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. Previous SectionNext Section The epidural space is usually located by the loss-of-resistance (LOR) technique, in which a change in compliance is detected by easier injection of air or saline associated with passage of the tip of the epidural needle from the ligamentum flavum into the epidural space. The technique was originally described in 1933 using a fluid-filled syringe.1 Subsequently, air was often substituted, perhaps as a way to avoid the technical difficulty associated with increased friction between the plunger and barrel of the LOR syringe.2 Anesthesiologists' preference for one medium or the other was largely dictated by experience during training3 rather than by objective evidence of superiority of either. In 1987, however, a case report described incomplete analgesia in 2 pediatric patients in whom the epidural space had been located by LOR to air. Imaging of the spine showed air bubbles adjacent to the unblocked nerve roots.4 Subsequently, several randomized clinical trials (RCTs) investigated the performance of the 2 techniques, and some suggested superiority of saline over air, particularly with respect to the incidence of incomplete analgesia.5–11 In our experience, however, few anesthesiologists use the 2 media interchangeably as they would in a RCT. Because it is impossible to mask the anesthesiologist to the medium used for LOR, we hypothesized that RCTs might overestimate the difference between air and saline by forcing the operator to use a less-preferred technique in half of the subjects. Therefore, we undertook an effectiveness study of the impact of LOR media choice on analgesic outcomes in laboring patients.
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