Clinical significance of oxyhemoglobin dissociation curves on the interpretation of pulse oximetry in sickle cell diseas

Some sickle cell patients have very abnormal SpO2 measurements yet appear to be clinically well. The clinician's interpretation of oxygenation status of a patient is based on the relation of SpO2 to pO2 that has been calibrated through experience to the HbAA oxyhemoglobin dissociation curve. To address the possibility that apparent severe hypoxia in some SCD patients is due to large differences in their individual oxyhemoglobin dissociation curves, we measured the oxyhemoglobin dissociation curves in SCD subjects and controls, determined the within-subject and population variance, and used the equations obtained from fitting population data and data from individual subjects to derive the differences between observed and corrected SaO2 in subjects with HbSS and HbSC phenotypes. There was excellent agreement (< 5 saturation points difference) between observed SpO2 and derived SaO2 for 87.5% of the measurements. However, about 12.5% of the saturation measurements would be more than 10% higher if HbAA were present instead of HbSS. This study confirms that SpO 2 is a reasonable estimate of oxygenation status in patients with sickle cell disease. However, in about 12% of the subjects, SpO2 values in the 80 to 90% range may significantly underestimate the oxygenation status of the patient because of individual differences in the oxyhemoglobin dissociation curve.