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AS - N929 Pharmacology II > Local Anesthetics > Flashcards

Local Anesthetics Flashcards

1
Q

LAs MOA

A

Reversibly block conduction/transmission along nerve fibers
Preferentially bind to open & inactive VGNa+ channels
- Binds internally to VGNa+
Also block K+, Ca2+, & GPCRs
Do not alter the resting transmembrane or threshold potential

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2
Q

What surrounds each nerve axon?

A

Schwann cells

Support & insulate each axon

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3
Q

What are the Nodes of Ranvier?

A

Periodic segments b/w Schwann cells along the axon that do not contain myelin
High VGNa+ channels concentration here

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4
Q

Saltatory Conduction

A

APs jump from node to node to increase the transmission speed

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5
Q

How many nodes must be block for LAs to inhibit channels in myelinated fibers?

A

3 successive nodes

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6
Q

Axon bundles are called ______

A

Fasciculi

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7
Q

Fasciculi are covered with 3 connective tissue layers

A
  1. Endoneurium - thin, delicate collagen that embeds the axon in the fascicle (innermost layer)
  2. Perineurium - flattened cells layers that bind fascicle groups together (middle layer)
  3. Epineurium - surrounds the perineurium; composed from connective tissue that holds the fascicles together to form a peripheral nerve
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8
Q

What must occur for LAs to exert effects?

A

Diffuse through all 3 connective tissue layers (endoneurium, perineurium, & epineurium)

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9
Q

What restores the RMP?

A

Na+/K+ pump

3 Na+ ions exit
2 K+ ions enter

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10
Q

Which nerve fibers are most difficult to block?

A

Larger nerves - conduct impulses faster & are harder to block

Smaller nerve lacking myelin enhance the sensitivity to LAs
LAs preferentially bind to smaller & unmyelinated fibers

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11
Q

What indicates a successful LA blockade?

A

Vasodilation ↓BP indicates pre-ganglionic fibers (SNS tone)

Type B light myelination 1st blocked

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12
Q

Nerve Fiber Blockade Onset

A

Type B pre-ganglionic (autonomic vasomotor) →
Type C fibers (sympathetic post-ganglionic vasomotor & dorsal root - pain, warm/cold, & touch) →
Type A β (touch & pressure) λ (muscle tone) Δ (pain, cold, & touch) →
Type A α (proprioception & motor)

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13
Q

LA Chemical Structure

A

Ester -COOCH or amide -N binds the aromatic ring to the Carbon group

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14
Q

Ester LAs

A

Shorter acting LA (Tetracaine longest-acting ester < 90min)
Plasma & tissue cholinesterase metabolism via hydrolysis
Para aminobenzoic acid (PABA)

LA allergies uncommon, but higher potential w/ esters
Ester allergy avoid ALL ester LAs d/t cross-reactivity

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15
Q

Amide LAs

A

Longer acting (more lipophilic & protein bound)
Require transport to the liver for metabolism
Hepatic CYP1A2 & CYP3A4

Amide allergies are extremely rare
No cross-allergy b/w ester & amide LAs

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16
Q

What increases LA onset?

A

Higher concentration injected → faster onset

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17
Q

What terminates LA effects?

A

Systemic absorption results in drug termination

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18
Q

What does lipid solubility correlate with?

A
  • Protein binding
  • ↑potency
  • Longer DOA
  • ↑severe cardiac toxicity

Amides are more lipid soluble

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19
Q

LAs are _____ _____

A

Weak bases

Basic drugs become more ionized when placed in a solution w/ pH < pKa
Drugs w/ pKa closer to physiologic pH → faster onset
Weak bases bind to α acid glycoprotein (lesser extent to albumin)

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20
Q

LA impact on smooth muscle:

A

RELAXATION
Vasodilation ↓DOA ↑plasma concentration & potential toxicity

Exceptions: Lidocaine, ropivacaine, & cocaine

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21
Q

Highest → lowest blood concentrations

A

IV → tracheal → caudal → paracervical → epidural → brachial → sciatic → SQ

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22
Q

Drugs to add to LAs spinal/epidural:

