Evidence for using air or fluid when identifying the epidural space

Lumbar epidural analgesia is frequently employed to provide pain relief for women during labor. Anesthesia providers use various methods to identify the epidural space. Some providers use air, some use fluid, and others use a combination of air and fluid during the loss of resistance technique. Loss of resistance to air has been speculated to result in a lesser quality of analgesia compared with loss of resistance to only fluid. A search strategy focusing on preappraised sources was used to locate evidence from interventional and observational studies. Four evidence sources were located, including a systematic review with meta-analysis of 4 older studies. The evidence reviewed was inconclusive in determining whether a difference in analgesia quality results from the use of air or fluid during the loss of resistance technique. Future studies should include an adequate number of subjects and address other problems such as operator experience, observer blinding, equivalence of subject characteristics, outcomes definition and measurement, and composition of epidural solution. Providers should consider other factors when selecting loss of resistance medium, such as the reported complications of large amounts of air injected into the epidural space and surrounding structures.