Local Anesthesia Flashcards
LA MOA
impair nerve signal transmission by blocking VG Na channels
Amino-esters
- metabolized by pseudocholinesterase except cocaine
- ester linkage connects aromatic ring to tertiary amine
- cocaine, procaine, chloroprocaine, tetracaine
Amino-amides
- metabolized by liver
- amide linkage connects aromatic ring to tertiary amine
- lidocaine, lignocaine, mepivacaine, prilocaine, bupivacaine, levobupivacaine, ropivacaine
structure of LA
- aromatic ring is lipophilic
- tertiary amine is hydrophilic
- more hydrophobic compounds are more portent and produce longer blockade
procaine
Duration: 45-60 min
pka 8.9
nonionized (7.2) 2; (7.6) 5
-low potency, slow onset, short duration
chlorprocaine
Duration: 30-45 min
pka 8.7
nonionized (7.2) 3; (7.6) 7
short duration, fast onset
Tetracaine
Duration: 60-180 min pka 8.5 nonionized (7.2) 5; (7.6) 11 more potent, long duration, slowly metabolized too toxic for use in peripheral blocks OK for long acting spinal blocks
lidocaine
Duration: 60-120 min
pka 7.9
nonionized (7.2) 17; (7.6) 33
rapid absorption parenteral, GI, respiratory
-intermediate duration
-1.5-2% for most regional blocks for surgery
-more dilute concentrations for pain management (no motor block)
mepivacaine
Duration: 90-180 min pka 7.6 nonionized (7.2) 28; (7.6) 50 -intermediate duration -similar to lidocaine -similar onset, longer duration than lidocaine -toxic to neonates
bupivicaine
Duration: 240-480 min
pka 8.1
nonionized (7.2) 11; (7.6) 24
-longer duration LA (longer with EPI)
-slow onset, longer/variable duration
-2-3 hr for spinal dose, 12-24 for peripheral nerve block
-cardiotoxic (direct injection into medulla will producce arrhythmias, difficult to dissociate bupivacaine from na+ channels)
-periph in dilute concen (0.5)
cont infusion usually 0.1% w or w/o opioid
-careful use in OB epidurals (0.25)
levobupivicaine
Duration: 240-480 min pka 8.1 nonionized (7.2) 11; (7.6) 24 -single enantiomer version of bupiv -less well studied -appears to work in similar fashion to ropivicaine, less toxic than bupivacaine, similar onset and duration as bupivacaine
prilocaine
Duration: 60-120 min pka 7.9 nonionized (7.2) 17; (7.6) 33 -intermediate duration -pharmacology similar to lidocaine -lacks vasodilation, increased Vd, limits CNS toxicity -causes methemogloinemia (req dose 8 mg/kg, rx with methylene blue 1-2 mg/kg) -uncommon for peripheral nerve blocks
ropivacaine
Duration: 180-360 min
pka 8.1
nonionized (7.2) 11; (7.6) 25
S–enantimor of bupivicaine: lower cardiac toxicity
-slower uptake and thus lower blood levels
-extensive hepatic metabolism
-less potent than bupivacaine at lower concentations (0.5%)
- at > 0.5% dense block with slightly shorter DOA than bupiv
at 0.75% onset is fast, CNS and cardiotoxicity are reduced, motor block is less than bupiv
-very popular for peripheral blocks
What partially determines DOA of LA
-pro binding fraction
bupivicane > etidocaine > ropivacaine > mepivacaine > lidocaine > procaine > 2 chloroprocaine
What other tertiary amines block Na channels
meperidine, morphine, codeine, heroin, TCA, amitryptyline, pufferfish toxin (blocks from outside)
Rate of diffusion across nerve sheath is determined by:
- concentration of LA
- degree of ionization
- hydrophobicity
- physical characteristics of tissue surrounding the nerve
Why are motor fibers blocked before sensory in large trunks
-due to location on outer portion of the nerve bundle
increased lipid solubility….
- increases potency and duration of LA
- increases toxicity and decreases TI
duration of a block is determined by:
- lipid solubility
- vascularity of the tissue
- presence of vsoconstrictors which prevent vascular uptake
order of loss of nerve sensations:
autonomic functions pain cold warmth touch pressure vibration proprioception motor function
cocaine
- blocks nerve impulses
- ester LA
- local vasoconstriction d/t inhib of local norepinephrine reuptake
- euphoria 2/2 blockade of dopamine reuptake in CNS
- good for LA in nasal passages
allergies to LA
-rare, but sometimes happens with esters or additives like PABA
etidocaine
- longer duration
- onset like lidocaine, duration like bupivicaine
- alkyl substitution on aliphatic group between hydrophilic amine and amide linkage increases lipid solubility with increases potency and increases duration
- prolonged motor blocks that outlasts sensory block
- not useful for peripheral nerve blocks
plasma concentration of LA is determined by:
- dose of drug administered
- rate of absorption of drug (systemic: interscalene, humeral, paravertebral, psoas, intercostal, femoral, sciatic more than superficial and intracutaneous)
- site of injection
- biotransformation and elimination of the drug
symptoms of LAST
drowsy–>parasthesias of mouth/tongue>tinnitis/auditory hallucinations > seizures > coma > resp arrest > cardiac arrest
rx of LAST
1) airway mangement/seizure precautions
2) BLS/ACLS (avoid lidocaine, Ca+ blockers, vasopressin, beta blockers, reduce epi doses to
