PNB - General Flashcards
peripheral nerve blood flow is regulated by?
sympathetic nervous system
what is the epineurium?
- enveloping external connective sheath
- contains many nerve fascicles
what is contained in a nerve fascicle?
-nerve fibers and capillaries in loose connective tissue
what is the perineurium?
-multi-layered epithelial sheath that surrounds the individual nerve fascicles
what type of junctions does the nerve capillary endothelium have?
- tight junctions
* similar to BBB
should you perform nerve blocks in patients that are sedated or under general anesthesia?
-no: cannot tell you if they are having pain on injection
nerve blocks - mechanism of injury
- mechanical: needle trauma, intraneuronal injection
- neuronal ischemia
- neurotoxicity of local anesthetics
- drug error
- infection
strategies to avoid mechanical injury
- use short beveled needle
- avoid high pressure injections
- avoid fast injections
the ‘perfect storm’ of neuronal ischemia
- endoneural injection
- epinephrine-containing local anesthetics
- prolonged tourniquet times
what is the best probe size for PNB?
25L
acoustic impedance
- measure of how the ultrasound transverses the tissue
- depends on the density of the medium and velocity of the ultrasound through the medium
acoustic impedance mismatch
- large difference in acoustic impedance
- the larger the mismatch, the more is reflected and less is transmitted
tissue attenuation
- loss of ultrasound as a medium is transversed
- due to absorption of ultrasound energy
- increases as distance from probe increases
what is a wavelength? what does a wavelength tell us?
- the distance between two areas of maximal compression (peak of wave)
- wavelength is proportional to penetration of ultrasound
- image resolution is no more than 1-2 wavelengths
what is frequency? high versus low frequency?
- the number of wavelengths that pass per unit of time
- higher frequency = clearer image, lower penetration
hypo-echoic
- reflect little amount of sound, absorb wave
- appear dark
- blood vessels
hyper-echoic
- reflect large amounts of sound, do not absorb wave
- appear bright
- bone, air
in-plane
- longitudinal view of entire length of probe
* best view
out-of-plane
-perpendicular to probe
which view is better, in plane or out of plane?
-in plane
short axis
-cross section of a structure
long axis
-longitudinal view of length of structure
where is the interscalene block completed?
-at the level of the roots of the brachial plexus
nerve anatomy levels
- roots
- trunks
- divisions
- cords
- terminal nerves
the brachial plexus is what roots?
-C5-T1
what do the brachial plexus roots pass through when coming off the spinal cord?
- posterior fascia of anterior scalene
- anterior fascia of middle scalene
where is the supraclavicular block completed?
-at the level of the divisions of the brachial plexus
trunks of the brachial plexus
- upper = C5-C6
- middle = C7
- lower = C8-T1
where do the trunks of the branchial plexus travel through?
- between anterior scalene and middle scalene
- subclavian artery lies anterior to nerve bundle
- pass behind the first rib and divide into divisions
brachial plexus divisions
- each trunk divides into an anterior and posterior division
- superior and posterior to the subclavian artery
where is the infraclavicular block completed?
-at the level of the cords of the brachial plexus
brachial plexus cords
- three cords branch from divisions at the level of the coracoid process
- lateral, medial, posterior cords
brachial plexus cords: lateral
- combines with a branch from median cord to form median nerve
- musculocutaneous nerve
- located in coracobrachialis muscle
brachial plexus cords: medial
- combines with a branch from lateral cord to form median nerve
- ulnar nerve
- located in subcutaneous tissues
brachial plexus cords: posterior
- radial nerve
- axillary nerve branches early
brachial plexus terminal nerves
- vary with anatomy
- remain close to axillary artery
brachial plexus terminal nerves: radial
-posterior to axillary artery
brachial plexus terminal nerves: median
-superior to axillary artery
brachial plexus terminal nerves: ulnar
-inferior to axillary artery
where is the axillary block completed?
-at the level of the terminal nerves of the brachial plexus
what nerve is consistently missed in an axillary block? why?
