Local Anesthetics Flashcards
Importance of local anesthetics
- Interrupts transmission of autonomic, sensory and motor neural impulses
- Decreases requirement for inhaled and IV anesthesia
- Expedites return to physiologic function
- Pain control
- Spontaneous return of nerve function with rare nerve damage
Types of nerve fibers
- A
- B
- C
A fibers
o Large, myelinated fibers
o Rapid conduction of motor and sensory impulses
o Vulnerable to injury (pressure, hypoxia from tourniquet, etc.)
B fibers
o Smaller diameter
o Slower conduction velocity
o Some autonomic function
Easiest fibers to block
Think “B” for “block” the nerve
C fibers
o Small diameter, unmyelinated fibers
o Slowest conduction velocity
o Pain and autonomic impulses
Spinothalamic tract
- System composed of small diameter nerve fibers
- Responsible for sharp pain, temperature, crude touch, noxious stimuli
- Originates in peripheral skin, travels to dorsal horn on contralateral side of thalamus
Review of nerve characteristics
- A = largest, myelinated
- B = moderate size, myelinated
- C = small, unmyelinated
Types of A fibers and function
o Alpha (α): proprioception, somatic motor o Beta (β): touch, pressure o Gamma (γ): muscle tone o Delta (δ): sharp pain, temperature, touch
Function of B fibers
- B = preganglionic autonomic nervous system
Function of C fibers
- C = dull pain reflex response, temperature, touch, post-ganglionic sympathetics
Phases of nerve propagation
1: Negative resting potential (-70 mV)
o Na+ is outside nerve cell
o Na+ enters the cell until threshold is reached
2: Depolarization (+35 mV)
o When threshold is reached, Na+ rushes into cell
o Nerve signal is transmitted
3: Return of negative resting potential
o Nerve cell becomes impermeable to Na+ and K+ moves into cell
o Negative resting potential is reinstated
Local anesthesia mechanism of action
- Inhibition of depolarization of nerve membrane
- Action potential is never produced because threshold is never reaches
- Most retard Na+ influx
- Local anesthetics are weak bases, water soluble
- Unionized/unprotonated form of local anesthetic diffuses across cell membrane readily and is trapped inside due to protonation
- Acidity (as seen in infection) reduces effectiveness and action of all local anesthetics because of protonation outside the cell preventing uptake into cell
Structure of local anesthetics
o Lipophilic unsaturated benzene ring
o Hydrophilic tertiary amine
o Proton acceptor
o Hydrocarbon chain – DETERMINES CLASS
Two broad classes of local anesthetics
o Esters: hydrocarbon chain is ester (=CO=)
o Amides: Hydrocarbon chain is amide (=HNC=)
Esters
o Example: procaine (one “i”) o Less commonly used o Greater potential for allergic reaction o Half-life is 40-80 seconds o Vasoconstrictors
Amides
o Example: lidocaine (two “i”s)
o More commonly used
o Less potential for allergic reaction
o Half-life is 3.5 hours in adults, 8.1 hours in neonates
o Toxicity possible, avoid systemic exposure, take care in hepatic impairment and CHF
Epinephrine in foot and ankle surgery
- Weak evidence recommending against epinephrine in digital surgery
- Allows surgery without tourniquet
- Duration of epinephrine: 20 minutes to 1 hour
- Circulation is not completely occluded to the digits
- Comes in 1:100,000 and 1:200,000 – both work well and are safe in surgery
Side effects of epi
- 1 mg or 100 mL of 1:100,000 solution
- In heart disease, decrease this to 0.2 mg or 20 mL of 1:100,000 solution
Contraindications to epi
- Pheochromocytoma
- Hyperthyroidism
- Severe hypertension
- Severe peripheral vascular disease
- Relative contraindications: pregnancy, psychological instability
Purpose of epinephrine
- Shortens onset of anesthesia
- Prolongs effect of anesthesia
- Produces vasoconstriction (decrease bleeding, slow absorption of anesthetic)
- Larger doses of local can be used with epi
1% lidocaine plain: dose, max mg, max mL
1% lidocaine plain
o Dose: 10 mg/mL
o Max mg: 300 mg
o Max mL: 30 mL
2% lidocaine plain: dose, max mg, max mL
2% lidocaine plain
o Dose: 20 mg/mL
o Max mg: 300 mg
o Max mL: 15 mL
1% lidocaine with epi: dose, max mg, max mL
1% lidocaine with epi
o Dose: 10 mg/mL
o Max mg: 500 mg
o Max mL: 50 mL
2% lidocaine with epi: dose, max mg, max mL
2% lidocaine with epi
o Dose: 20 mg/mL
o Max mg: 500 mg
o Max mL: 25 mL
0.5% marcaine plain: dose, max mg, max mL
0.5% marcaine plain
o Dose: 5 mg/mL
o Max mg: 175 mg
o Max mL: 35 mL
0.5% marcaine with epi: dose, max mg, max mL
0.5% marcaine with epi
o Dose: 5 mg/mL
o Max mg: 225 mg
o Max mL: 45 mL
