Diagnostic perineural and joint blocks Flashcards
(36 cards)
What is the mechanism of action of local anesthetics
-Pain travels to the CNS via thinly myelinated A delta and nonmyelinated C fibers
-Inhibit excitation of nerve endings by reversibly binding to and inactivating sodium channels
-Nerve blocks act by preventing the increase in membrane permeability to the sodium ions that occur when nerves are stimulated. Prevent rapid influx of sodium ions thereby inhibiting depolarization of the membrane
-Onset of nerve fiber desensitization: Sensory>Sympathetic>Motor
What pharmacological properties affect how a LA works
-PKa, lipid solubility and protein binding all affect duration
-Lipid solubility also affects potency, ability to cross membrane, and onset
-Speed of onset is determined by the pKa ( the pH at which the ionized and unionized forms are present in equal amounts
-low the pKa the greater the non-ionized form in solution and the faster the speed of onset
What are the positive and negative effects of adrenaline added to LAs
Positive effects: increased duration, decreased local anesthetic plasma concentration, vasoconstriction/less bleeding
Negative effects: tissue ischemia and necrosis, white hair formation, dehiscence (due to reduced blood supply)
What types of LAs can be used
-Topical anesthetic: splash block on mucosa
-Local anesthesia: ring block, direct infiltration, line block
-Regional anesthesia: perineural anesthesia (nerve blocks), intrasynovial anesthesia (articular, bursal, thecal), IVRA, epidural anesthesia
Local anesthesia techniques
Ring block: anesthetic injected subcutaneously in a ring above area to be blocked. Structures distal to block are anesthetized (pastern ring block, hindlimb low 6 point)
Direct infiltration: anesthetic injected directly at site to be blocked, bony prominences (splint bones), insertion of ligaments or tendons
Line block: anesthetic injected in a line cranial/dorsal to area to be blocked, lacerations, surgical incisions
What are common sites of perineural anesthesia
-Palmar/plantar digital nerve
-abaxial sesamoid
- low 4 point
-Nerve block injected directly over nerve, anesthetizes at and distal to site
What are common sites of perineural anesthesia
-Palmar/plantar digital nerve
-abaxial sesamoid
- low 4 point
-Nerve block injected directly over nerve, anesthetizes at and distal to site
What types of intrasynovial anesthesia/blocks are there
-Joint blocks: anesthetic injected directly into the joint. Anesthetizes joint capsule, synovial membrane, does not anesthetize structures outside the joint efficiently, subchondral bone/SL branches. Common sites: distal interphalangeal (coffin) joint, fetlock joint, carpal joints (intercarpal and radiocarpal), distal tarsal joint (TMT and DIT), stifle joints (femoralorotibial and femoropatellar)
-Bursal blocks: common sites- navicular bursa and bicipital bursa
-Thecal blocks: common sites- digital flexor tendon sheath
What are some important principles of diagnostic anesthesia
-Nerve blocks: structures distal to block are anesthetized, start dismally and work proximally, always check block (test skin sensation)
-Intrasynovial blocks: must be sterile, localized to particular structure, soft tissue injuries within DFTS may only partially block, subchondral bone pain not always blocked
What equipment is needed for diagnostic anesthesia
-Sterile disposable needles: perineural (smallest gauge possible- 23 to 25 g), intra-synovial: big enough for synovial fluid (20g)
-Sterile non-lure lock syringes
-Local anesthetic solutions
-Gloves: exam, sterile
-Scrub: chlorhexidine(.05%), povidine iodine (.2%), alcohol 70%
What should you look for in diagnostic anesthetic
-Improvement in lameness: check block has taken(skin sensation), sound, improved but still lame on original limb, >50%improvement with intrasynovial block considered a positive response, switch to another limb
