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Local Anesthetics - Coggle Diagram
Local Anesthetics
Delivery Techniques
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Field Block
dental extraction, operation on limbs, herniorraphy etc
Nerve Block
- small amount of drug produces large area of anesthesia.
- Intercostal, sciatic, brachial plexus, facial, & phrenic N blocks
Surface Anesthesia
Surface Anesthesia
- Tetracaine, Benzocaine, Lignocaine, Proparacaine
- Effect is superficial & of short duration & seen only on mucosa or abraded skin
- Drug is absorbed systemically – esp tetracaine from tracheobronchial tree
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Epidural
Epidural- Inject LA into epidural space
- Safer than spinal but technique is more complicated
- Slower in onset than spinal
- Requires larger volume of drug than spinal
- Can ↓requirement for GA
- Anesthetic can given continuously thro catheter, does not reach CSF, provides long term anesthesia & pain relief
- Can be used for labor analgesia & post-op analgesia – Epidural Analgesia
- Labor – analgesia w/o significant motor blockade. Mother is able to bear down – max with bupivacaine/ ropivacaine
Intravenous Regional Anesthesia/ Biers Block
- Max 60 minutes
- Min 15 minutes – toxic amounts of anesthetic enter the circulation
- avoid Bupivacaine & LA + Adrenaline
Spinal
Spinal
- into subarachnoid space - single dose of LA
- into L2-L3 intervertebral space
- Duration depends on agent used & it’s concentration- Lignocaine/ Bupivacaine common
- solution made hyperbaric with 8.25% dextrose in water
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- Nerve roots take up & retain the LA – concn in CSF falls quickly after inj
- Level of anesthesia does not change after 10 mins – becomes fixed
Complications –
- Post Dural Puncture Headache (prevent by use of very fine needle)
- post-op urinary retention
- Pain/ paresthesia in lower limbs – esp with lignocaine – but no neurological deficit
- respiratory paralysis (intercostal muscle paralysis)
- ↓BP – Spinal hypotension – most common complication - preload with crystallods, raise foot end of bed, ephedrine, mephentermine
- Bradycardia
- Nausea, vomiting
- Septic meningitis
- Cauda equina syndrome – rare – prolonged loss of control over bladder & bowel – traumatic damage to N roots or chorionic arachnoiditis due to inadvertent inj of antiseptic into CSF
Advantages – no loss of consciousness, good analgesia & muscle relaxation, safer for pts with pulmonary/ cardiac disease
C/I – hypotension, bleeding, ↑ICT, vertebral anomaly
Adverse Effects
Allergic reactions
- more with esters
- allergy to one ester rules out use of all other esters
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CNS stimulation followed by depression – restlessness, tremor, headache, drowsiness, confusion, convulsions, respiratory depression
1st symptoms – circumoral numbness, abnormal sensation in tongue
Treatment of LA induced seizure
Diazepam, followed by SCh, followed by Propofol
CVS – bradycardia, hypotension
Except cocaine – sympathomimetic – tachycardia, HT
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Classification
Coca plant - Erythroxylum coca
- grown as cash crop in South America
- Leaves used by Incans for working long periods, setting bones, childbirth, nose bleeds, cure for many illnesses
- Good surface anesthetic, easily absorbed from mucus membrane
COCAINE
- Alkaloid extracted from coca leaves
- Highly addictive CNS stimulant & anorexiant
- Unique drug of abuse – no tolerance on repeated use
- 1st used as ocular anesthetic – 1884
SURFACE ANESTHETICS
Lignocaine, Tetracaine, Benzocaine, Oxethazine, Proparacaine
Proparacaine
- Tonometry can be performed 30 seconds after instilling 1 drop in the eye
- Minimal ocular irritation
Benzocaine
- Long lasting numbness on topical application
- Not absorbed – no systemic toxicity
- lozenge, dusting powder, suppository
Oxethazine
- Does not ionize at low pH – unique
- Anesthetizes gastric mucosa
Tetracaine
- used only for corneal anesthesia
- Slow metabolism by pseudocholinesterase & ↑systemic toxicity
- Not to be used for bronchoscopy – rapid absorption yields concentration equal to systemic concentrations
Dibucaine
- Longest acting. Most potent. Most toxic
- only for use on anal mucus membrane
INJECTABLE ANESTHETICS
Short Acting & Low Potency
Procaine, Chlorprocaine
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Intermediate Acting & Potency
Lignocaine, Prilocaine, Mepivacaine
Lignocaine
- most widely used
- also a Class Ib anti-arrhythmic – ventricular arrhythmia
- Transdermal – postherpetic neuralgia
Prilocaine
- no vasodilation
- can cause methemoglobinemia due to O-toluidine metabolite
Long Acting & Highly Potent
Tetracaine, Bupivacaine, Ropivacaine
Bupivacaine
- widely used for spinal, epidural, N block.
- Cardiotoxic - ↓HR & ↑BP, ↑QT interval, VT, cardiac depression
- Levobupivacaine
Ropivacaine
- less cardiotoxic
- Blocks pain fibers more than motor fibers
Esters
Procaine, Chlorprocaine, Tetracaine, Benzocaine, Cocaine
- Metabolized by plasma butrylcholinesterase
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Cocaine - only ester not metabolized by pseudocholinesterase, but in the liver
- Poor tissue penetration – no surface action
- Allergy more common
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Amides
Lignocaine, Prilocaine, Mepivacaine, Bupivacaine, Ropivacaine, Articaine
- Good tissue penetration
- Rapid onset of action
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Pharmacokinetics
- All are weak bases – HCl salt added to make them water soluble & injectable
- Equilibrium of ionized (injectable) & unionized (lipophilic) in aqueous solution
- Degree of lipid solubility enables diffusion through nerve membrane – thus directly related to potency
- Onset of action related to pKa – lower pKa like lignocaine are 30-40% unionized at pH 7.4 & have faster onset of action
- pH drops in infected tissues & onset of action is delayed or even prevented.
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