NOTES ON THE APPARENT DISCORDANCE OF PULSE OXIMETRY AND MULTIWAVELENGTH HEMOGLOBIN PHOTOMETRY

Multi-wavelength photometers, blood gas analysers and pulse oximeters are widely used to measure various oxygen-related quantities. The defi nitions of these quantities are not always correct. This paper gives i nsight in the various definitions for oxygen quantities. Furthermore, the possible influences of dyshaemoglobins and fetal haemoglobin on th e accuracy of pulse oximetry are discussed. As pulse oximeters are con structed for the determination of arterial oxygen saturation, they sho uld be validated with sample oxygen saturation values and not with the oxyhaemoglobin fraction. The influence of carboxyhaemoglobin is insub stantial over an oxygen saturation range of 0% to 100%. Through the pr esence of methaemoglobin, pulse oximetry will give an underestimation above 70% and an overestimation below 70% oxygen saturation. The influ ence of fetal haemoglobin is insignificant in the neonatal use of puls e oximetry, in the range of 75% to 100% arterial oxygen saturation. Ho wever, a pulse oximeter underestimates the arterial oxygen saturation at the 25% level with 5%, if the pulse oximeter has been calibrated in human adults. Such a low level of arterial oxygen saturation can be p resent in the fetus during labor.