FROM OXYGEN-CONTENT TO PULSE OXIMETRY - COMPLETING THE PICTURE IN THENEWBORN

In recent years clinicians caring for sick preterm infants have come t o depend on pulse oximetry to avoid hyperoxia, which means assuming sa turation values for critical levels of oxygen tension. This prediction is made difficult by the shape of the haemoglobin-oxygen dissociation curve at critical values for arterial pO(2) and by the effects of cha nges in acid-base balance on p50. Combined blood gas and co-oximetry m easurements can be used to determine critical limits for pulse oximetr y. Fetal haemoglobin has slightly different light absorption character istics from adult haemoglobin. To adjust for this, adult and fetal mat rices are available in the OSM(TM)3 HEMOXIMETER(TM) (Radiometer Medica l A/S, Denmark) but the measurement requires an extra preliminary step to estimate fetal haemoglobin concentration. We sought to determine t he importance of this extra procedure for measuring the saturation of newborn blood, and to determine whether the adult or fetal mode should be used for determining saturation for comparison with pulse oximeter s. We measured the effect of the correction for fetal haemoglobin by o btaining absorbances from the co-oximeter and multiplying them by the adult and fetal matrices. We demonstrated that, at 90% saturation, fai lure to use the fetal correction in the presence of high levels of fet al haemoglobin result in a 4% overestimate of saturation, with resulta nt underestimation of the safe range for pulse oximetry. Published val ues for extinction coefficients for fetal and adult blood at wavelengt hs used by pulse oximeters are inconsistent, but it appears that fetal haemoglobin does not bias pulse oximetry readings. Determining satura tion limits by co-oximetry for use with pulse oximeters in preterm inf ants requires the description of the haemoglobin-oxygen dissociation c urve with the correction for fetal haemoglobin.