1. IVC compression and hyperventilation decrease lumbar CSF volume
Tom Archer, MD

2. Lateral position– IVC is not compressed.
Manbit images

3. Supine position– IVC is compressed and epidural veins are engorged.

4. Abdominal compression narrows the IVC in non-pregnant volunteers.
Lee RR et al Spine 2001;26:1172–1178

5. Hyperventilation (B) decreases lumbar CSF volume.
Lee RR et al Spine 2001;26:1172–1178

6. Hyperventilation (B) decreases lumbar CSF volume.
Lee RR et al Spine 2001;26:1172–1178

7. Hyperventilation alone decreases lumbar CSF volume by 10%, abdominal compression alone decreases it by 28 %. Both together decrease CSF volume by 41%.
Lee RR et al Spine 2001;26:1172–1178

8. Summary
Volume of lumbar thecal sac and lumbar epidural blood is dynamic and respond to:
1) IVC compression and
2) hyperventilation– perhaps d/t decrease in intracranial blood volume with shift of CSF into the head.

9. Summary
IVC compression in pregnancy, with engorgement of epidural veins and decrease in lumbar CSF volume, probably explains 30% decrease in local anesthetic required for spinal anesthesia in pregnancy.
Rare instances may occur, as in this case, of a “tight” lumbar epidural space with nerve compression and ischemia with epidural medication injection.