Comparing the minimum local anesthetic dose of ropivacaine in real-time ultrasound-guided spinal anesthesia and traditional landmark-guided spinal anesthesia: a randomized controlled trial of knee surgery patients.

BACKGROUND: Through previous studies and clinical practice, we have found that real-time ultrasound-guided (UG) spinal anesthesia (SA) and traditional landmark-guided (LG) SA each require a different minimum local anesthetic dose (MLAD) of ropivacaine. For this study, we used Dixon's up-and-down sequential method to analyze and compare the MLAD of different ropivacaine concentrations required for the UG and LG SA methods. METHODS: A total of 120 patients undergoing knee surgery were consecutively recruited and randomly divided into four groups (30 patients per group). These groups were categorized as follows: Group I: high ropivacaine ultrasound-guided (HRUG), Group II: low ropivacaine ultrasound-guided (LRUG), Group III: high ropivacaine landmark-guided (HRLG), and Group IV: low ropivacaine landmark-guided (LRLG). SA was established by a bolus administration of up-and-down doses of 0.75% or 0.5% plain ropivacaine. Initial doses of 16, 18, 12, and 14 mg were administered to groups I-IV, and after that, increased or decreased by 1.5 mg according to dose effectiveness. Upon identifying the intervertebral puncture level, a lumbar X-ray was performed with metal markers, and actual radiographic findings were identified and compared to the initial markings. RESULTS: For UG groups, the MLAD in the LRUG group was significantly higher than in the HRUG group [20.192 mg (95% CI, 19.256-21.174) versus 17.176 mg (95% CI, 16.276-18.124), respectively; P<0.001]. For LG groups, the MLAD in the LRLG group was significantly higher than in the HLRG group [14.478 mg (95% CI, 13.364-15.500) versus 13.201 mg (95% CI, 11.959-14.571), respectively; P=0.047]. When comparing both high ropivacaine groups (HRGs: I/III) to the low ropivacaine groups (LRGs: II/IV), we found that both UG subgroups (I/II) had a significantly higher MLAD than LG subgroups (III/IV) (P<0.001). US identified L4-5 in up to 90% of cases. Comparatively, palpation was successful in only 33.3% of patients. The rates of cephalad localization by US and palpation were 6.67% vs. 66.67%, respectively (P=0.002). CONCLUSIONS: We found a higher MLAD of ropivacaine was required for UG SA at the L4-5 level due to the method providing a more accurate (less cephalad) localization than traditional LG SA. TRIAL REGISTRATION: Chinese Clinical Trial Registry ChiCTR2000033158.