All have at least two i's in their name. Include Lidocaine, Bupivicaine, Mepivicaine, and Ropivicaine.
Different from esters with regards to distribution. Taken up by highly perfused tissues from which redistribution takes place
Also metabolized and excreted by the kidney, and metabolized by liver microsomal esterase (diff. from esters).
The closer the pH of the tissue is to the pKa the more rapid the onset(lipid solubility and conc. of total dose also matter).
Procaine and Benzocaine(very hydrophobic and only used topically). Just like amides except with regards to distribution/elimination: it is rapidly degraded in blood by plasma cholinesterase
generally cause more allergic rxns. Really only used in 2 situations. 1. Benzocaine as a topical anesthetic(only causes local rxn so can be in otc preparations) 2. Procaine when a person's liver dz prohibits amides
Local anesthetic side effects
toxicity potential is directly proportional to lipid solubility
CNS toxicity: tonic-clonic seizures, hypoventilation, resp. arrest
Cardiac toxicity: direct myocardial depressants, can cause profound block. Tachycardia can lead to more profound block(less time in resting phase). ECG changes include bradycardia, widened QRS, V. fib
Epinephrine, sodium bicarb and local anesthetics
Epi prolongs the effect. Bc you get vasoconstriction, there is decreased systemic absorption. But avoids in areas lacking collateral flow.
bicarb hastens the onset of action by raising the pH so more of the drug is in the non-ionized, more lipid soluble state. But if pH is raised too much, drug precipitates.
pH and local anesthetics
The uncharged is more membrane-permeable, but the charged form is a better blocker. So can penetrate in uncharged form then switch to charged in axoplasm
If pH > pKa, mostly uncharged drug
If pH < pKa, mostly charged drug
Inflamed/necrotic tissue has lower pH, so diffusion of local anesthetic is slowed
Order of what local anesthetic numbs
Small diameter fibers > large diameter and myelinated fibers > unmyelinated fibers
Overall, small, myelinated fibers > small unmyelinated fibers > large myelinated fibers > large unmyelinated fibers
Order of loss = pain(lose first) > temp > touch > pressure (lose last)