Effect of Digital Block on Pulse Oximeter Signal Detection during General Anesthesia

Pulse oximetry is dependent upon the presence of a pulsating vascular bed. Low pulsatile blood flow in the digits is caused by various factors. Regardless of the cause, if there is sufficient arterial pulse pressure, blocking the sympathetic nerve should cause a local increase in capillary flow and pulse volume. This study was performed to evaluate the effect of digital nerve block on pulse oximetric signal detection (SpO2, Lag time and amplitude of plethismographic wave) during general anesthesia. After induction of general anesthesia in 105 patients, SpO2, Lag time and amplitude of plethismographic wave in index fingers of two hands were determined by pulse oximeter. Then 2 ml of 2% lidocaine was injected to index fingers of one hand and these parameters were determined at 10, 20, 30, 40 and 60 minutes after digital nerve block, in blocked and unblocked fingers. The data were compared between two groups. Mean lag time of plethismographic wave in 10, 20, 30, 40 and 60 minutes after digital block was shorter in blocked fingers than unblocked fingers (p<0.05). Mean amplitude of plethismographic wave in these times after digital block was greater in blocked fingers than unblocked fingers (p<0.05). No statistical difference in SpO2 was determined between blocked and unblocked fingers. In conclusion, the digital block is an effective and safe technique for better pulse oximetric signal detection during general anesthesia. Short lag time and high amplitude of plethismographic wave probably can facilitate the accuracy of SpO2 calculation and therefore rapid detection of hypoxemia.