Local anesthetics Flashcards
How can you tell the difference between Esters and Amides
Esters: only 1 “I”
Amides: two “I”’s
What are the esters
Short: Procaine
Long: Tetracaine
Surface action: Benzocaine, Cocaine
What are the amides
Medium: Lidocaine, Mepivacaine
Long: Bupivacaine, Ropivacaine
What local anesthetics have the highest potency
Tetracaine: 16
Bupivacaine, Ropivacaine: 16
What is the onset time of certain local anesthetics
Lidocaine, Prilocaine: <2 Procaine: 2-5 Mepivacaine: 3-5 Ropivacaine: 10-30 Tetracaine: <15
How do local anesthetics work (generally)
-Sensory transmission from a local area of the body to the CNS is blocked= loss of sensation
What is the chemical MOA of local anesthetics
Block voltage dependent sodium channels
reduce influx of sodium ions
prevent depolarization of membrane
block conduction of action potential
What are the roles of non-ionized and ionized drugs
Non-ion: help reach the receptor site
Ionized: cause the effect
Onset of local anesthetic action can be accelerated by
sodium bicarbonate, to enhance IC access of weakly basic compounds
What can help prolong the duration of the local anesthetic
Epinephrine (alpha agonist), a vasoconstrictor
Less blood flow= drug stays around longer
*Longer acting agents usually don’t need epi
How are Esters metabolized
Metabolized by plasma cholinesterases AKA very rapid
How are Amides metabolized
by the liver
higher risk of toxicity with liver dysfunction
What fibers are blocked by local anesthetics
Small fibers: more easily blocked than large
Myelinated fibers: more easily blocked than unmyelinated
Fibers in periphery: blocked quicker than fibers in the core of a thick nerve bundle
What are the types of pain fibers
A: Alpha, Beta, Gamma, Delta (heavy myelination)
B: preganglionic autonomic (light myelination)
C: dorsal root (pain) (unmyelinated)
What do Type A fibers do
Alpha: proprioception, motor
Beta: touch, pressure
Gamma: muscle spindles
Delta: pain, temp
The difference in type A and C nerves is
A: sharp pain
C: dull pain
Local anesthetics can block
all nerve types!
actions are not limited to the desired loss of sensation from sites of noxious stimuli
Central neural techniques (epidural) cause
impaired respiratory activity if motor paralysis is blocked
Hypotension if autonomic nerves are blocked
When is motor paralysis desired
during surgery!
How can motor paralysis lead to undesirable effects
Epidural during labor: may limit ability to “bear down” and push baby out
Post-op analgesia: mess with ability to ambulate without assistance, interfere with bladder function (urine retention)
What is the order of components blocked
Sympathetic transmission Temperature Pain Light touch Motor block (surgical anesthesia may require loss of touch, not just ablation of pain)
Added vasoconstrictors (epi) can also help
- extend duration of post-op pain control, and lower need for total anesthetic
- Decrease risk of toxic effects by lowering peak blood levels of anesthetics
When incorporated into spinal anesthetics, Epi
- contributes to prolongation of local anesthetic effect 2/2 vasoconstrictor properties
- Exerts direct analgesic effect mediated by post-synaptic a2 adrenoreceptors in spinal cord
What are the types of spinal anesthetics
Epidural: injected extradurally
Caudal block: type of epidural w/ injection into caudal canal
Perineural blocks: injection around peripheral nerves
Spinal block: injection into CSF
FDA banned the use of this in labor
0.75% Bupivacaine, 2/2 cardiotoxicity
can be reversed w/ Lipid resuscitation
Clinical uses of local anesthetics include
minor surgical procedures
spinal anesthesia
autonomic blockade in ischemic conditions
slow epidural infusion at low concentrations (post-op anesthesia)
IV to reduce pain perioperatively
Oral/parenteral in neuropathic pain
On a bottle, Epinephrine is…
RED!!
Toxicities of Amides include
CNS excitation, seizures Vasodilation, hypotension, heart block arrhythmias (bupivacaine) light headed, restless, nystagmus *Amides rarely used IV, mostly topical*
Toxicities of Esters include
Cocaine vasoconstricts
cocaine abuse= HTN, seizure, cardiac arrhythmia
How is cocaine different than other esters
it has intrinsic sympathomimetic actions!
What is the avg half life of lidocaine
1.6 hours in normal patients
6+ hours in those with severe liver disease
Longer in those with reduced hepatic blood flow, or if anesthetized with volatile agents that reduce blood flow
Also longer in those with CHF (impaired hepatic blood flow)
If you cant take an ester or amide, you may use
Benadryl!!
How do you treat toxicity
No antidotes; can manage convulsions with Diazepam*
What is EMLA cream
(Eutectic Micture of Local Anesthetics)
2.5% Lidocaine and 2.5% Prilocaine
Allows penetration of keratinized skin= local numbness
Can be used in peds prior to venipuncture (IV or cath placement)