A
  • Clonidine
  • Dexmedetomidine
  • Epinephrine
  • Opioids
  • Na+ bicarb
  • Ketorolac
  • Dexamethasone
  • Hyaluronidase
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23
Q

Epinephrine

A

Vasoconstriction ↓vascular absorption rate
↑duration & block potency
↓systemic toxicity risk

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24
Q

Sodium Bicarbonate

A

Commonly used in epidural anesthesia
Theoretically ↑LA solution pH → more drug in the non-ionized state
↓pain on injection
Limitation = precipitation

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25
What receives the highest LA plasma concentration initially?
Highly perfused tissues - brain, heart, & lungs | Risk → toxic levels
26
What receives the most secondary LA after re-distribution?
Muscles receive the most after 2° distribution
27
What LAs are impacted with severe hepatic disease?
Amide LAs | Hepatic metabolism CYP450
28
How does renal dysfunction affect LA clearance?
Impacts protein binding to α1 glycoprotein & albumin | Renal dysfunction affects clearance far less than hepatic failure
29
Pregnancy Physiological Changes
Engorged epidural veins ↓epidural space | Hormonal changes → progesterone levels affect LA sensitivity
30
Local Anesthetics | SEs & Complications
LAST Cauda equina syndrome Transient neurologic symptoms
31
LAST
Local anesthetic systemic toxicity Rare, but serious *ALWAYS ASPIRATE*
32
What most commonly causes LAST?
Inadvertent IV injection - Blocks inhibitory neurons thought to cause seizures - Blocks cardiac ion channels → bradycardia
33
What the most serious LAST complication?
Ventricular fibrillation
34
What LAs are thought to be less cardiotoxic?
Shorter acting LAs | - More potent agents higher lipid solubility & protein binding
35
LAST Clinical Presentation
Rapid onset usually w/in one minute S/S: Tinnitus, metallic taste, agitation, blurred vision, & circumoral numbness Muscle twitching, unconsciousness, & seizures TOXIC LEVELS → cardiac & respiratory arrest
36
LAST Incident Rate
0.4 per 10,000
37
LAST most common following what procedures?
1. Epidural - veins & dura 2. Interscalene - carotid artery & IJ vein 3. Axillary less common
38
How to prevent LAST
Test dose to ensure in the right spot NOT in the vein or dura Incremental injection w/ aspiration Use pharmacologic markers Ultrasound
39
LAST Treatment
Prompt recognition & diagnosis 1° airway management - adequate ventilation & oxygenation Seizure suppression - benzodiazepines & Succinylcholine Hyperventilation ↑seizure threshold Prevent hypoxia & acidosis Lipid emulsion 20% 1.5mL/kg rapid infusion over 2-3min → 0.25mL/kg/min IBW NOT propofol Vasopressors - Epi < 1mg/kg NOT vasopressin
40
Lipid Emulsion Therapy MOA
1. Captures LA in blood → lipid sink 2. ↑fatty acid uptake by mitochondria 3. Interferes w/ Na+ channel binding 4. Promotes Ca2+ entry 5. Accelerated shunting
41
Methemoglobinemia
Ferris (Fe2+) Hgb → Ferric (Fe3+) Hgb ↓oxygen-carrying capacity → tissue hypoxia ↓SpO2 not responsive to therapy Benzocaine-induced methemoglobinemia
42
Methemoglobinemia Treatment
Methylene blue 1-2 mg/kg over 3-10 minutes High levels may require transfusion or dialysis
43
Cauda Equina Syndrome
Cause - LA or nerve compression | Manifests as bowel & bladder dysfunction w/ LE weakness & sensory impairment r/t cord ischemia
44
Transient Neurologic Symptoms
