Flashcards in Local Anesthetics Deck (71)
In general, the _______ the pKa of the LA, the ________ the proportion of LA in nonionized form at pH= 7.4, and the _______ the onset of the conduction block.
lower; greater; faster
What local anesthetic is the MOST nonionized at pH 7.4?
Mepivacaine b\c some sources say the pKa is 7.6.... some say 7.7.... which if that is the case, then lidocaine and etidocaine have the same
Why does chloroprocaine have a fast onset despite its high pKa?
d\t the high concentration of the drug injected
How do LA's block nerve conduction?
by blocking (impairing) propagation of the action potential along axons--> by directly acting on sodium channels and inhibiting sodium influx
What determines the SPEED of onset?
LA's with a lower pKa will have a faster onset
Remember is you take LSD you'll have to Pee Pee Pee.
P-----S (pKa=speed of onset)
L-----P (lipid solubility=potency)
P-----D (protein binding=duration of action)
What determines the POTENCY of LA's?
a LA with a high lipid solubility is very potent
What determines the DURATION OF ACTION of LA's?
LA's that are highly protein bound will have a prolonged DOA
The ______ the oil:water partition coefficient, the ______ the lipid solubility.
The _______ coefficient provides a measure of lipid solubility.
oil:water partition coefficient
Why do you need a higher concentration of lidocaine (ex: 2%) vs bupivacaine (ex: 0.25%)?
b\c bupivacaine has a higher oil:water partition coefficient (30 compared to 4)--> so it is more lipid soluble, therefore potent... so you need less of it to provide the effect
______ serve as storage depots for the LA's; as anesthetic diffuses, more become unbound maintaining the supply of anesthetic to nerve axons.
proteins (protein binding)---> directly correlated to the duration of action
What is responsible for the binding of weak bases?
alpha-1 acid glycoprotein
What is the second factor that determines the duration of action of local anesthetics?
*when injected, the LA with higher lipid solubility will dissolve to a greater extent into surrounding lipids--> the lipids act as a reservoir for lipid soluble agents
What single change in a property of a LA will result in a more potent and longer acting agent?
increase lipid solubility will increase DOA and potency
Name two short acting LA's.
Name two moderate acting LA's.
Name 4 long acting LA's.
Loss of an anesthetic from the injection site is primarily by ________.
What 2 factors can influence the rate of absorption of LA from injection sites?
1) presence of vasoconstrictor--> EPI may increase the duration of spinal anesthesia by 75-100% (decreases BF and slows removal)
2) high blood flow to tissue where anesthetic is injected--> the greater the blood flow, the faster the agent is absorbed and washed away from the site
Is blood concentration higher after epidural anesthesia or a subarachnoid block?
epidural--> receives higher blood flow--> higher toxicity risk
Rank the tissues from highest to lowest blood flow.
7) brachial plexus
8) subarachnoid, sciatic, femoral
*greatest to least for risk of LA toxicity
*I Think I Can Push Each Bolus SSlowly For Safety
What are 3 non-physiochemical properties that prolong conduction blocks?
1) presence of vasoconstrictor
2) concentration of LA injected (>concentration = > DOA)
3) blood flow--> lower blood flow in the tissues = > DOA
Is the ionized or nonionized form of the local anesthetic needed to block nerve conduction?
BOTH= nonionized LA diffuses into the nerve axon---> ionized form binds to receptors on the Na channel when the channel is in the inactivated state--> action potentials cannot be generated
For myelinated axons, ______ nodes of Ranvier must be blocked to stop nerve conduction.
Conduction block is _______ dependent.
*the > the frequency of action potentials, the faster the nerve is blocked by LA--> the LA must bind to the Na channel when it is in the inactivated state--> the faster the nerve is firing, the more opportunities the LA will have to catch the Na channel in the inactivated state
Voltage gated Na channels are found only in ______.
the nerve's axon
Sympathetic block is ____ to _____ dermatomes ______ than sensory block.
2-6 higher than sensory
Motor block is ______ dermatomes _______ than sensory block.
2 lower than sensory