Finger blood content, light transmission, and pulse oximetry errors

Abstract

SummaryThe changes in light emitting diode current necessary to maintain a constant level of light incident upon a photodetector were measured in 20 volunteers at the two wavelengths employed by pulse oximeters. Three states of finger blood content were assessed; exsanguinated, hyperaemic. and normal. The changes in light emitting diode current with changes in finger blood content were small and are not thought to represent a significant source of error in saturation as measured by pulse oximetry. Key wordsEquipment; pulse oximeter. Haemoglobin; anaemia.Light emitting diodes (LEDs) are used as light sources in pulse oximeters as they produce a narrow band of wavelengths. The stability of the emitted wavelengths is critical to the accuracy of pulse oximeters as the absorption spectra of reduced and oxygenated haemoglobin exhibit large changes for small changes in wavelength. A change in LED wavelength during the operation of a pulse oximeter may thus result in inaccurate oximeter readings. For example, if the wavelength of the red LED is decreased, the absorption of red light by reduced haemoglobin increases, resulting in an apparent fall in saturation as measured by the oximeter.Severinghaus and Koh have recently shown that pulse oximeters under-read at low saturations [ 11 and speculated that at low saturations the relatively high absorption of red light causes a reduction in the amount of red light incident upon the detector (photodiode). Pulse oximeters alter the intensity of their LEDs according to the thickness and optical density of the finger or earlobe in order to maintain the intensity of light reaching the photodiode in a narrow range. A reduction in red light transmission thus leads to an increase in current at the red LED. These authors suggest that this heats the diode and results in a reduction of the peak wavelength of light emitted by the LED. This in turn causes the oximeter to under-read at low saturations. Paradoxically, this is worse in anaemic subjects.This explanation for pulse oximeters under-reading at low saturations seems unlikely to be correct for two reasons. Firstly, blood represents only a small proportion of the tissue of the finger or ear. Unless blood is the major light absorbing component of tissue, it is unlikely that changes in saturation will alter the total light absorption of the finger or ear sufficient to cause major changes in LED current. Secondly, increasing LED output from 10% to 100% of the manufacturer's maximum specified current has been shown to increase, not decrease, the peak wavelength. This increase in wavelength amounted to only 8 nm when LED current was increased ninefold [2].The contribution of blood to the total optical density of the finger has not been evaluated. The following study was undertaken to assess our hypothesis that the level of haemoglobin in the finger has no effect on LED current and is thus unlikely to affect pulse oximeter accuracy. MethodsChanges in light absorption were measured using a locally made two-wavelength photometer designed to...