Bviii: Describe the pharmacokinetics of drugs in epidural and subarachnoid space

Paraspinous m

Epidural fat

Ligaments

Across spinal meninges to CSF → Arachnoid matter is primary site of meningeal resistance

Distribution to CSF depends on:

Ionisation – only unionised fraction diffuses

 Dose – concentration gradient via Fick’s law

 Volume injected – greater volume allows greater surface area and will determine level of block

Baricity and Gravity – hyperbaric solutions are heavier than CSF and will distribute according to patient position

 Fick’s Law:

Rate of diffusion

Proportional

Meningeal SA

C2-C1

Inversely proportional:

Meningeal thickness

Drug MW

NB in pregnancy, decreased CSF volume 2 engorged epidural veins, therefore decreases diffusion distance