Background and Objectives. During performance of epidural injection, entry into the epidural space has traditionally been determined by identification of negative pressure to the advancing needle by indirect means such as hanging drop; loss of resistance to air, saline, or water; or use of a MacIntosh balloon. Confusion in the literature regarding entry pressures versus postentry (baseline) pressures and zero reference pressure was noted.

Methods. Baseline epidural pressure changes were examined using a closed system zeroed to the dorsal spine during and after injection of local anesthetics in 39 patients referred to the Pain Treatment Center, located at SUNY Health Science Center, Syracuse, New York, for epidural injections. The course of epidural pressure changes after injection and the temporal relation of epidural pressure waves to the arterial and venous wave forms were examined.

Results. In contrast to previous investigations, subatmospheric pressure was found in the epidural space in only one patient. Baseline pressure for all patients was 7.7 ± 3.9 mmHg. There were significant differences in baseline pressure in patients who had undergone back surgery compared with patients who had not undergone such surgery: 11.8 ± 3.4 as opposed to 7.0 ± 3.5 mmHg, respectively ( p < 0.005). Three minutes after a 2-ml injection of local anesthetic into the epidural space, the pressure returned to baseline. This contrasted to the effects of a 6-ml injection, which resulted in the pressure remaining above baseline after 3 minutes. The epidural pressure wave forms more closely paralleled the radial artery wave form than the central venous pressure.

Conclusion. Lumbar epidural pressure is greater than atmospheric pressure when referenced to zero at the dorsal spine level.