Local Anaesthesia Flashcards
What is a local anaesthetic ?
a drug or agent which reversibly blocks neuronal transmission in the applied region causing a temporary loss of sensation/pain, without affecting consciousness
What is a general anaesthetic?
a drug or agent which produces a loss of response to painful stimulation (analgesia) and a loss of reflexes (motor and autonomic) with a reversible loss of consciousness
What is an analgesic?
a drug or agent which relieves and prevents pain without a loss of consciousness
What is a sedative?
a drug or agent which reduces irritability, excitement or nervousness
Where is LA injected?
Near nerve branches innervating the small, specific area of the body near the surgical site
Briefly state some reasons why LA is clinically useful
temporary and rapid in action, completely reversible
sufficient potency to provide complete anaesthesia
sufficient in duration to complete procedure comfortably
not irritating to tissues
does not produce allergic reaction (hypoallergenic)
not addictive
low degree of systemic toxicity
high therapeutic ratio
What was the first naturally occurring local anaesthetic
cocaine
Found in coca leaves
Who was the first clinician to inject cocaine to perform a peripheral nerve block? When did this take place?
William Halsted (1884)
What does a low therapeutic index refer to ?
This is when there is a small difference between the therapeutic and toxic doses of a drug
Give an example of a toxic effect of cocaine
respiratory depression
There is a potential for abuse an addiction when using cocaine. Briefly state why this is the case
CNS stimulant, psychological dependence
crave drugs euphoric and stimulatory effects
What are the short term CNS effects of cocaine?
Mydriasis (dilated pupils)
headache
nausea
vomiting
muscle tremors
twitching
pseudohallucinations (cocaine bugs)
What are the long term CNS effects of cocaine?
Generalised seizures
hallucinations
gross muscle tremors and twitching
decreased responsiveness to stimuli
increased deep tendon reflexes
incontinence
What are effects of a cocaine overdose on the CNS?
Generalised convulsions
cerebral infarction and haemorrhage
pupils fixed and dilated
flaccid paralysis
loss of vital support functions
CNS depression
coma
What are the short term cardiovascular system effect of cocaine?
vasoconstriction
hypertension
tachycardia
pallor
What are the long term cardiovascular effects of cocaine?
cardiac dysrhythmias
hypertension
tachycardia
peripheral cyanosis
What is cyanosis?
blue/grey lips or skin; happens when there is not enough blood supply to these areas
What are effects of a cocaine overdose on the cardiovascular system?
aortic dissection
MI
cardial arrest
What are short term respiratory effects of cocaine?
increase in breathing rate and depth
What are long term respiratory effects of cocaine?
abnormally rapid breathing (tachypnoea)
shortness of breath/gasping (dypnoea)
irregular breathing pattern
What are the effects on a cocaine overdose of the respiratory system?
respiratory depression
respiratory failure
cyanosis (lack of oxygen)
gross pulmonary oedema
paralysis of respiration
What are the short term effect of cocaine on the body temperature?
elevation
What are the long term effects of cocaine on the body temperature?
sever hyperthermia
What is the effect of a cocaine overdose on the body temperature?
Severe hyperthermia
What are the short term behaviour effect of cocaine?
euphoria
elation
excitation
restlessness (increased motor activity)
Garrulousness (excessively talkative)
alert
energetic
strong
paranoia
What are the short term behaviour effect of cocaine?
social maladjustment
paranoia
depression
agitation
difficulties with emotion regulation and impulse control
violent or suicidal tendencies
What is the effect of a cocaine overdose on behaviour?
death
Synthetic local anaesthetics are broadly split into ________ and _______.
Esters
Amide
State a method that can be used to identify ester and amide local anaesthetics
Amides have an “i” in the prefix before the “caine” whilst esters do not
Name some examples of ester LA
Cocaine
Procaine
Tetracaine
Benzocaine
Give some examples of amide LA
Bupivacaine
Lidocaine (lignocaine)
Articaine (has an ester side-chain)
Prilocaine
Regarding the structure of amides and esters; they both contain an ________
aromatic ring
Ester LAs are metabolised by …
plasma esterases
Amide LAs are metabolised by…
hepatic enzymes
What does pharmacokinetics refer to ?
what the body does to the drug
ADME
What does pharmacodynamics refer to?
what the drug does to the body
Local anaesthetic drugs are classified as …
Weak bases
BH+ <—-> B + H+
B exists as the unionised form
At the physiological pH of 7.4, all local anaesthetics will exist in what form ?
more will be in the ionised form as opposed to the unionised from
What affects the proportions of ionised V unionised forms of LA at physiological pH 7.4?
The pKa
How much exist in ionised form
How much exist in unionised form
What percentage of lidocaine exists in the unionised form?
25%
What percentage of bupivacaine exists in the unionised form?
15%
Between lidocaine and bupivacaine, which one of these would reach their target site more quickly and why?
lidocaine reaches the target site more quickly and has a faster onset of action because more of it exists in the unionised form and thus can cross the membrane and reach the target site much quicker
Why would LA delivery be less effective in a patient with irreversible pulptitis?
“Hot tooth” will have infected/inflamed tissue; infected environments tend to have a lower (acidic) pH; this means that a greater proportion of LA will exist in the ionised form (BH+)
the ionised form is unable to penetrate the cell membrane and therefore is less effective
Areas of inflammation also have an increased body supply due to vasodilation and thus this might increase “wash-out” before it can reach the site of action on the neurone
What are the 4 principles of local anaesthetics?
Lipid solubility
Dissociation constant
Chemical linkage
Protein binding
Lipid solubility correlates to the _______ of LA
potency
The dissociation constant (pKa) correlates to the ___________.
time of onset
The chemical linkage correlates to the ___________ of LA
metabolism
Protein binding correlated to the ___________ of LA
duration
Briefly state why the potency (lipid solubility) of LA is important
greater lipid solubility enhances diffusion through neuronal coverings (myelin sheath) and the cell membrane, thus allowing lower milligram dosage
What is the importance of the pKa of LA
It determines the portion of an administered dose that exists in the lipid soluble, unionised form at given pH
LA with a lower pKa have a _________ proportion in the unionised form. What is the benefit of this
Greater proportion in unionised form
this leads to a quicker onset
Ester LAs are hydrolysed by …
plasma cholinesterases
The duration of action of LA is dependent on _________
Protein binding at the receptor site (Na channels)
A higher affinity for proteins means that the LA remains at the site of action for longer period of time
The affinity of LA for proteins at the receptor site (with sodium channels) corresponds to…
Their affinity for plasma proteins
if they bind tightly to plasma proteins then they will do the same at the receptor site
What is the effect of local anaesthetic on neurones?
They disrupt the ion channel function within the neuronal cell membrane.
Thus preventing the transmission of the neuronal action potential
What is the MOA of local anaesthetics?
Once the unionise form has diffused into the neuron
The ionised form then blocks the voltage gated Na+ channels via the open gate from the intracellular side of the membrane
It acts as a physical plug which repels positively charged Na+ (the ionised form is also positively charged)
Therefore blocks initiation and propagation of action potentials
When do LAs gain access to the voltage gated Na+ channel? What is the consequence of this on the depth of the block observed?
In its open state
Therefore, the depth of the block increasease with action potential firing
Use dependence
What is use dependence?
This is the selective inhibition of hyper active neurons while minimising the effects on normal neuronal activity
LA differentially blocks neurons. Nerve fibres associated with what sensory modality are blocked first?
Nerve fibres associated with pain are blocked before other sensory modalities such as touch, pressure and proprioception.