Local Anaestetics Flashcards
Analgesia vs anaesthesia
Analgesia is the loss of sense of pain without loss of sensation while anaesthesia is loss of both pain and sensation
Local anaesthesia induces absence of sensation in specific part of the body leading ot
Analgesia and paralysis
How does local anesthesia cause absence of sensation
Block afferent activity in peripheral and central nervous system.
What technique is used to deliver local anesthesia to the maxilla and mandible
Infiltration. Small terminal nerve endings are flooded with LA solution
What s a plexus block
A plexus block is when you deposit LA solution in close proximity to the main nerve trunk, preventing nerve impulses from travelling beyond that point
How is spinal local anesthesia administer
Subarachnoid block. Administered directly into fluid sac
What are the localised adverse effects of prolonged local anesthesia
There may be severing of nerves and support tissue due to infection, hematoma, or excessive fluid pressure in the confined cavity
What are some systemic adverse effects of local anesthesia
Local anesthesia may enter systemic circulation due to prolonged delivery or because LA was accidentally injected directly into blood
This can lead to systemic toxicity
- Depressed CNS: sleepiness, lightheadedness, metallic taste, tongue numbness, seizure, coma
- Cardiovascular effect. Depress myocardial contractility, systemic hypotension, arrthymia, arteriolar dilatation
- Hematology. Methaglobinema due to accumulation of prilocaine metabolite
- Allergic reaction due to metabolites of ester LAs
What are the amide local anesthesics
Lidocaine Mepivacaine Prilocaine (methemoglobinema) Endocrine Bupivacaine
What is lidocaine
Medium acting amide local anesthesic
What is bupivacaine
Long acting amide local anesthesic
What is procaine
Short acting ester local anesthesic
What are some ester local anesthesics
Cocaine, procaine, chloroprocaine, tetracaine
What is the structure of local anesthesics
Lipophilic group connected via ester/amide to ionisable group
Ester vs amide which has shorter duration of action and why
Esters have a shorter duration of action as they are more prone to hydrolysis (by butyrylcholinesterases)
Are local anesthesics acidic or basic? Charged or uncharged?
Weak bases. Active when charged as cannot readily exit ion channels. Uncharged state allows for rapid penetration of lipid membranes
Why are local anesthesics less effective when injected into infected/ischemic tissues
Infected/ischemic tissues are acidic. LAs are weakly basic. Hence lower proportion is non ionised ie more ionised form, hence less available for diffusion into nerves
Name some factors that affect the systemic absorption of local anesthetics
Dosage, site of action eg is it highly vascularised, local blood flow, use of vasoconstrictors to prolong action
How are amide local anesthesics eliminated
Metabolised in the liver by microsomal cytochrome P450 isoenzymes
Hence decreased metabolism in patient with liver disease, in patient with decreased hepatic blood flow, if patient is taking cytochrome p450 inhibitor drugs
How are ester local anesthesics eliminated
In the blood (vs liver for amides) by butyrlylcholinesterases
What is the mechanism of action of local anesthetic
- LA enters the cell through the phospholipid bilayer (uncharged form)
- Binds to receptor near intracellular end of the sodium channel to increase threshold for excitation (charged form)
- Causes closing of gate at intracellular end, inactivated state, na+ unable to pass through the channel, block conduction of impulses
Reduce depolarisation and prolong depolarisation
What fibres are more susceptible to effects of local anesthetics
Rapidly firing fibres are more likely to become hyperpolarised. Use dependent block. Drug has better access to binding site when the channel opens frequently
Smaller fibre diameter. Unmyelinated and passively propagate electrical impulses over short distance. LA only need to block short segment vs large fibres where LA need to block at least 2-3 nodes of ranvier
Fibres located circumferentially in nerve trunk is first to be exposed to LA. Proximal sensory fibres first, proximal before distal sensory loss
What kind of LA technique do i use to numb the upper limb
Nerve block of the brachial plexus
What does epidural spinal blockage cause
Numb feeling in lower half of the body
What is a bier block
Intravenous administration of LA. Inject into distal vein isolated by proximal tourniquet. Inject LA into venous system of extremity exsanguinated by compression of gravity, isolated from central circulation
How to prolong effect of local anesthetic
Increase dose, use vasoconstrictor eg adrenaline
How to accelerate action of local anesthetic
Add sodium bicarbonate into the solution. More alkaline, more likely to be unionised so that can penetrate lipid membrane and diffuse into nerves
Patient reports history of allergic reaction to procaine. What LA should you use?
Clarify with patient what she means by allergic reaction —.difficulty breathing, anaphylaxis, swelling
Procaine is ester type. Use lidocaine (amide, allergic reaction unlikely)
Patient given bupivacaine. Complains of light headed ness, loses consciousness and develops tonic clinic movement of extremities. How to respond?
Systemic toxicity. Stop injection of local anesthetic to limit toxicity.
Check breathing and maintain airway
Administer small dose of benzodiazepine
