Flashcards in Local Anaesthetics Deck (20)
Describe when local anaesthetics are used
- When there is no need for patient to be unconscious
- As an adjustment to surgery to avoid general anaesthetics
- Post operative analgesia (pain relief)
- Used with sedation for general surgery.
What are the two types of structural local anaesthetics and give a small amount of information about them
Ester linked or Amide linked.
Amide linked are more commonly used are they are far more stable and are longer lasting.
Ester linked are more easily broken down so have a shorter duration of life. The breakdown products of ester linked are more associated to allergic reactions
Name some of the most commonly used amide containing local anaesthetics
Lidocaine - medium acting, rapid onset
Prilocaine - medium acting, no vasodilation
Bupivacaine - long acting, slow onset
Articane - rapid and short acting
Name some of the Ester-containing local anaesthetics
These are very rarely used
Tetracaine - long acting, very slow onset
Choloprocaine - medium-acting
Benzocaine - atypical mechanism of onset
Describe the mechanism of action
They work by reversibly blocking voltage gated sodium channels. This stabilised excitable cell membranes and prevents membranes from depolarising.
Describe the order of loss when using local anaesthetics
-loss of pain
-loss of temperature
-loss of proprioception (awareness of body)
-loss of skeletal muscle tone
What can effect the LA's ability to block an impulse
Diameter of the nerve fibre, smaller nerves are blocked more easily,
Myelination status, myelinated fibres are blocked more easily,
Length of the nerve,
Length of time exposed to drug,
Concentration of drug.
What form of local anaesthetics will block the sodium pump?
Local anaesthetics are only able to block the sodium pump in their ionised form. However they cannot pass through the membrane in their ionised form.
What type of acid/base are local anaesthetics?
They are a weak base meaning they only partially dissociate in an aqueous solution.
Define pKa and what are the pKa values for LA
pKa is the pH at which 50% of the drug is dissociated. The value for most LAs is between 8 and 9.
What is the onset of local anaesthetics determined by?
Since they find to open or inactive channels, the onset is determined by the rate at which neurons fire. Faster neuronal firing, faster the onset
What is the duration of the drug determined by?
The rate of removal. This can be influenced by;
blood flow, action of plasma easterases (for ester linked only)
and the hydrophobicity of the drug.
How can you increase the duration of action?
By decreasing the blood flow to the area. This can be done by administering adrenaline to cause vasoconstriction. This can double the duration, and decrease bleeding during surgery however can cause ischaemic injury.
What LAs do not cause vasodilation
Prilocaine and cocaine (causes vasoconstriction)
Describe some of the routes of administering local anaesthetics
Surface (nose, mouth, urinary tract)
Infiltration (injection into tissues to reach nerve branch)
Nerve block (injecting around the nerve)
Intravenous regional (Double-cuff method to contain LA to limb)
Extradural/epidural (Particularly bupivacaine, used in thoracic, lumbar and sacral regions)
Subarachnoid (Drug injected into subarachnoid space)
Describe some features of an eutectic mixture of local anaesthetics
It is used for dermal anaesthetics and contains a mixture of lignocaine and prilocaine.
The melting point of the mixture is lower than that of the chemicals alone so a higher concentration can be used and there is a greater chance of LAs crossing the skin.
Describe some of the side effects of local anaesthetics
-Occurs when LAs entre the systemic circulation,
In the CVS; Dysrhythmias and sudden fall in blood pressure
In the CNS; Tremors, restlessness, convulstions, respiratory centre depression and death.
Why are log scales used?
Because they allows for big concentration differences to be more easily compared
What equation is used to determine the concentration of the ionised/unionised version of the drug
pKa-pH = log 10 (BH+/B)
With BH+ being the ionised form and B being the unionised form.