Accuracy of Perfusion Index and Perfusion Index Ratio as a Predictor of a Successful Low Interscalene Brachial Plexus Block: A Prospective Observational Study.

Background: The perfusion index has previously been used to predict the success of brachial plexus blocks but to our knowledge has not yet been studied for the low interscalene block approach. This study evaluated the accuracy of the perfusion index and perfusion index ratio in predicting the success of a low interscalene brachial plexus block in patients undergoing upper limb surgery.

Methods: The study included 70 patients undergoing upper limb surgeries under ultrasound-guided low interscalene brachial plexus block. After local anesthetic injection, patients' sensory and motor blocks were assessed every 5 minutes. The perfusion index was recorded at baseline and then at 5-minute intervals until 30 minutes after anesthetic injection in both the blocked and unblocked limbs. The perfusion index ratio (perfusion index at a specific timepoint/baseline perfusion index) was calculated and recorded at each 5-minute time point. Receiver operating characteristic curves were constructed to determine the accurate value of the perfusion index that indicated block success.

Results: The mean perfusion index and perfusion index ratio were higher in the blocked limb vs the unblocked limb at all time points. At the 10-minute time point, the cutoff values for the perfusion index and perfusion index ratio for a successful block were 3.24 and 3.54, respectively. At the 10-minute time point, the sensitivities of the perfusion index and perfusion index ratio in predicting a successful block were 95.2% and 100%, respectively; specificity was 100% for both the perfusion index and perfusion index ratio.

Conclusion: The perfusion index and perfusion index ratio can accurately predict the success of a low interscalene brachial plexus block well before surgical anesthesia is fully established, thereby saving operating room time. A perfusion index ratio of 3.54 or more at 10 minutes is a reliable predictor of block success.