An entry of needle into the epidural space has been identified by the development of negative pressure to the advancing needle; hanging drop, loss of resistance, or use of Macintosh balloon. But it is suspicious that epidural space is a true negative pressure space.

We measured the epidural pressures of pregnant patients in right lateral decubitus (RLD) and sitting position and compared with the values of non-pregnant patients of respective position. 17G Tuohy needle with closed system was used, the pressure of which was zeroed to the puncture level at third lumbar intervertebral space.

Epidural pressures were positive in all subjects. Values of measurement were 13±4mmHg, 15±7mmHg, 16±6mmHg and 25±7mmHg for pregnant-RLD, pregnant-sitting, non-pregnant-RLD, and non-pregnant-sitting group, respectively.

There was a significant difference between RLD and sitting position in non-pregnant patient only (p<0.05). It is suggested that the hydrostatic pressure of CSF above puncture level and the hydrostatic pressure of valveless epidural venous plexus blood influenced the epidural pressure of non-pregnant group in sitting position. But in pregnant group, abdominal mass may have elicited a draining effect on epidural venous plexus during lumbar flexion in sitting position and partially nullified the hydrostatic pressure of CSF and blood.