Locoregional Flashcards
Peripheral Nerve Stimulation
Monophasic square wave electric current: nerves selectively stimulated depending on size
Large diameter axons (>0.15mA) more easily excited than small (>0.3mA), motor nerves stimulated at lower currents than sensory
Closer needle is to target nerve, lower electrical current required to elicit muscle response
At 0.2-0.6mA
if stimulating needle able to elicit contractions of effector muscle or groups, assumed that needle to nerve distance small enough that when LA injected sensory blockade will occur
PNS MOA
Negative electrode (black) serves as searching electrode, connected to the insulated needle
Positive (red) electrode connected to patient’s skin
When used with configuration, current will induce depolarization of target nerve, resulting in obvious muscle twitches
Minimal Intensity Required to Depolarize a Specific Nerve…
depends on distance between the tip of the needle, the nerve
Settings on the Nerve Stimulator
2Hz, initially set at 1-1.5mA then decrease current to 0 and confirm twitches decrease in intensity before twitch elicited at 0.5 – frequency 0.1-0.3ms
USG for LRA
high frequency linear array transducers: 10-15mHz
Higher frequency transducers achieve lower penetration, better resolution
Function of Gain?
amplification of signal that returns to transducer, optimizes image obtained so structures that are identified
Regulates brightness, contrast of received signal to modify black and white balance
Horner’s Syndrome DT LRA
Blockade of the sympathetic trunk at cervical dermatomes
Respiratory Depression DT LTA
Excessive migration of injectate
Intercostal nerve blockade
Phrenic nerve blockade - C5, C6, C7
Circulatory insufficiency at resp center
MOA - epidural
nerve roots as leave SC, travel out from intervertebral foramina
o Drug access to site of action depends on drug’s physical, chemical properties, volume, gravity, permeability, etc
What is contained within the epidural space?
potential space of nerves, blood vessels, fat
MOA - intrathecal
non-selectively affect roots of spinal nerves located in dural sac
o Distribution, extension affected by baricity of drug and patient positioning
Layers for Epidural or Intrathecal Injection
o Skin
o Subcutaneous Tissue
o Supraspinous ligament
o Interspinous Ligament
o Interacuate ligament, yellow ligament or ligamentum flavum
o Epidural Space
o (Dura Mater)
o (Arachnoid membrane)
o Subarachnoid space = intrathecal, CSF, intricate trabecular network
Will you reliably see CSF with intrathecal injections?
Will not always see CSF with dural puncture
Hydrostatic pressure within dural sac 4-5mm Hg, cannot overcome height of needle when in sternal recumbency
Do not aspirate - negative pressure will suck nerve roots into needle, false negative
Techniques to Confirm Correct Placement in Epidural Space
hanging drop
LOR
US
ENS
Running Drip
Length of block distribution: occipital-coccygeal length
o Measure OCL (cm) in dogs
o Define desired block extension in cm
o Calculate % of column to be blocked (cm to be blocked/OCL)*100%
o Obtain dose from nomogram
Bier Block
–Admin of LA into peripheral vein, surgical anesthesia for up to 90’
–Distal limb wrapped tightly to exsanguinate, tourniquet then applied proximal to injection site
o Tourniquet in place 20-25’ minutes
–Tissue distal will be desensitized
–Early release of tourniquet may result in transient increases in LA plasma concentration CNS, myocardial system toxicities
