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LA and Regionals

Barry - Local Anesthetics and Regionals

QuestionAnswer
Polarized resting state of nerve cell? (-) charges on interior of cell, (+) charges on the extracellular surface
What makes the cell polarized? Na ions > extracellular, K ions > intracellular, 3 Na out while only 2 K in - making interior more (-)
What propagates the nerve impulse along an axon? Synapse receives neurotransmitters from nearby nerve endings.
What follows the nerve impules? It increases the nerves permeability for Na influx - which lowers the voltage difference until threshold potential occurs.
What does lowering the threshold potential do to the cell? It opens the Na channels and massive amounts of Na ions enter the cell - as the interior of the cell becomes more (+) action potential develops.
When is the cell membrane refractory? During repolarization.
Local MOA: Block Na channels by binding directly to intracellular voltage dependent Na channels - cell cannot depolarize due to blocking Na influx.
***Does blocking the Na channels alter the membrane potential? NO - the resting membrane potential is still negative
What are the 3 states of the membrane? Activated (open), resting, and inactive.
When do LA have the greatest affinity for Na channel activation? In the activated and inactive states. So LA activity is both voltage and time dependent
4 things that affect sensitivity to blockade: 1)Axonal diameter (smaller more sensitive), 2)Degree of myelenation, 3)Conduction velocity (the more rapid firing nerves are more sensitive), 4)Relative location in nerve bundle (closer to the mantle [outside] of the nerve bundle more sensitive)
Chemistry of LA: ALL have a lipophilic portion (benzene ring), and a hydrophilic portion (tertiary amine).
What separates the benzene ring from the tertiary amine? Either an ester or amide linkage.
LA are ***ALL weak bases: And they carry a slightly (+) charge at physiologic pH.
Ester metabolism: By pseudocholinesterase - bi-product of some are PABA so if pts. are allergic it is to the PABA bi-product
CSF lacks esterase enzyme: Intrathecal injections rely on their absorption into the blood stream for metabolism - most often Tetracaine
Amide metabolism: By liver - decreases in function or blood flow will decrease metabolism.
What is Methemoglobinemia? Normal Hb has iron in the ferrous state (Fe2+), Met-Hb has iron in the ferric state (Fe3+)
What causes Methemoglobinemia? Metabolites of prilocaine and to a lesser extent topical benzocaine.
How do you treat methemoglobinemia? Methylene blue 1-2mg/kg of 1% soln over 5 minutes.
Lipid solubility correlates with... potency - the more lipophilic the more readily it will cross the membrane and lesser molecules are needed for blockade. It is also less likely to be cleared by blood so increased DOA
Protein binding correlates with... Duration of action (DOA) - bound to a-1 acid glycoprotein and albumin prolongs their elimination.
pKa correlates with... Onset of action
Define pKa: the pH at which the specific drug is 50% in the ionized and 50% in the unionized state. The ratio varies with the pH of the environment.
What state do the molecules need to be in to cross the membrane? Unionized. The closer the pKa is to physiological pH the higher the concentration of nonionized that will cross the cell membrane.
pKa of LA: All LA have a pKa 7.6-9.4 except benzocaine. Assume environment has pH of 7.4 unless told otherwise. Less than 50% of LA are in unionized form.
Do LA work better in acidic or basic environments? All LA are weak bases so they are more unionized in a basic environment, meaning they will work better. Infected areas are acidic so LA will not work as well due to more of the drug being in the ionized state.
List Ester locals: Novacaine (procaine), Nesacaine (Chloroprocaine), Pontocaine (tetracaine), Cocaine
List Amide locals: Lidocaine (xylocaine), Mepivicaine (carbocaine), Ropivicaine, Bupivicaine (marcaine), Etidocaine (duranest 1%) - they all have 2 I's in their name but know both names.
Novocaine/procaine dose: Max: 12mg/kg DOA: 30-60m DOA with epi: 30-90m
Nesacaine/Chloroprocaine dose: Max: 12mg/kg Max with EPI: 14mg/kg DOA: 30-60m DOA with EPI: 30-90m
Pontocaine/tetracaine dose: Max: 3mg/kg DOA: 90m-6h
Cocaine dose: Max: 3mg/kg DOA: 30-60m
Lidocaine/xylocaine dose: Max: 4mg/kg (3-5) Max with EPI: 7mg/kg DOA: 30-120m DOA with EPI: 120-360m
Mepivicaine/carbocaine dose: Max: 4mg/kg Max with EPI: 7mg/kg DOA: 45-90m DOA with EPI: 120-360m
Ropivicaine dose: Max: 3mg/kg DOA: 90-240m
Bupivicaine/marcaine dose: Max: 2.5mg/kg Max with EPI: 3.2mg/kg DOA: 120-240m DOA with EPI: 180-420m
Etidocaine/duranest 1% dose: Max: 6mg/kg Max with Epi: 8mg/kg DOA: 120-180m DOA with EPI: 180-420m
Why is max dose with EPI higher? Due to vasoconstriction so it is not being picked up by blood as quickly so less of a concern for LA toxicity.
Bier Block: IV regional anesthetic on upper or lower extremity.
What supplies are needed for a Bier Block? 2 IV's - one in hand of surgical arm, monitors, o2, and sedation. Esmarch to wrap arm from fingers to tourniquet.
What is the purpose of using a double tourniquet for a Bier Block? Pt. may get pain at site of tourniquet so the lower portion can be inflated and the upper portion deflated.
