Abstract
This study investigates the relation between changes in pulse oximeter oxygen saturation (SpO2) and changes in arterial oxygen saturation (SaO2) in the critically ill, and the effects of acidosis and anaemia on precision of using pulse oximetry to predict SaO2. 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 micromol/l) or inadequate pulse oximetry tracing were excluded. A total of 1085 paired readings demonstrated only moderate correlation (r= 0.606; P < 0.01) between changes in SpO2 and those in SaO2, and the pulse oximeter tended to overestimate actual changes in SaO2. Anaemia increased the degree of positive bias whereas acidosis reduced it. However, the magnitude of these changes was small. Changes in SpO2 do not reliably predict equivalent changes in SaO2 in the critically ill. Neither anaemia nor acidosis alters the relation between SpO2 and SaO2 to any clinically important extent.
Highlights
This study investigates the relation between changes in pulse oximeter oxygen saturation (SpO2) and changes in arterial oxygen saturation (SaO2) in the critically ill, and the effects of acidosis and anaemia on precision of using pulse oximetry to predict SaO2
During a 2-month period all patients admitted to our intensive care unit (ICU) who had an arterial line for the measurement of blood gases and who were being monitored by continuous pulse oximetry were recruited
We showed that SpO2 underestimates SaO2 to a greater extent with progressive anaemia, whereas acidosis increases the SpO2 estimate of SaO2
Summary
This study investigates the relation between changes in pulse oximeter oxygen saturation (SpO2) and changes in arterial oxygen saturation (SaO2) in the critically ill, and the effects of acidosis and anaemia on precision of using pulse oximetry to predict SaO2. Previous studies investigating the use of pulse oximeter oxygen saturation (SpO2) in intensive care patients have reported that the minimum SpO2 levels to maintain SaO2 at 90% range between 92% and 96% [4,6,7] These studies have not answered the question of whether, after achieving a target SaO2, a subsequent change in SpO2 predicts a corresponding change in SaO2 in the critically ill.
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