-No improvement: keep working proximally, check if correct limb, wrong drug, mechanical lameness, other diagnostic (nuclear scintigraphy)
Side effects of diagnostic anesthetic
-Soft tissue swelling: needle trauma, hematoma, cellulitis, small gauge needle breakage
-Synovitis: non-septic flare, septic
-Toxicity: lidocaine- 10-12 mg/kg, max dose in 500kg horse is 250mls of 2% lidocaine
-Skin sloughing: adrenaline containing solutions
What nerve blocks do we need to know how to do for final year
-Palmar digital nerve block
-Abaxial sesamoid
-low 4 point (low volar)
What position, nerves, and location are relevant for the palmar digital nerve block
-Position: standing or foot elevated, facing back, 1 hand holding limb, facing forward with foot between your knees
-Nerves blocked: medial palmar/plantar digital nerve, lateral palmar/plantar digital nerve
-Location: palmaromedial and palmarolateral pastern region, 1cm above collateral cartilage, palpate VAN, nerve palmaroaxial to vein and artery
What regions are blocked in the palmar digital nerve block
-Heel region, palmar skin, navicular bursa, distal DDFT, palmar coffin joint, palmar processes of the pedal bone, collateral cartilage, bars/frog/entire sole, laminar corium, digital cushion
-Desensitizes: coffin joint
What regions, nerves, and location are affected by the abaxial nerve block
-Position: standing or foot elevated, facing back with 1 hand holding limb, facing forward with foot between your knees
-Nerve blocks: medial palmar/ plantar digital nerve, lateral palmar/plantar digital nerve
-Location: distal border of proximal sesamoid bones, palpate VAN, nerve axial to vein and artery
What regions are blocked in an abaxial nerve block
-Foot, P3, P2 and most of P1, coffin joint, pastern joint, part of the DDFT and digital flexor tendon sheath, distal sesamoidean ligaments, extensor branch of the suspensory ligament, extensor tendon insertion
-May also block: proximal sesamoid bones, fetlock joint
What nerves are blocked in the low 4 and 6 point nerve blocks
Low 4 point forelimb: medial and lateral palmar nerves, medial and lateral palmar metacarpal nerve
Low 6 point hindlimb: medial and lateral plantar nerves, medial and lateral plantar metatarsal nerves, medial and lateral dorsal metatarsal nerves
What locations are the palmar metacarpal, the palmar, and the dorsal metatarsal nerves
-Palmar metacarpal: just distal to button of splint bone, caution of palmar fetlock joint pouch
-Palmar: between DDFT and suspensory ligament, 3-5 cm proximal to button of splint, caution of DFTS
-Dorsal metatarsal: either side of extensor tendon or subcut dorsal ring at level of plantar metatarsal nerves
What regions are blocked by the low 4 point nerve block
Regions: fetlock joint, proximal sesamoid bones, flexor tendons distal to block, digital flexor tendon sheath, entire distal limb
What joint blocks do we need to know
-Distal interphalangeal (coffin)
-Metacarpophalangeal (only one well do in final year
What is the position and location of the distal interphalangeal joint block
-Position: standing
-Location: 1)dorsal pouch: 1 cm dorsal to coronary band, 45 degree angle distal. 2) dorsal pouch: just above coronary band, midline, needle directed horizontally. 3)palmar pouch: abaxial, just above collateral cartilage, needle directed 45 degree angle dorsally or axially
What structures are blocked with a distal interphalangeal joint block
-Coffin joint, navicular bursa (note that LA can diffuse from DIP joint to navicular bursa in 100% of horses in 15 minutes)
-May also block insertion of DDFT and sole
What is the position and location of fetlock joint block
-Position: standing or limb elevated
-Location: a) proximopalmar: 2 cm distal to splint button, proximal to collateral sesamoidean ligaments, needle directed 45 degrees palmaroproximal to palmarodistal. B) dorsal: abaxial to CDE tendon. C) distopalmar: depression proximal to proximal palmar process of PI. D) palmar: between dorsal sesamoid and palmar distal MC3, flex fetlock
