Decreased circulation time in the upper limb reduces the lag time of the finger pulse oximeter response

To observe the influence of circulatory changes on the lag time of the pulse oximeter response, eight healthy patients scheduled for hand surgery were studied. After breath holding, the patients took a breath of oxygen and the time to an increase in SpO2 was measured before and after axillary brachial plexus block. It was found that the lag time with finger probe decreased from 28.6 +/- 7.1 sec to 15.8 +/- 1.1 sec (mean +/- SD) following brachial block (P less than 0.01). There was no change in arterial blood pressure. The results suggest that the lag time of the finger pulse oximeter response is primarily determined by blood flow.