Local Anesthetics Flashcards
Amide
ie Lidocaine
binds to plasma protein
has n-alkylation
ester
ie Tetracaine
is hydrolyzed by plasma esterases
Local anesthetic mech of action
block Na channels from cytoplasmic mouth
stabalize inactivated state of chanel
Factors that influence effectiveness of Local Anesthetic
- Diameter of nerve
- Firing rate, action potential duration
- position of fiber in nerve bundle
- local blood flow (epinephrine can help or hurt)
- drug size and hydrophobicity
- liver function (for amides)
- tissue pH (inflammation reduces effectiveness)
order of Block
- pain
- cold
- touch
- deep pressure
- motor function
LA toxicity to CNS
initially drowsiness and excitation (up to seizures), followed by general depression, coma
LA toxicity to cardiac (especially bupivacaine)
- depressed pacemaker activity
- decreased conduction
- arrhythmias
- cardiac toxicity is potentiated by hyperkalemia
- amides especially can synergize with anti-arrhythmic drugs to induce arrhythmias
LA toxicity to peripheral blood vessels
vasodilation leading to hypotension (except cocaine-hypertension caused by block of NE reuptake)
LA Allergies
Esters- PABA derivatives
amides- preservatives
Lidocaine duration as LA
2 hour spinal
30-60 min for topical or infiltration
Lidocaine metabolism
- Metabolized by liver
- higher systemic toxicity than ester-linked drugs
- careful with concurrent use of amiodarone
Procaine
introduced in 1905
short acting ester (40 sec plasma half life)
low toxicity
mainly used for infiltration and regional anesthesia
interaction: PABA blocks sulfonamide action
Tetracaine
intermediate duration ester ( slowly hydrolyzed, 2 hour spinal, 30-60 min for topical)
higher systemic toxicity than other esters
causes mucous membrane irritation, urticaria, burning
main uses- spinal and topical anesthesia of nose and throat for diagnostic procedures
Bupivacaine
- long duration amide (3-9 hrs regional, plasma 1/2 life 3.5 hr adult, >8 for neonate)
- concurrent epidural use with opiates in labor can reduce amt of opiate needed
- more cardia effects than lidocaine, dissociates slower from cardiac Na channels
- synergistic with anti-arrhythmics on heart- can induce arrythmias.
LA for short (20-45min) block
procaine (ester), benzocaine (ester)
