Air or nitrous oxide for loss-of-resistance epidural technique?

To compare the spinal-epidural spread of gas following loss of resistance (LOR) technique for detection of the epidural space using air or nitrous oxide (N2O).

Comparison of the spread of air and N2O in the epidural space following LOR technique was performed by using Magnetic Resonance Imaging (MRI). Ten adult patients ASA grade I served as their own control in this prospective study. A control MRI (MRI Contr-1) of the dorsolumbar spine was performed. Then, an 18 gauge epidural needle was introduced at the L3-4 intervertebral space using 0.14 ml x kg(-1) N2O for LOR and the MRI (MRI-N2O) was repeated. Forty eight hours later, an MRI scan (Contr-2 MRI) was performed and, subsequently, an 18 gauge epidural needle was introduced, using 0.14 ml x kg(-1) air for LOR followed by an MRI (MRI-Air) scan. The volumetric measurements of gas pockets were done using a formula.

Gas bubbles after N2O were few and small compared with larger gas pockets occupying up to three vertebral segments after the use of air for LOR. The volume of air in the epidural space was 2.96+/-0.93 ml compared with 0.35+/-0.32 ml N2O.

The use of N2O for LOR technique of detecting the epidural space produced very small bubbles detected by MRI compared with the use of air under similar conditions.