Pulse Oximeter Perfusion Index as an Early Predictor of Successful Ultrasound-Guided Axillary Brachial Plexus Block: A Prospective Observational Study

Abstract

Background: Objective assessment of ultrasound-guided axillary brachial plexus block (AXB) success is challenging due to the subjective nature of conventional sensory and motor evaluations. The perfusion index (PI), a non-invasive marker derived from pulse oximetry, may provide an early, objective indicator of block efficacy.

Methods: This prospective study included 30 adult patients (ASA I–II) undergoing upper limb surgery with ultrasound-guided AXB. PI measurements were taken on the middle finger of the blocked arm at baseline, every 2 minutes up to 10 minutes, and every 5 minutes up to 30 minutes post-block. PI ratio was calculated as PI at 10 minutes divided by baseline PI. Sensory and motor blocks were evaluated at 5-minute intervals for 30 minutes. Statistical analysis used ROC curve assessment to determine optimal PI cut-off values for predicting block success.

Results: Successful AXB was achieved in the majority of patients. PI values significantly increased from baseline as early as 5 minutes post-block in successful cases (p < 0.001). At 10 minutes, mean PI and PI ratio were significantly higher in successful blocks. ROC analysis identified optimal cut-offs of PI ≥2.6 and PI ratio ≥1.9 at 10 minutes, with high sensitivity (96% and 93.3%) and specificity (94% and 100%), and AUROC values of 0.97 and 1.0, respectively.

Conclusion: PI and PI ratio are reliable predictors of successful ultrasound-guided AXB, reflecting sympathetic blockade-induced vasodilation. Incorporating PI monitoring may enhance block assessment, improve operating room efficiency, and reduce reliance on subjective tests.