A Retrospective Effectiveness Study of Loss of Resistance to Air or Saline for Identification of the Epidural Space

BACKGROUND: Randomized trials comparing air to saline for loss of resistance (LOR) for identification of the epidural space have suggested the superiority of saline. We hypothesized that, in actual clinical practice, anesthesiologists using their preferred technique would produce similar analgesic outcomes with either air or saline. METHODS: The labor analgesia records for 929 parturients requesting neuraxial analgesia were reviewed with respect to technique (epidural or combined spinal-epidural; air or saline for LOR), analgesic outcomes (initial comfort, asymmetry of the block, need for physician top-up during patient-controlled epidural analgesia, and catheter replacement), and complications (paresthesia, IV or intrathecal catheter placement, and unintentional dural puncture). RESULTS: Of 929 labor analgesics analyzed, 52.6% were performed with LOR to air and 47.4% to saline. Among anesthesiologists who performed at least 10 blocks, 82% used 1 medium at least 70% of the time. There were no differences between the air and saline groups in patient characteristics, analgesic technique, or block success. Among operators with a preference for 1 medium, use of the preferred technique was associated with fewer attempts (1.3 +/- 0.7 vs 1.6 +/- 0.8, P = 0.001), fewer paresthesias (8.7% vs 18.5%, odds ratio = 0.42, P = 0.007), and fewer unintentional dural punctures (1.0% vs 4.4%, odds ratio = 0.23, P = 0.03). CONCLUSIONS: When used at the anesthesiologist's discretion, there is no significant difference in block success between air and saline for localization of the epidural space by LOR.