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Background and Aims Background and Aims: Achieving rapid onset of surgical anaesthesia after a popliteal sciatic nerve block remains a challenge, which this study aimed to determine using a twin subparaneural injection below its divergence with alkalinized lignocaine

Methods After ethical approval 20 ASA I-III patients scheduled for elective foot and ankle surgery, under an ultrasound-guided popliteal sciatic nerve block (PSNB), were recruited for this non-randomized study of intervention. All patients received two separate injections into the individual subparaneural compartments of the common peroneal (CPN) and tibial nerve (TN) below the divergence, each with 14.5 ml of 1.5% lignocaine and 0.5 ml of 8.4% sodium bicarbonate. To achieve this, the subparaneural compartment of the sciatic nerve was initially distended with normal saline at its divergence. Sensory and motor blockade was assessed using a numeric rating scale (0-100; 0=anaesthesia) and Likert scale (0-2; 0=paralysis) respectively. Time to achieve complete anaesthesia (sensory score 0/100 and motor score 0/2) and duration of sensory-motor blockade were the primary and secondary outcomes, respectively.

Results The median [IQR] time to complete anaesthesia was 10[5-15] min for the CPN and 15[10-25] min for the TN and it was effective for surgery in all patients studied. The median[IQR] duration of self-reported sensory motor blockade was 7.4 [5.9-9.7] hours.

Conclusions An USG subparaneural PSNB when performed as two separate injections below the divergence of the sciatic nerve, with alkalinized lignocaine, produces surgical anaesthesia within 15 minutes. We believe this is the fastest onset-time reported for a PSNB in the literature.