Time course in thoracic epidural pressure measurement.

The purpose of this study was to measure thoracic epidural pressure at the time of insertion of an epidural needle. The pressure was measured using a closed pressure measurement system after insertion of a Tuohy needle at the T7-8 intervertebral level. This system is unique because it has a pressure transducer equipped with a device which regulates flow through the transducer and prevents the heparinized saline in the system from being evacuated into the epidural space due to sudden pressure change. As reported previously, high negative epidural pressures, up to -60 mmHg, were observed at the moment of epidural puncture. However, the pressure became positive in about 30 sec in 12 of the 13 patients, and stabilized at +3.7 +/- 3.2 mmHg (mean +/- SD) within 90 sec. Similar results were observed when the procedure was repeated within a few minutes to the same patients. This suggests that negative epidural pressures at the moment of epidural puncture are artifacts induced by tenting of the dural membrane. Subsequent adaptation of the surrounding tissue results in restoration of the normal positive epidural pressure.