Regional and Anatomy

Epidural fat causes opioid sequestration: lipid soluble opioids such as fentanyl and sufentanil have decreased net transfer from epidural to intrathecal space. Lipid solubility reduces bioavailability.

Similarly, etidocaine, which is 7x more potent than lidocaine but more lipid soluble, is effective equipotent in the epidural space. 

• CSF volume is 100-160ml in adults and produced 500ml/day. CSF has very little movement and does not distribute drugs effectively. 

• Spinal cord extends to sacrum in utero and shrinks to L3 at birth to L1 as an adult. Spinal cord gives off 31 pairs of spinal nerves that are made up of an anterior motor root and posterior sensory root. T1-L2 intermediolateral gray matter contains cell bodies of preganglionic sympathetic neurons. 
• Epidural test dose with 3ml of local anesthetic containing 1:200,000 of epinephrine is used to detect increase in HR of 30 in 30. Pts who are beta-blocked may use BP elevation of >20mmHg. Women in labor who have natural HR elevations 2/2 contractions can use isoproterenol. 

Baricity and Positioning is the ratio of the density of a local anesthetic solution divided by density of CSF. Baricity affects block height and thoracic blocks can be achieved with hyperbaric solution in supine or trendelenberg positioning. Hypobaric with sitting position is not used because it results in too high of a block. Saddle blocks can be achieved with hyperbaric solution in sitting or hypobaric in prone. Hypobaric solution given in lateral supine position is also useful as unilateral block on the upward side. Distribution by gravity becomes fixed after enough dilution in CSF (can take 60 minutes). Isobaric solutions tend to have variable block concentrated in the lower thoracic. 

Dose, volume, and concentration

In isobaric solutions: Volume and concentration have no effect. Lower total dose can produce lower blocks (at least in bupivacaine but not tetracaine).

In hyperbaric solutions: positioning is king. But lower total doses again produces lower blocks in bupivacaine but not tetracaine (2.5 dermatomes lower). 

Dosing does affect duration of block with larger doses creating longer blocks. 

Injection site

Only in isobaric solutions does this matter. Decreasing injection site by 1 vertebrae can lower height by 2 dermatomes with bupivacaine.

• CSF volume down (higher block) - pregnancy, older, kyphoscoliosis, ascites (engorgement of epidural veins). Obesity increase CSF volume!
• Adding fentanyl to spinal quickens onset and deepens sensory block
• Intrathecal fentanyl compared to morphine: less respiratory depression, less urinary retention, less n/v
• Chassaignac's tubercle is C6 transverse process, which is the landmark for stellate ganglion block. Remember stellate ganglion is C7-T1 ganglion but you can't inject directly because overly vertebral artery.
• Infraclavicular is most dangerous for pneumothorax and chylothorax
• Axillary block has 3 nerves surrounding the axillary artery. The one across from the artery is radial, and one closest is median. Remember to block musculocutaneous nerve and also intercostobrachial nerves.
• Medial knee is covered obturator nerve
• Remember peroneal nerve wraps around lateral fibula, while tibial is medial to that
• 5 nerves in ankle block, superficial peroneal, deep peroneal, sural, tibial, and femoral

Anatomy
V3 - Anterior Tongue
IX - Posterior Tongue, Soft palate, oropharynx,
Internal branch of X - Hypopharynx above VC, VC
Recurrent branch of X - VC, Larynx, Trachea

Celiac plexus at L1
Artery of Adamkiewicz T8-L1