Tourniquet use: Always inflate upper portion prior to surgery. Use 250mmHg for upper ext. and 300-350mmHg for lower extremity - may need higher pressure for HTN
Cuff time: Minumum of 20 minutes - if less then have to let cuff down slowly to prevent LA toxicity. Maximum 1.5-2 hours.
Bier Block medication? Lidocaine 0.5% without EPI, 40-50cc = 200-250mg then remove IV.
Bier Block complications: Mechanical failure of tourniquet - LA toxicity, tissue necrosis.
***What are the s/s of LA toxicity? Circumoral and tongue numbness, lightheadedness, tinnnitus, visual disturbances, muscle twitching, unconciousness, convultions, coma, respiratory arrest, CV collapse.
How do you treat LA toxicity? O2 to raise the seizure threshold, ETT, hyperventilate (to decrease CO2 and decrease blood flow to brain), benzos, barbs, support CV collapse.
Which LA is the most carditoxic? Bupivicaine due to long 1/2 life, blocks cardiac Na channels and dissociates very slowly.
What are peripheral nerve blocks used for? Digits, ankles, brachial plexus, lower extremities, penile.
Can you use Epi with digits or penile blocks? NO - it can cause necrosis.
List structures you will go through with Epidurals: Skin, Sq tissue, Supraspinous ligament, Intraspinous ligament, ***Ligamentum flavum (crunchy), Epidural space (will feel pop), Spinal meninges (dura mater, arachnoid mater, pia mater)
What is the hanging drop method? Put drop of fluid in hub of needle and insert slowly and when fluid gets sucked in you are in epidural space.
What is loss of resistance (LOR) technique? Use a touhy syringe with a few cc's air or preservative free saline. Slowly advance and tap on plunger in increments until there is a loss of resistance in epidural space.
How far do you insert epidural catheter? 3-7cm
What is the epidural test dose? 3cc's of 1.5% lidocaine with EPI 1/200K = Lido 45mg and EPI 15mcg.
What is the purpose of the test dose? To make sure you are not in the subarachnoid space or intravascular space.
What are ss if in IV space with test dose? EPI will increase HR 20-30% and Lidocaine will cause ringing in ears, metalic taste and numbness in mouth.
What are ss if in subarachnoid space? Pt will exhibit ss of spinal - they will have a block.
Once the epidural is established can you use freely? No - always pull back 1st and then give test dose.
***What is the dose per segment? 1-2cc per dermatome.
Where is the epidural placed? Between L2-L5. L4-L5 is at level of iliac crest - when advancing touhy will feel crunch of Ligamentum Flavum.
Drugs used for epidurals: Chloroprocaine 2-3%, Lidocaine 1-2%, Mepivicaine 1-2%, Bupivicaine 0.25-0.75%, Ropivicaine 0.1-0.5%
What is a Caudal? Entering epidural space thru the sacral hiatus in kids. Penetrates sacrococcygeal ligament - between sacral cornua above the coccyx.
What drugs are used for Caudals? Bupivicaine, Ropivicaine, Opioids, Lidocaine s EPI
List the vertebrae: 33 total - 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, 4 coccygeal.
What types of needles are used for spinals? Quinke (flat), Sprotte, whitacre (pencil point).
List the spinal medications: Bupivicaine (marcaine) 0.75%, Lidocaine 5%, Tetracaine 0.5-1%, Procaine 10%
Spinal med baracity: Hyperbaric (sinks) - mixed with 8.75% dextrose to make heavier than CSF, Isobaric (stays at level)- mixed with CSF, Hypobaric (floats) - mixed with sterile water
***Key landmarks L4-5: At superior iliac crest.
T4: Nipples.
T6: Xiphoid process.
T10: Umbilicus.
Absolute contraindications: Pt. refusal, uncorrected coagulopathies, Severe aortic stenosis (will drop afterload), Hypovolemic shock, Increased ICP, Infection at site.
Relative contraindications: Sepsis, pt. uncooperative, pre-existing neuro deficits, severe spinal deformity, stenotic valve lesions.
Controversial contraindications: Prior back sx, communication issues, complicated sx.
What are the CV effects of spinals and epidurals? NS blockade resulting in vasodilation. Decreased CO and BP due to decreased venous return. Decreased HR if block higher than T4. Hypotension can cause nausea. Vasodilation can cause hypothermia.
What are the pulmonary effects of spinals and epidurals? Blockade of intercostal muscles.
Order "differential" of blockade: STPTPMVP Sympathetic, temp., pain, touch, pressure, motor, vibration, proprioception.
What is the dermatome blockade? SNS blocks 2 dermatomes higher than sensory, motor blockade is 2 dermatomes lower than sensory.
What are some of the complications of spinals and epidurals? Hypotension, N&V (treat hypotension), PDPH (post dura puncture headache or wet tap)
How do you treat PDPH? BR, analgesics, caffeine, abd. binder, blood patch.
What are the s/s of PDPH? Severe frontal and occipital HA, tinnitus, diplopia (light sensitivity) - worsens with sitting and standing.
How do you prevent PDPH? Use smaller needle, paramedian approach (to side), bevel to side, pre-hydrate
What is a high spinal? Above T4 decreases HR, tingling of the fingers - C8=middle, C7=pointer, C6=thumbs
Where are the cardioaccelerator fibers located? T1-4
Created by: Renetta on 2010-04-07



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