Chapter 11: Local anaesthetics history and pharmacology Flashcards
Definition of local anaesthetics
Loss of sensation in a circumscribed area of the body caused by a depression of excitation in nerve endings to an inhibition of the conduction process in peripheral nerves
What happened in the year 1884?
Koller: ophthalmologic meeting in Vienna: cocaine as LA
What happened in the year 1885?
Halsted injects cocaine into the mandibular nerve (through Six spine) and brachial plexus
Objective of local anaesthetics
Use of a substance that induces a transient and completely reversible state of anaesthesia
What happens when local anaesthetics are used for pain control?
They cease to provide a clinical effect when they are absorbed from the site of injection into the circulation
What is the prime factor of LA?
The redistribution of the LA from the nerve fibre into the cardiovascular system
What are the desirable properties of LA?
- Not irritating to the tissue
- To not cause any permanent alteration of nerve structure
- Low systemic toxicity
- Effective regardless of whether it is injected into tissue or applied to the mucous membrane
- Time of onset of anaesthesia should always be as short as possible
- The duration must be long enough to permit the completion of the procedure
Action of LA
- LA prevent the generation and conduction of nerve impulses
- Provoke a chemical roadblock between the source of impulse and the brain
- The neutron is the structural unit of the nervous system
Physiology of the nerves
- Nerves carry impulses
- Impulses are irritated by the chemical, thermal, mechanical or electrical stimuli
What are the mode and site of action of LA?
- Altering the basic resting potential of the nerve membrane
- Altering the threshold potential
- Decreasing the rate of depolarisation
- Prolonging the rate of repolarisation
- Primary effects of LA occur during the depolarisation phase
- LA binds to specific receptors in the sodium channels
When do the primary effects of LA occur?
During the depolarisation phase
Do LA increase or decrease the depolarisation?
They decrease it
LA binds to specific receptors in the _____ channels
Sodium
pH of LA solutions is between _____
5-5.7
What happens when LA is injected into the tissue (regarding the pH)
The buffering capacity of the tissue fluids returns the pH at the site to a normal pH of 7.4
Most LA are secondary amines. True or False
False. Most LA are tertiary amines
Tissue pH value
7.4
LA are basic compounds poorly soluble in water. True or False
True
LA pKa value
From 7.5-10
LA combine with _____ to LA _____
- acids
- salts (hydrochloric salts dissolved in sterile water or saline solution)
When pKa is close to the pH of tissue, we have a more ____ onset because we have a high portion of ______ ______ that can cross the ionised membrane.
- rapid
- non-ionised base
What does the pKa affect?
The action
What does the lipid solubility affect?
The anaesthetic potency
What does protein binding affect?
The duration
