local anesthetics

Question 1

what are local anesthetics?

Answer 1

Causing loss of sensation in a specific area of the body without the loss of consciousness.
Mechanism: Blockage of Na+ influx thereby limiting the propagation of AP.

It may come with:
• supression of somatic and visceral reflexes
• muscle paralysis.

In clinical practice used for:
• Complete loss of sensory modalities
• Analgesia

Contrast to general anesthesia:
• LAs delivered directly to the target organ
• Systemic circulation serves only for termination of the effect.

Question 2

Esters and amides as local anesthetics

Answer 2

All local anesthetics are weak bases. Chemical structure of local anesthetics have an amine group on one end connect to an aromatic ring on the other and an amine group on the right side.
The amine end is hydrophilic (soluble in water) and the aromatic part is lipophilic (soluble in lipids).

Two classes of local anethstetics are amino amides (such as lidocane) and amino esters (such as novocaine/procane).

Esters are highly allergenic and amides are not very allergenic (hypoallergenic).

Question 3

local anesthetic permeating the cell…

Answer 3

Local anesthetics must permeate the cell membrane or through the channel to block the Na channel from inside.

Most local anesthetics are Basic (Pka 8-9) and since skin has a Ph of 7 it will perameate the cell membrane (non-ionized form).

Once inside the cell, it becomes charged and blocks the channel.

Incase of inflammation, tissues become more acidic making ph drop to around 4-6–> more local anesthetia is ionized and less effective.

Question 4

epinephrine and clonidine

Answer 4

epinephrine is given with local anesthesia to produce vasoconstriction of the vessels and prolonging the effect of the anesthesia (by alpha 1 mechanism).

Incase of spinal cord anesthesia, clonidine (alpha 2 receptor agonist) and epinephrine are given to increase the anesthetic effects.

Question 5

routes of administration

Answer 5

local anesthetisa (topical).
infiltration anesthesia (perineurial infiltration, used in dentistry), regional anesthesia (nerve block), subarachnoidal (spinal) anesthesia (obstetrics and gynecology), epidual anesthesia, intravenous and more..

Question 6

Toxins that blocks the VGSCs from the extracellular side.

Answer 6

Tetrodotoxin & saxitoxin

Question 7

Susceptibility of different types of nerve fibers to LAs

Answer 7

Level of susceptibility from highest to lowest. 
Type C (pain) are very susceptible.
Type B are also equally susceptible. 
Type A-delta (pain temp)
Type A- gamma- muscle spindles.
Type A beta (Touch pressure)
Type A alpha (proprioception, motor)

Order of disappearance of sensations:

1. Pain
2. Temperature
3. Touch
4. Deep pressure
5. Motor function