Clinical Lecture: Local Anaesthesia Flashcards
Give forms of non-pharmacological local anaesthesia.
- Cold Temperature
- Pressure
- Hypoxia e.g. tight tourniquet causing the area to go numb
What is a local anaesthetic?
A drug that can reversibility prevent transmission of the nerve impulse in the region in which it is applied without affecting consciousness.
How do local anaesthetics work?
They block sodium channels where applied to impede flow of action potentials.
What layers of connective tissue must a LA penetrate through at the level of the nerve?
LA must penetrate the epineurium, perineum and endoneurium to work. As a result it can take some time for LAs to work.
What are the two structures of local anaesthetics?
Two basic classes of local anesthetics exist, the amino amides and the amino esters. Amino amides have an intermediate amide link between the aromatic chain and the hydrophilic group, whereas amino esters have an ester link between the aromatic residue and hydrophilic residue.
How do we differentiate between amide and ester LAs?
All local anaesthetics end in “caine.” If this suffix preceded by an “i” anywhere it is an amide. An example of this include lidocaine or bupivacaine. If there is no “i” before the suffix the LA is an ester. An example of ester Las include chloroprocaine and procaine.
Which drug has the faster onset of action:
(a) Lidocaine pKa 7.8
(b) Procaine pKa 7.7
(c) Ropivacaine pKa 8.1
(d) Bupivacaine pKa 8.1
Body pH ~ 7.4
Pus pH ~ 6.9
The greater the concentration of unionised form of a drug correlates with a faster onset of action. This is as only the unionised form of a drug can pass through the membrane.
When the pKa = pH then the unionised form of the drug = ionised from. If pKa > pH then the unionised form > ionised form. The further away from the pKa of the LA is from the body pH the lesser is the unionised form.
Therefore the answer = Procaine
Why is the effectiveness of a potentially LA reduced in infection?
Procaine here works the quickest. If there is pus or inflammation, the pH in the area is reduced. The difference between the pH and pKa goes up. The amount of unionised drug decreases significantly. The effectiveness of LA is reduced. If we inject LA into already infected tissue LA may not work well or not at all.
Why is the clinical onset not the same for LAs with the same pKA?
- The individual local anaesthetic ability to diffuse through connective tissue may not be the same
- The closer the pKA ti physiologically pH the faster the onset generally. There are some exceptions such as chloroprocaine and benzocaine
- LAs are less effective in inflamed tissue. Inflammation results in local acidosis which decreases the pH of the surrounding tissue
What does the duration of action of an LA depend on?
Duration of action depends on protein binding. The more protein bound, the longer the duration of the action.
– Lignocaine - 65%
– Bupivacaine is 95%
– Procaine 6%
Therefore Bupivacaine has a longer duration of action - 12-14 hours.
The length of the intermediate chain joining the aromatic and amine group also determines the duration of action.
What does potency of an LA depend on?
Lipid solubility - The more lipid soluble the drug the more it is able to penetrate the cell membrane. This means a smaller amount is required to produce a given effect.
What is the most common additive added to LAs?
Vasoconstrictors. These are used to prolong action, reduce plasma levels, reduced operative haemorrhage and reduce the dose. The most common include adrenaline felypressin (analogue of vasopressin).
Vasoconstrictors mean that less of the LA is absorbed into the blood and stay in the area for longer. Areas supplied by only one artery such as fingers, penis and ear lobule cannot have vasoconstrictors added to the LA. This is as it can lead to necrosis.
What are side effects of LAs?
- Hypersesnitivity - this is rare with amides though
- Methaemoglobinaemia
- Toxicity- starts with arrhythmia, headaches and tinnitus, and progresses to conclusions, coma and then respiratory arrest and CVS depression. Since the CVS system is last to be affected we can spot the affects of toxicity before.
