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Anaesthetics > Local Anaesthetics > Flashcards

Local Anaesthetics Flashcards

1
Q

What are the two groups of local anaesthetics?

A

Amides
Esters
Both consist of an aromatic ring which is lipophillic. This is connected by a link (determining group) and then a hydrophilic amine

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2
Q

What is a local anaesthetic?

A

Drug that produces temporary reversible blockage of AP/ neuronal transmission when applied to a nerve fibre.
Depending on volume and concentration, small autonomic to large motor fibres are affected.

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3
Q

List 3 amide LA?

A

Lidocaine (xylocaine)
Prilocaine (Biers block use)
Bupivicane (Marcaine)
Ropivicaine (Naropin)

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4
Q

List 3 ester LA?

A

Cocaine, procaine and amethocaine

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5
Q

What are the differences between amides and ester LA?

A

Structural link -O-CO-, -NH-CO-
Functional - stability in solution
Esters are unstable but amides can remain stable for up to 2 years
Pharmacokinetics - distribution varies as esters are minimally bound to proteins and amides are extensively bound to protein
Metabolism - esters are rapidly hydrolysed by pseudocholinesterases in the plasma to inactive compounds, amides are hepatic
Allergy - higher with amides due to para-aminobenzoate metabolite

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6
Q

How do LA work?

A

Block influx of sodium at VG Na channels inside nerve axons/ nerve cell membrane preventing depolarisation and propagation of neuronal actions potential. Higher affinity for open or inactived Na channels. Block from inside of neurone by crossing lipid membrane positive ion binding receptor.

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7
Q

What is pKa?

A

pH at which 50% of drug is ionised and 50% unionised

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8
Q

What is the Henderson hassellbach equation?

A

Acid: pH = pKa + log (ionised form)/ (unionised form)
Base: pH = pKa + log (unionised)/(ionised)

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9
Q

What is the significance of pKa in LA?

A

LA exist in equilibrium between unionised and ionised forms
Their pKa and pH DETERMINE percentage ionised v unionised
Weak base pH below pKa - a greater proportion of base exists in ionised form (acidic)
pKa determines speed of onset of action, lower pKa = quicker due to high unionised proportion of drug

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10
Q

List pKa of lidocaine, bupivicaine and cocaine

A

Lidocaine 7.9 (2-4min onset of action)
Bupivicaine 8.1 ( 5-8mins onset of action)
Cocaine 8.6

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11
Q

What is physiological pH and why is this important in LA mechanism of action?

A

pH of body 7.4 - below LA pKa therefore exist as greater proportion unionised form the lower the pKa (lidocaine higher proportion unionised therefore quicker onset of action!)

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12
Q

Lipid solubility is related to what?

A

Potency of a drug
Bupivicane is 95% protein bound
Lidocaine is 75% protein bound

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13
Q

What else affects potency of a drug?

A

Tissue distribution and vasodilatation (control volume available at membrane)

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14
Q

What affects duration of action of LA?

A

Protein binding - highly bound = longer duration
Drug potency
Vasodilation reduces duration of action

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15
Q

What LA vasoconstricts?

A

Cocaine

Other agents in high concentrations of with adrenaline

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16
Q

What are toxic doses for common LA?

A

Lidocaine 3- 5mg/kg, 7mg/kg with adrenaline

Bupivicaine 2mg/kg in any state

17
Q

What is the elimination half life of lidocaine?

A

Plasma cholinesterases break down lidocaine into inactive PABA metabolite, 100 minutes
This is compared to 160 mins bupivicaine

18
Q

What LA would be suitable for a sciatic nerve block?

A

Ropivicaine
slower onset of action due to reduced lipid solubility and therefore lower penetration of large myelinated nerve fibres
reduced potency and shorter duration of action
Less cardiotoxic due to higher toxic plasma level of 4ug/ml

19
Q

How do LA affect the heart?

A

Block sodium channels therefore phase 0 of cardiac AP increases more slowly . This delays arrival at threshold for spontaneous depolarisation. Prolong refractory period.
PR and QRS prolonged
Bupivicaine takes 10x longer to move away from the myocardium leading to arrythmias and VF
Ropivicaine is structurally different and diffuses away more quickly

20
Q

Why does LA not work in infected tissue?

A

Acidic environment
Low pH reduced unionised fragment making diffusion across lipid membrane longer.
Vasodilatation of surrounding tissue and increased blood supply takes away LA more quickly

21
Q

What concentrations of LA are used in spinal anaesthesia?

A
  1. 5% bupivicaine spinal

0. 1% in epidurals as no motor block required

22
Q

What is heavy Marcaine?

A

Bupivicaine contains glucose 80mg/ml to provide a denser block that descends with gravity and sits above CSF

23
Q

What are features of LA toxicity?

A

CNS - tingling, dizziness, tinnitus, visual disturbance, seizures, coma
CVS - chest pain, sob, arrythmias VF

24
Q

What factors increase risk of LA toxicity?

A

Highly vascular area
- brachial, intercostal, caudal, epidural, paracervical
Higher concentration or volume
Acidic or hypoxic environment a protein binding decreases

Vasoconstrictor added aid prevention of systemic absorption
Adipose helps to protein bind LA reducing toxicity risk

25
Q

How would you manage LA toxicity?

A 
ABCDE
STOP infusion
Call for help
Intralipid 20% - refer to AAGBI guideline (1.5ml/kg bolus followed by 0.25ml/kg/min)
Cardiac arrest algorithms
26
Q

EMLA is what?

A

Eutectic mixture of LA
2 compounds mixed together (2.5% lidocaine and 2.5% prilocaine) to produce a substance with single set of characteristics
White oil:water emulsion
5-30mg onto skin covered with clear dressing for 60mins
Lower melting point absorption is quicker

27
Q

When do you avoid EMLA?

A

Those with methaemglobinaemia risk (o-toludine metabolite induces this)
Not on mucous membranes
Class I anti-arrythmics as toxic effects can be synergistic

28
Q

How are LA presented?

A

A weak base in HCl

Soluble in water

29
Q

What is the future of LA?

A

Centribucide - 6-8x more potent but few CVS complications

Liposomal bupivicaine - emulsion

30
Q

What is the dibucaine number?

A

Given to allergic reactions
Irreversible blocker of pseudocholinesterases
Potent LA

31
Q

What is ametop?

A

Amethocaine

32
Q

What 4 ways can you give LA?

A

Topical
IV
Intrathecal or epidural
Infiltration

33
Q

What other effects do LA have?

A

Anti inflammatory
Prevent axonal sprouting and transport
Bacteriostatic
Decrease hyperalgesia by inhibition of glutamate and glycine channels
Inhibit fatty acid oxidation
Block nicotine AchR
Anti-apoptotic effect on myocytes and neurones

34
Q

What is baricity?

A

Comparable in certain respects to specific gravity but expressed as a ratio of the densities of LA and CSF st 37C

35
Q

Specific gravity is?

A

Ratio of density of a substance to a std. LA at 20C related to water at 4C

36
Q

Density is?

A

The ratio of a mass of a substance to its volume. Varies with temperature so must be stated.
Density of CSF is 1.0003g/l
Lower in women and pregnancy and premenopausal

37
Q

Is CSF isotonic?

A

Yes, it is isotonic and aqueous medium similar to isotonic fluid.

38
Q

How is a solution hypobaric to CSF?

A

Below 0.9990 to be hypobaric to CSF.

Plain bupivicaine

39
Q

What are the characteristics of intrathecal injection?

A 
Baricity
Vol/dose/conc
Temperature of LA
Viscosity
Additives

Anaesthetics (12 decks)

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