Do changes in pulse oximeter oxygen saturation predict equivalent changes in arterial oxygen saturation?

Introduction This study investigates the relation between changes in pulse oximeter oxygen saturation (SpO(2)) and changes in arterial oxygen saturation (SaO(2)) in the critically ill, and the effects of acidosis and anaemia on precision of using pulse oximetry to predict SaO(2). Patients and methods Forty-one consecutive patients were recruited from a nine-bed general intensive care unit into a 2-month study. Patients with significant jaundice (bilirubin > 40 mumol/l) or inadequate pulse oximetry tracing were excluded. Results A total of 1085 paired readings demonstrated only moderate correlation (r=0.606; P<0.01) between changes in SpO(2) and those in SaO(2), and the pulse oximeter tended to overestimate actual changes in SaO(2). Anaemia increased the degree of positive bias whereas acidosis reduced it. However, the magnitude of these changes was small. Conclusion Changes in SpO(2) do not reliably predict equivalent changes in SaO(2) in the critically ill. Neither anaemia nor acidosis alters the relation between SpO(2) and SaO(2) to any clinically important extent.