous fluids, portable devices without a reliance on electrical energy could be used to reduce the incidence of hypothermia in trauma patients. Methods Rapid infusion of red cells into patients was timed to sample typical clinical flow rates. An approved dry heat blood warmer was compared with a prototype blood warmer using a supercooled liquid latent heat storage material, to warm red cells whilst monitoring inlet and outlet temperatures. To determine the effect of warming on red cell integrity compared to the normal storage lesion of blood, extracellular concentrations of potassium, lactate dehydrogenase and haemoglobin were measured in blood which had been warmed after storage at 2°C – 6°C for 1 to 42 days. Results A prototype latent heat fluid warmer consistently warmed red cells from approximately 4°C to approximately 35°C at typical clinical flow rates. Warming of stored blood with latent heat did not affect red cell integrity more than the approved dry heat blood warmer. Conclusion Using latent heat as an energy source can satisfactorily warm cold blood or other intravenous fluids to near body temperature, without any adverse affects.

2007-08-01T00:00:00Z