Local Anesthetics Flashcards
(22 cards)
Local Anesthetic metabolism
LAs are Amides, which are metabolized in the liver where hydrolysis breaks the ester bonds. It is then excreted in the kidneys.
LA mechanism of action
Local Anesthetics work on voltage gated sodium channels. They bind to the receptors and either open the channel to allow equilibrium inside and outside the cell, or they will completely deactivate the ion channel all together. In both circumstances, they inhibit the action potentials of nerve cells.
Quaternary lidocaine is ineffective from outside the cell
Type A (alpha) nerve fibers
alpha = proprioception motor = heavy myelination
12-20 micrometers in diameter
Has a fast conduction velocity 70-120 m/s
Low sensitivity to block (1/4)
Type A (Beta) nerve fibers
Beta = touch, pressure = heavy myelination
5-12 micrometers in diameter
Has conduction velocity of 30-70 m/s
Moderate sensitivity to blockage (2/4)
Type A (gamma) nerve fibers
gamma = muscle spindles = heavy myelination
3-6 micrometers in diameter
Has conduction velocity of 15-30 m/s
Moderate sensitivity to blockage (2/4)
Type A (delta) nerve fibers
delta = pain, temperature = heavy myelination
2-5 micrometers in diameter
Has conduction velocity of 12-30 m/s
High sensitivity to blockage (3/4)
Type B nerve fibers
Preganglionic fibers = light myelination
Less than 3 micrometers in diameter
Conduction velocity of 3-15 m/s
Extreme sensitivity to blockage (4/4)
Type C dorsal root fibers
Dorsal root = pain = zero myelination
0.4-1.2 micrometers in diameter
Conduction velocity of 0.5-2.3 m/s
Extreme sensitivity to blockage (4/4)
Type C sympathetic fibers
Sympathetic = Postganglionic = zero myelination
0.3-1.3 micrometers in diameter
Conduction velocity of 0.7-2.3 m/s
Extreme sensitivity to blockage (4/4)
Order of Nerve Block Onset
Pain > Cold > Touch > Deep Pressure > Motor Function
PCTDM:
Please Capture The Damn Mouse
Factors influencing effectiveness of LA
Diameter and myelination of fiber
Firing rate and AP duration (use-dependent block)
Position of fiber in bundle
Blood flow (danger of epi)
Size and hydrophobicity (hydrophobic = fast penetration)
Liver function (metabolism)
Tissue pH (inflammation)
pH and inflammation affecting effectiveness of LA
Inflammation of tissue means lower pH in tissue.
Lower pH means more drug is charged (H+ addition).
Only uncharged drug crosses nerve membrane.
SO, LOWER pH MEANS LOWER EFFICACY OF BLOCK.
Toxicity of Amides (CNS)
CNS:
Drowsiness leading up to seizure, followed by general depression and coma.
Toxicity of Amides (Cardiac)
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
Toxicity of Amides (Peripheral Vessels)
Peripheral Vessels:
Vasodilation leading to hypotension (except cocaine - hypertension caused by block of NE reuptake)
Toxicity of Amides (Allergies)
Esters: Para-aminobenzoic acid (PABA) derivatives
Amides: Preservatives
Lidocaine Info
Most widely used Anesthetic
Intermediate duration (approximately 2 hrs for spinals and 30-60 minutes for topical or infiltration)
Higher systemic toxicity than esters
Metabolized by the liver
Can be used in spinal, epidural, local, and topical
Be careful when using with Amiodarone as it will activate sodium, NE, and many other receptors in heart. Can cause dangerous arrhythmias. (must ask if patient stopped more than 4 months ago)
Procaine (Novocaine)
Introduced in 1905
Hydrolyzed by plasma esterases into PABA
Short acting Ester (40 sec half life in plasma)
Low toxicity
Used in infiltration and regional anesthesia
Interaction: PABA blocks sulfonamide action
Tetracaine (Pontocaine)
Intermediate duration of action
High systemic toxicity
Tends to irritate mucous membrane
Used in spinal (2hrs) and topical (30-60 min) in nose and throat
Bupivacaine (Marcaine)
Widely used Amide Long duration of action (lipophilic) Reduces amount of opiates needed More cardiac effects than Lidocaine Synergistic w/ antiarrhythmics, can induce arrhythmias
LA section Duration of Block
CAPS = AMIDE lowercase = ester
Short (20-45 min): procaine, benzocaine
Medium (1-2 hrs): LIDOCAINE
Medium to Long (3-9 hrs): tetracaine, BUPIVACAINE
LA selection Desired area of Block
CAPS = AMIDE lowercase = ester
Topical: benzocaine, proparacaine
Local: LIDOCAINE, procaine
Regional: tetracaine, BUPIVACAINE