- musculocutaneous nerve
- its branches from the terminal branches early
interscalene block - site blocked
-most reliable for C5-C7
-not reliable for C8-T1 (ulnar side)
-sensory anesthesia to cervical plexus (C2-C4)
-shoulder and upper arm
surgery
*caution for phrenic nerve paralysis (affects by volume injected)
interscalene block - approach
- supine with head turned away
- landmarks: cricoid cartilage, lateral border of sternocleidomastoid muscle, interscalene groove
- always aim needle caudal and lateral
supraclavicular block - site blocked
- forearm and hand, below mid-humerus level
- nerves are tightly packed = onset is fast and deep
- risk of pneumothorax
- 20 mL to avoid phrenic nerve paralysis (60%)
supraclavicular block - approach
- arm at side
- palpate clavicle and scalene muscles
- place probe at midpoint of clavicle to locate subclavian artery
- plexus is located superior and posterior to subclavian artery
infraclavicular block - site blocked
- below the mid-humerus
- musculocutaneous nerve may need separate injection because it can diverge higher
- longer onset
infraclavicular block - approach
- supine with head turned away
- place probe interior to clavicle and medial to the coracoid
- visualize axillary artery deep in pectoralis - cords surround it
- always advance needle posterior and caudal to avoid pneumothorax
axillary block - nerves missed
- musculocutaneous
- medial-brachial cutaneous
- medial ante-brachial cutaneous
axillary block - site blocked
- hand surgeries
- multiple injections because of separation of terminal nerves
axillary block - approach
- supine with arm abducted 90 degrees and flexed at elbow
- identify axillary artery
axillary block - non-ultrasound approaches
- perivascular: advance needle close to artery while aspirating, withdraw and insert 3-4 mL, repeat twice and then complete on opposite side
- trans-arterial approach: insert through vessel, inject 10 mL when there is no longer blood return, pull needle back through artery, inject 10 mL where there is no longer blood return
mid-humeral approach: median
- place PNS next to brachial artery on superior side in direction of axilla
- flexion of wrist/fingers, pronation of forearm
mid-humeral approach: ulnar
- redirect needle posterior and inferior from brachial artery
- flexion of wrist/ring finger + pinky, adduction of thumb
mid-humeral approach: radial
- close to ulnar
- extension of wrist/fingers
mid-humeral approach: musculocutaneous
- redirect needle superior to artery in coracobrachialis muscle
- flexion of elbow
Horner’s syndrome is associated with which block? what are the symptoms
- interscalene
- ptosis, miosis, anhydrosis
evaluation of brachial plexus block
- push: radial
- pull: musculocutaneous
- pinch: lateral = ulnar
- pinch: medial = median
femoral nerve block -approach
- supine with leg neutral
- mark inguinal crease
- lateral to femoral artery, superior to iliopsoas muscle
- done out of plane
intercostal nerve block - anatomy
- VAN: vein, artery, nerve
- thoracic nerve divided into ventral (intercostal nerves) and dorsal (posterior sensation to skin, muscles, and bones of back)
intercostal nerve block - considerations
- need to block above and below site because of crossover of intercostal nerves
- good for analgesia, not anesthesia
- local anesthetics are rapidly absorbed from intercostal space
- blocks above T7 are difficult
intercostal nerve block - approach
- angle of the rib=6-8 cm lateral from spinous process
- angle needle 20 degrees cephalad and advance until you hit the rib
- readjust needle caudal, and repeat until you do not hit the rib
- advance a few mm until you feel a pop when entering the neurovascular bundle
- aspirate and inject 3-4 mL
which nerves are blocked more easily?
small myelinated nerves
potency of LA is determined by?
lipid solubility
duration of LA is determined by?
protein binding
onset of LA is determined by?
pKa
nerve blocked by local anesthetics, in order of being blocked
c: pain (dull, pressure), temperature
B: autonomics
A delta: pain (sharp), touch
what nerve innervates the posterior thigh?
-posterior femoral cutaneous nerve (S1-3)
