Regional Lecture I part II Flashcards
mechanisms of nerve injury?
mechanical
stretch
pressure/compression
chemical
vascular (prolonged disruption of BF to nerve)
are LA drugs neurotoxic?
yes they all are to some degree. Cause histological changes, but usually not clinically significant
how does lidocaine affect blood flow?
inhibits neural blood flow
have neural injury rates changed with introduction of ultrasound?
no
are nerve injuries primarily from intraneural injections?
no, we used to think so though.
does intrafascular injection equal nerve injury?
yes
is it okay to inject into a nerve?
No, it might not cause injury (unless intrafasicular) but we still want to avoid this.
where do axons run?
in the fassicules
what is epinerium?
protective connective tissue forming the outer covering of nerves, as well as inner supportive tissue
how would an intrafasicular injection feel to the anesthesia provider?
there is higher injection pressure
why do risks of nerve injury decrease distally?
there are only a few large fascicles bound by a sheath proximally. these are easy to needle
distally there are many small fascicles without a sheath so needles have a hard time entering the fasicle.
two kinds of vascular nerve injuries?
intrinsic within epineurium and extrinsic around nerve
key factors in post op neuropathy mgmt?
communication (ensure pt you are on it and don’t blow it off)
surgeon - possible procedural component
neurology - involve them earlier than later (can do electrophysiological testing)
when should you immediately involve neurology for post op neuropathy?
motor involvement in nerve injury
how long should you follow patient after post op nerve injuyr?
until symptoms resolve or stabilize
how quickly do sensory symptoms resolve in post-op neuropathy?
95% in 4-6 weeks
99% in 1 year
Techniques for finding nerves?
landmarks
paresthesia
nerve stimulator
ultrasound
fluoroscopy
CT guided
what kind of images to fluroscopy and CT provide?
still and live images
when is fluroscopy for PNB used?
mostly in pain blocks and is expensive
when is CT used to guide PNBs?
rarely in pain blocks, is extremely expensive
What is the paresthesia technique and how is it done?
old technique that creates a feeling of tingling, tickling, burning, prickly or buzzing.
goal is to place needle in direct contact with the nerve to produce a paresthesia, then slightly withdraw the needle and inject.
this has a risk of neural injury and higher block failure rates
should you inject LA if the pat has sharp pain or paresthesia?
NO!!!
how does nerve stimulator technique affect motor or sensory nerves?
motor nerve - muscle twitch
sensory nerve - paresthesia over target nerve distribution
what kind of impulse does nerve stimulator use?
negative polarity impulse
this neutralizes the positive current outside the nerve dropping the membrane potential
what does black lead attach to?
the needle
what does red lead attach to?
the skin
are highly myelinated nerves motor or sensory?
motor
are unmyelinated nerves motor or sensory?
sensory
which nerves have the lowest threshold of external stimulation to generate an AP?
(highly myelinated) motor nerves
which nerves have the highest threshold for external stimulation to generate an AP?
(unmyelinated) sensory nerves
what is amplitude?
strength of an electrical stimulus
what impulses are best for discrimination of distance?
short duration
current duration for motor?
0.1ms
current duration for sensory?
0.3ms (longer duration needed to reach threshold level)
frequency used with nerve stimulator?
1-2 Hz
amplitude for nerve stimulator?
start at 1.5 mA
what is the goal with nerve stimulator technique?
loss of motor response at 0.3-0.5mA
this indicates needle is in correct position
is it okay to inject at 0.29mA?
No! never inject at less than 0.3mA (0.2mA by some sources)
is the nerve stimulator technique reliable?
No, only 74.5% sensitivity for needle to nerve contact
how often do you get no muscle twitch even wheen needle is actually touching the nerve with the nerve stimulator technique?
25% of the time
what does it mean if you have a twitch present at 0.2-0.3mA?
you are ALWAYS intranural
if > 0.3mA can you be in a nerve?
No
What is SENSe Mode?
sequential electrical nerve stimulation
how does SENSe mode work?
2 short at 0.1ms
0.2ms @ 0.3 mA
0.42ms @ 1 mA
0.84ms @ 2 mA
longer pulse reaches further in tissue
what is the goal with SENSe mode?
current at 0.3-0.5 mA with 3 twitches
is insulated/coated or non insulated needle ideal for nerve stimulation technique?
insulated is best
what kind of bevel do stimulating needles use?
blunt bevel
how does piezoelectric material in US probe work?
converts electricty to sound waves adn then sound waves back to elecricity. Sends out cyclical pulses of US energy
how many crystals in ultrasound probe?
100-300
sum of all crystals creates the US beam
how much does US probe talk vs listen?
talk 2%
Listen 98%
where does the majority of US energy go?
gets converted to heat
4 things that can happen to US waves?
reflection
scatter
absorption
attenuation (degredation)
what frequency is commonly used for ultrasound?
2-15 MHz
what is the primary determinant of lateral and axial resolution?
wavelength
what is primary determinant of temporal resolution?
frame rate (typically 30 frames/sec)
is curvelinear probe low or high frequency?
low frequency
best for deep structures
linear ( high frequency) probe can be used for up to what depth?
6cm
curvelinear ( low frequency) probe can be used for up to what depth?
up to 14cm
another name for curvilinear probe?
phase aray
What is B-mode?
brightness
2D image that is typically used
what is M-mode used for?
useful in assessment of specific tissues
heart valves, lung
is high pitch moving towards or away from the probe? What color?
toward the probe (red color)
is low pitch moving towards or away from the probe? What color?
away (blue color)
what is gain?
brightness of image on the screen
when is doppler mode used?
aide in detecting vascular structures
what shape are nerves on US?
round, oval, or triangular
how do nerves appear on US?
hyper or hypo echoic, or honeycomb
what can US waves not penetrate?
air or bone
what kinds of artifact can you get with US?
shadowing
enhancement
reverberation
mirror image
velocity error
what is enhancement?
overly intense echogenicity behind an object.
blood vessel or cyst
what is reverberation?
equally spaced bright linear echoes below an object, like your US needle
when is out of plane mode used?
mostly for vascular access
what plane would you need for direct visualization of LA injection?
needle in plane mode
Pros and cons of single shot PNB?
easier to do
fewer risks
effects generally limited to < 24 hours
how long can perineural catheter stay in?
up to 72 hours
do continuous catheters work well in femoral location?
Eh, just associated with quad weakness and falls
what area are continuous perineural catheters being used more often?
adductor canal
why are ergonomics so important, especially when you are learning PNBs?
so you don’t fatigue and can actually finish the block
what is the triad of safety for PNBs?
nerve stimulator
injection monitoring
ultra sound guidance