Associated w/ intrathecal lidocaine Presentation - burning, aching, cramping pain in the lower back that radiates down the thighs up to 5 days postop Risk factors include lithotomy position & outpatient surgery
45
Lidocaine HCl (Xylocaine)
Weak base amide LA Rapid onset Protein binding 64-70% ↓DOA
46
Lidocaine Uses
- Antiarrhythmic - Topical - Induction to blunt SNS - Nebulized - Multimodal pain management - Regional anesthetic
47
Lidocaine | Antiarrhythmic
``` ACLS algorithm Class IB Depress myocardial automaticity Vtach or Vfib 1-1.5 mg/kg IV Refractory 0.5-0.75 mg/kg Total dose 3 mg/kg Maintenance infusion 1-4 mg/min (30-50 mcg/kg/min) ```
48
Lidocaine | Topical
Eutectic mixture of LA (EMLA) Lidocaine:Prilocaine 1:1 mixture Contraindicated use on mucous membranes, broken skin, infants < 1mos old, methemoglobinemia history
49
Lidocaine | Induction
1-1.5 mg/kg IBW ↓pain associated w/ Propofol Admin 1-3 min prior to laryngoscopy attenuate HTN Blunt CV sympathetic response to intubation ↓CBF → attenuates ↑ICP in patients w/ ↓compliance Block reflex bronchoconstriction
50
What causes the pain associated with Propofol on injection?
Phenol
51
How best to prevent the pain associated with Propofol?
Large bore IV AC vein Tourniquet 60 second veno occlusion Opioid dose
52
Lidocaine LTA
Laryngotracheal topicalization anesthesia Decrease emergence phenomenon - coughing, sore throat, & dysphonia Lidocaine 4% pre-filled 4mL syringe (remove excess → easier to inject exact dose) DL then insert LTA holding the catheter at the end near the syringe Place LTA w/ black marking at the vocal cord level then inject
53
Lidocaine | Airway Block
Nebulized lidocaine 4% direct to the oropharynx Swish & spit or swallow lidocaine Tongue depressor w/ lidocaine gel Spray as you go Transtracheal block inject 4% through the cricothyroid membrane
54
Lidocaine | Multimodal
Pain management Infusion 2 mg/kg/hr ↓narcotic requirements MOA unknown Reduces postop pain w/ open & laparoscopic GI surgeries ↓pain associated w/ improved functional outcomes in prostatectomy, thoracic, & spine procedures
55
Lidocaine | Regional
Peripheral nerve blocks - Bier block Neuraxial anesthesia - spinal & epidural
56
Bier Block
Short procedures Lidocaine 0.5% 25-50mL Onset 5-10 minutes Tourniquet pain at 20 minutes
57
Lidocaine Dose-Dependent Effects | Plasma Concentration 1-5 μ/mL
Analgesia
58
Lidocaine Dose-Dependent Effects | Plasma Concentration 5-10 μ/mL
Tinnitus, circumoral numbness, skeletal muscle twitching, systemic HoTN, & myocardial depression
59
Lidocaine Dose-Dependent Effects | Plasma Concentration 10-15 μ/mL
Seizures | Unconsciousness
60
Lidocaine Dose-Dependent Effects | Plasma Concentration 15-25 μ/mL
Apnea & coma
61
Lidocaine Dose-Dependent Effects | Plasma Concentration > 25 μ/mL
Cardiovascular depression
62
Exparel
Liposomal bupivacaine *Only mix w/ NS 0.9% or bupivacaine Lidocaine breaks down the capsule → bupivacaine bolus ↑toxicity risk Caution in patients w/ hepatic disease
63
Exparel SEs
N/V | Dizziness, headache, bradycardia, tachycardia, somnolence, hypoesthesia, & lethargy
64
Cocaine
Original LA derived from the cocoa plant Only naturally occurring Blocks the monoamine transporter in the adrenergic system Unable to reuptake catecholamines → vasoconstriction 1° topical nose & throat Max dose 5mL 5% Caution w/ other epi containing solutions, MAOIs, & tricyclics

AS - N929 Pharmacology II (10 decks)

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