Assessment of parasacral ischial plane block success using sonographically measured regional hemodynamic variables and perfusion index: an observational diagnostic accuracy study

Traditional methods of assessing block success pose challenges in patients with traumatic brain injuries, who are often uncooperative and non-compliant to subjective assessment. We thus planned this study with the aim of evaluating effectiveness of objective parameters, including perfusion index(PI) and ultrasound-guided regional hemodynamic parameters, for indicating success of ultrasound-guided parasacral ischial plane(PIP) block, a lower extremity fascial plane block. We enrolled 89 consecutive adult trauma patients scheduled for lower-limb orthopaedic surgery under ultrasound-guided PIP block (plus a femoral block), and insonated the popliteal artery for the following parameters blood‑flow velocities: peak systolic velocity (PSV), end diastolic velocity (EDV), Time-averaged maximum velocity (Tmax), Time averaged mean velocity (Tmean) with Doppler ultrasound and perfusion index (PI) at the big toe at baseline and 10, 20, 30, 40 min after the block. We assessed block success with conventional sensory and motor tests (pinprick and strength) — this was the “clinical reference standard” and looked at how well the percentage changes in PI and Doppler measurements predicted the clinical (subjective) determination of block success. Our secondary objectives compared objective parameters with subjective parameters in terms of sensitivity, specificity, positive predictive value(PPV) and negative predictive value(NPV). Of the 89 patients, 83 (93%) had a successful block, tracked by significant increases in regional hemodynamic variables and PI at 20 min. Successful blocks showed significant increases in regional doppler measures and perfusion index by 20 min. At 20 min, the optimal thresholds were a 38.2% rise in popliteal PSV and a 46.5% rise in PI. The AUROCs were 0.92 (95% CI 0.84–0.99) for %ΔPI and 0.86 (95% CI 0.74–0.99) for %ΔPSV. In this sample the specificity and positive predictive value at these thresholds were 100%, but these estimates are imprecise because of the small number of failed blocks. Regional hemodynamic parameters, more so percentage increase in PSV and PI, can be used as early and reliable adjunctive indicators of a successful ultrasound-guided parasacral ischial plane block. The trial was prospectively registered with Clinical Trial Registry (https//ctri.nic.in/Clinicaltrials/rmaindet.php?trialid=84902&EncHid=63217.30513&modid=1&compid=19; Registration number CTRI/2023/06/053375 Date of Registration01062023 Date of first patient enrolment15062023).