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What is the aim of local anaesthetics, with regards to the pain pathway?
The aim of local anaesthetics is to prevent the transmission of pain up to the CNS; to block the pain pathway.
What is the resting membrane potential?
What is a) depolarisation b) repolarisation?
- a) When more Na+ moves into the cell
- b) When Na+ channels become inactive and K+ channels open
What effect do local anaesthetics have on voltage gated Na+ channels? What does this do to an AP?
They block voltage gated Na+ channels which blocks an AP and stops the signal being sent through the CNS
List some of the clinical uses of local anaesthetics
- Topical anaesthesia
- Local infiltration
- Instillation into a cavity or wound
- Intravenous regional anaesthesia
- Peripheral nerve blocks
- Epidural block
- Systemic administration
Local anaesthetics are weak acids/bases?
Describe the mechanism of action of local anaesthetics
The uncharged weak base exists in equilibrium with the charged form. Acidic environments pushes the equilibrium to the charged form. The channels are too narrow for the drug to move into them so the drug has to pass through the axonal membrane to get into the channel. Only the uncharged form can cross the cell membrane. It must them pick up an H+ ion as only the charged form can block the Na+ channel.
What three states can channels exist in?
Open, closed or inactivated
Describe the principle of use dependence in local anaesthetics
The depth of local anaesthetic blockage increases with AP frequency - the more frequently the channel is used the better the action of the drug
What type of channels have a higher affinity for local anaesthetics?
Open and inactivated channels have a higher affinity for LAs than closed channels
What type of nerves are more susceptible to the effects of local anaesthetics?
Nerves which carry pain signals are more susceptible than other types of nerves. This means you can block pain transmission whilst maintaining a reasonable degree of motor function.
Describe the chemical structure of the local anaesthetics
Local anaesthetics are weak base. They consist of an aromatic region and a basic amine side chain. These are combined by an ester or amide bond.
With regards to pH, why will local anaesthetics not work as well in areas of inflammation?
In areas of inflammation the pH tends to fall so there are more H+ ions present and more ionisation of the local anaesthetics. The drug will not be able to pass through the membrane of the inflamed tissue as well because there is more of it in the charged form.
Describe the pharmacokinetics of the local anaesthetics
- The speed of onset is related to the degree of ionisation - the more uncharged molecules present the more easily the drug passes through the membrane and the faster the onset
- Duration is related to protein binding - LAs with a higher degree of protein binding have a longer duration of action
- Potency is related to lipid solubility - the more lipid soluble the higher the potency and more rapid the onset
- Other drugs may be added e.g. bicarbonate (increases pH), adrenaline (vasoconstricts)
Describe how local anaesthetics are eliminated from the body
- Ester linked drugs are rapidly hydrolysed by non-specific cholinesterases
- Amide linked drugs are metabolised in the liver
Which has a longer half life: ester linked or amide linked LAs?
Amide linked local anaesthetics
What side effects can occur with the local anaesthetics?
- CNS - initial stimulation leading to convulsions, later depression leading to coma and death
- CVS - myocardial contractility and heart rate fall, peripheral vasodilation
- MISC - allergica reactions are rare, methaemoglobinaemia
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