Week 6 - Peripheral Nerve Phys/Local Anesthetics Flashcards
What are the “things” that affect local anesthetics doing their job?
- Molecular Structure
- Type of Nerve Fiber
- Activity of Nerve Fiber
- pH of Patient
- pKa of LA
- Metabolism of LA
What is the general structure of a local anesthetic molecule?
Aromatic Group (Lipophilic Portion)
Tertiary Amine (Hydrophilic Portion)
Intermediate Bond – Amide linkage OR Ester linkage
How do local anesthetics impair nerve signal transmission?
By blocking voltage gated Na+ channels
What type of compounds are more potent and produce longer blocakdes?
More hydrophobic compounds are more potent and produce longer blockade than less hydrophobic ones
What are the Amino-Ester local anesthetics?
Cocaine, Procaine, Cloroprocaine, Tetracaine
*Ester linkage connects the aromatic ring to the tertiary amine
What are the Amino-Amide local anesthetics?
Lidocaine, Lignocaine, Mepivicaine, Prilocaine, Bupivacaine, Levobupivacaine, Etidocaine, Ropivacine
*Amide linkage connects the aromatic ring to the tertiary amine
What do weak acids bind with?
Positively charged ions like Na+, Mg++, and Ca++
What do weak bases bind with?
Negatively charged ions like Cl- and Sulfate
What are important determinants of LA function?
pKa and pH
What is pKa?
the pH at which 50% is ionized and 50% is nonionized
*different for different local anesthetics
Ex: pKa of Lidocaine is 7.7-7.9 so at pH of 7.4 Lidocaine is more ionized (75% ionized) than at pH of 7.7
How does pKa affect the onset of LA block?
lower pKa means more LA in non-ionized form and faster onset of block
*LA with pKa nearest to physiologic pH (7.35-7.45) have the most rapid onset of action
As pH increases do weak bases become more or less non-ionized?
weak bases become MORE non-ionized as the pH increases
- increased pH = decreased H+ ions = increased non-ionized LA (good)
- LA must be in basic form to be unionized and capable of penetrating the cell membrane to block the Na+ channel
- LA will move across membranes faster in an alkalotic patient
Why will LAST be worse as the patient becomes acidotic?
Because the local anesthetic moved into Na+ channels and get trapped there secondary to the acidosis
How does the addition of Sodium Bicarb affect the local anesthetic?
increases pH resulting in more non-ionized form, which speeds onset
*adding too much will result in precipitation of the drug and render it useless
How does the addition of Epinephrine affect the local anesthetic?
LA premixed with Epi are more acidic (pH 3.5) to prevent breakdown of the Epi which increases the ionization of LA
How does protein binding affect local anesthetics?
Protein binding = Duration — High Protein Binding LA = Long Duration of action LA
- most LA have very high protein binding
- must release from protein to cause Na+ channel block
What is protein binding of LA dependent on and influenced by?
Protein binding is concentration dependent and influenced by plasma pH
- as pH decreases, bound drug % decreases
- very important to not let a suspected LAST patient become acidotic since the proportion of free LA molecules will increase markedly
ex: Bupivacaine 95% protein bound normally and decreases to ~70% with acidosis
How does lipid solubility affect local anesthetics?
The more lipid soluble = the more potent
- increasing the lipid solubility of any LA molecule will increase the potency of the LA and the duration of action
- increased lipid solubility also increases toxicity and decreases the therapeutic index of LA
What factor is more important for duration of action of local anesthetics?
Protein binding is more important than lipid solubility for duration of action
*the drug remains in the channel and surrounding areas for a longer time producing prolonged conduction blockade
How do local anesthetics block the Na+ channel?
- LA placed near a nerve diffuses (in non-ionized form) from the outer surface toward the core via the concentration gradient (fibers in outer mantle - proximal structures/motor fibers - are blocked first) (if enough drug present is will diffuse into the core and block the distal structures)
- Binds with cation to become ionized then binds to the Na+ channel
- Prevents opening and Na+ inrush
- Inhibits action potential
- Blocking the Na+ channel blocks conduction of the nerve impulse propagation to the brain
*No pain impulse = No pain sensation
What is the structure of the Na+ channel?
