Success rate and block characteristics of L5-S1 spinal anesthesia with hypobaric bupivacaine in total hip and knee arthroplasty

Background: Spinal anesthesia in older patients can be technically challenging due to degenerative spinal disease. The L5-S1 interspace often remains accessible and is a useful fallback; however, it has been associated with higher rates of therapeutic failure due to inadequate block height. Injection of hypobaric local anesthetic may promote cranial intrathecal spread and address this limitation. We investigated this by evaluating the efficacy and block characteristics of low-dose hypobaric bupivacaine injected at the L5-S1 level in a prospective observational study. Methods: Fifty-four patients undergoing elective total hip or knee arthroplasty received ultrasound-assisted spinal anesthesia at L5-S1 with 10 mg of 0.33% hypobaric bupivacaine. The primary outcome was surgical completion without conversion to general anesthesia, supplemental opioids, or local anesthetic infiltration. Secondary outcomes included adequate anesthesia for surgical incision, block pharmacodynamics, hemodynamic stability, adverse effects, patient satisfaction, and surgeon-perceived quality of anesthesia. Results: Surgical completion without anesthetic supplementation was successful in 50 patients (92.6%, 95% CI 82.1-97.8%). Adequate anesthesia for surgical incision was achieved in 53 patients (98.2%, 95% CI 90.1-100%). Four patients required supplemental opioids: one for surgical incision, and three for surgical completion following prolonged injection-to-incision intervals (47-59 minutes) due to unanticipated delays in operating room readiness. No patients required conversion to general anesthesia. Full motor recovery occurred within 198 ± 53 minutes. Patient and surgeon satisfaction with quality of anesthesia was high. Conclusions: Hypobaric 0.33% bupivacaine injected at L5-S1 may be a useful fallback option in lower-extremity arthroplasty if challenging spinal anatomy renders other interspaces inaccessible.