Exam 1 Flashcards
Benefits of PNB for regional anesthesia
Reduced narcotics
Decreased length of stay
Improved satisfaction scores
Decreased turnover times
Indications for PNB
- mod to severe pain in 1st 24 hours post op
- do not want to put to sleep for GETA (diff intubation, cardiac instability)
- analgesia postop
Contraindications for regional anesthesia
- pt refusal
- preexisting neurologic injury (esp. if acute injury)
- coagulopathies (esp deep plexus blocks and neuraxial blocks)
- infection at site of injection
Covering around each fascicle
Perineurium
Covering around each axon
Endoneurium
Covers the entire nerve
Epineurium
Terminal point where LA will work to block motor and/or sensory
Axon
Where want to inject LA
External epinurium
What is focused more on internal portion of a nerve
Blood vessels and motor components
Where do we want do inject the block
External epinurium
- want to fill the space between connective tissue sheath and external epinurium
LA targets Nodes of Ranvier. Why?
Highest amount of voltage gated Na channels
3 methods to prevent nerve injury with PNB
Ultrasound
Pressure monitor
Nerve stimulator
What are we trying to prevent by using US, nerve stimulator, and pressure monitor
Neural injury
Neurotoxicity
LAST (local anesthetic systemic toxicity)
Where do you place the red and black lead on nerve stimulator for PNB
Red - on skin close to where putting block
Black- block needle
Which is negative/positive red or black
Red positive (cathode)
Black negative (anode)
If reverse red and black lead on nerve stimulator what can occur
Hyperpolarization and need more input to stimulate nerve
Initial settings for nerve stimulator
<2 Ma output
1-2 Hz. Frequency of stimulation
0.1ms pulse duration
Most effective settings to elicit and action potential in a motor fiber
Low amplitude and short pulse duration
Law relative to proximity of needle tip to nerve is directly correlated with voltage required to stimulate
Coulombs’s Law
3 parts of Coulombs’s Law
- minimum current required to stimulate nerve is directly proportional to square of distance from the nerve
- low intensity current- nerve will only get stimulated when needle tip is very close to it
- current required to initiate nerve response determines distance of nerve to needle
< 0.2 Ma stimulus produces a response. What does this mean
Intraneural
High specificity, low sensitivity
0.2-0.4 Ma stimulus elicits a motor response. What does this mean?
Ideal block position. Inside neural sheath
0.4 Ma stimulus elicits motor response. What does this mean?
Outside neural sheath. Poor block success
For superficial blocks starting voltage should be?
Deep blocks
1 mA for superficial
1.5 mA for deep
When needling if you lose a twitch what should you do
Advance or reposition needle until twitch in 0.2-0.4Ma range
When needling if twitch continues at voltage <0.2Ma what should you do?
Reposition needle. You are too close to the nerve/in the nerve.
Withdraw until twitch occurs in 0.2-0.4Ma range
Coating/insulating needles results in
Improved voltage and the distance to nerve interrelationship
More focused/accurate
Ideal block needle should be
Echogenic
Insulated to the tip
More difficult to get into fascicle but cause more damage when do
Short bevel needle
Easier to enter fascicle but it do cause less damage
Long bevel needle
What are the initial settings when using a nerve stimulator
0.1ms. 1-2 Hz. ,2Ma
What complications might occur if stimulator set at 0.5 Hz
Or 4 Hz
0.5 Hz twitch too slow so could pass the nerve and not realize it
4 Hz. Too fast, the muscle could fatigue before get near it
Goal of pressure monitor to prevent intraneural injection
<15psi
Sudden high to low pressure drop with pressure monitor in PNB signifies what
Injection is intrafascicular
If psi > 15 on pressure monitor with PNB this means
Could be in bone, tendon, clotted needle, or nerve
Pressure monitor r/t specificity and sensitivity
Low specificity but high sensitivity
In regards to ultrasound 1 cycle/sec means
1 Hertz (Hz)
Ultrasound frequency
5-20 MHz
Transducer basic function
Converts electrical energy into acoustic sound waves and acoustic is what gets measured
US- better the sound gets reflected means
Lighter the image
Strength of the reflection of the sound wave is directly related to
The density of the tissue or the regularity of the surface being penetrated
How to determine if linear or curved probe based on image
Linear has a square boundary (square)
Curved has outer boundary (looks like a pie)
Short axis view is the preferred view because
Provides the thickest view of the nerve
Hyperechoic image
Bight gray
Denser image
High reflection of US waves
What appears hyperchoic on US
Fascial planes
Tendons
Bone edges
Nerves in periphery
Hypoechoic on US means
More waves are absorbed
Less dense the tissue is
Appears as mix of dark and lighter grays
What appears hypoechoic on US
Muscle
Anechoic on US means
No waves reflected
Appear completely blackened
What appears anechoic on US
Blood vessels and nerves above the clavicle
Why are peripheral nerves more hyperechoic in the periphery
More connective tissue and less axonal density
Blue flow on color Doppler indicates
Blood flow moving away from transducer
Red blood flow on color Doppler means
Blood flow moving toward transducer
4 causes of air artifact on US
Poor probe contact
Not enough pressure on probe
More conducting gel needed
Drop-out effect
Acoustic reverberations are caused by
Very solid objects
Lower frequency on US results in
Low resolution
Longer depth
Lower frequency means
Longer wavelength
Greater penetration
High frequency results in
High resolution
Shorter depth
Higher frequency means
Shorter wavelength
Better axial resolution
Linear probe
8-13 MHZ results in
Which blocks used for
Higher resolution, less depth
Good for interscalene, supraclavicular, femoral, ankle
Linear probe at 6-10 MHz
Results in
Blocks used for
Poorer resolution
Good for deeper blocks
Infraclavivular, subgluteal, TAP blocks
Curved probe 2-5 MHz
Used for
Very deep blocks
Transgluteal sciatic, lumbar plexus
Adjusting gain on US results in
Increase gain makes brighter
Decreasing gain makes darker
Grays in regards to US
Greys are good
5 ways to improve US image
Pressure Alignment Rotation Rock Toggle or Tilt
Goal of angle of intonation
Want angle parallel to nerve
What do you move to locate the needle when performing PNB
Always find the needle by moving the probe
Never move the needle without locating the tip
3 methods to locate needle with US
Spinning the needle
Hydrolocating with D5W or NS
Rocking the probe
LA work by
Blocking voltage gated sodium channels
What fibers blocked easier
Myelinated
Small
Motor fiber blockade
Blocked last but recover the quickest
Block onset and recovery order
Onset:
B-Adelta-Agamma-Abeta-Aalpha-C
Recovery is reverse
1st to disappear with LA blockage
Type A delta
Most resistant to LA
Type C
5 factors affecting duration, onset, and binding properties of LA
Lipid solubility Concentration Proximity to nerve Char of surrounding tissue (vascular) PH
Form of LA that crosses lipid bilayer
Non-ionized (unprotonated)
Effect of adding NaHCO3 to LA
Speeds onset bc increases amount of LA in non-ionized (unprotonated)form
LA primarily bind to which type of channels
Inactivated voltage gated Na channels
Senses stretch and speed of stretch
Muscle spindle
Senses tension
Golgi tendon organ
% of drug bound ______ as pH of plasma decreases
Means
%bound decreases as pH decreases
Amount of free drug increases