- It is a dynamic transmembrane protein
- Large sodium-conducting pore (alpha subunit) that is further divided into 4 subunits (DI-DIV)
- H is the alpha subunit that allows ion conduction and binds to local anesthetics
- Beta subunits may modulate LA binding to the alpha subunit
What are the conformational states of a Na+ channel and in which state do local anesthetics bind?
- Open (Activated): m gate opens, allowing Na+ influx, h gate begins to close
- Closed (Inactivated): h gate closed, stops Na+ influx
- Resting (Closed): h gate open and m gate closed preventing any Na+ through
- LA selectively bind to Na+ channel (“h gate”) in the closed (inactivated) state stabilizing the channel in this configuration and prevent their change to the resting/open states in response to nerve impulse – also binds to beta portion of the channel
- Inactivated state = not permeable to sodium
*binding appears to be weak and to reflect a relatively poor fit of the LA molecule with the receptor
What is the rate of diffusion of LA across the nerve sheath determined by?
- Concentration of local anesthetic (0.5%, 1%, 5%)
- Degree of ionization (non-ionized diffuses faster)
- Hydrophobicity
- Physical characteristics of tissue surrounding the nerve (more vascular = more uptake)
- Small fibers block easier than large fibers
- Myelinated fibers block easier than unmyelinated (LA pools near the axonal membrane - lipid soluble)
What is Differential Conduction Blockade of local anesthetics?
Illustrated by selective blockade of preganglionic SNS B fibers using low concentrations of LA — slightly higher concentrations of LA interrupt conduction in small C fibers and small/medium sized A fibers (loss of pain sensation and temp)
*Touch, proprioception, and motor function are still present such that the pt will sense pressure but not pain with surgical stimulation
How do LA change during pregnancy?
Increased sensitivity (more rapid onset)
*Alterations in protein binding characteristics of bupivacaine may result in increased concentrations of active unbound drug in pregnancy
_____ local anesthetics block faster than ____ & _____ at the same concentration.
Moderately hydrophobic LA block faster than hydrophilic & highly hydrophobic LA at the same concentration
- Mod hydrophobic less bound to tissue, more membrane permeable
- Highly hydrophobic have increased potency (use lower concentrations)
Describe type A-alpha nerve fibers
Function: proprioception and motor
Location: efferent to muscle
Heavy myelination
Very Fast conduction (70-120m/s)
Fiber size = 12-20 um
Sensitivity to Block = +
Describe type A-beta nerve fibers
Function: touch, pressure
Location: afferent from skin/joint
Heavy myelination
Very fast conduction (30-70 m/s)
Fiber size = 5-12 um
Sensitivity to Block = ++
Describe type A-gamma nerve fibers
Function: muscle tone (muscle spindles)
Location: efferent to muscle
Heavy myelination
Fast conduction (15-30 m/s)
Fiber size = 3-6 um
Sensitivity to Block = ++
Describe type A-delta nerve fibers
Function: pain, temp, touch
Location: afferent sensory
Heavy myelination
Fast conduction (5-25 m/s)
Fiber size = 2-5 um
Sensitivity to Block = +++
Describe type B nerve fibers
Function: preganglionic autonomic
Location: preganglionic SNS
Light myelination
Medium conduction (3-15 m/s)
Fiber size = <3 um
Sensitivity to Block = ++++
Describe type C nerve fibers
Function: dorsal root (dC)= pain – sympathetic (sC) = postganglionic autonomic
Location: dC = afferent sensory – sC = postganglionic SNS
No myelination
Slow conduction (dC 0.5-2.3 m/s – sC 0.7-2.3 m/s)
Fiber size = 0.3-1.3 um
Sensitivity to Block = ++++
What is the minimum concentration (Cm) of LA and what affects it?
The minimum concentration of LA necessary to produce conduction blockade of nerve impulses
Affected by:
- Nerve fiber diameter (larger fibers require higher concentrations of LA) – Cm of motor = 2x Cm of sensory
- pH (increased pH decreases Cm)
Why must a minimal length of myelinated nerve fiber be exposed to an adequate concentration of LA for conduction blockade to occur?
Because if only one node of Ranvier is blocked, the nerve impulse can jump (skip) across this node and conduction blockade doesn’t occur
- For blockade of A fiber to occur, it is necessary to expose at least 2 and preferably 3 successive nodes of Ranvier to LA
- Both types of pain fibers (A-delta/C fibers) are blocked by similar concentrations of LA
- Preganglionic B fibers are more readily blocked by LA than any fiber