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(IOP Publishing, 2009) McEwen, Mark; Bull, Geoff; Reynolds, Karen Jane
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.