13. Local anaesthetics Flashcards
(14 cards)
Drug groups of local anaesthetics
Esters and amides
Active substances of esters
- Procaine
- Oxybuprocaine
- Tetracaine
Active substances of amides
- Lidocaine
- Bupivacaine
- Mepivacaine
- Ropivacaine
- Prilocaine
- Articaine
Mechanism of action of local anaesthetics
Local anaesthetics are weak bases
1. they are in unionized when crossing barriers - Easier for uncharged molecules to pass through the barriers
2. Once inside the nerve cells, they switch to inoized form, and they bind to the sodium channels and block them
3. By blocking these channels, the sodium cannot enter the nerve cells and the nerve cannot depolarize and create an electric signal (Action potential)
4. The nerve cannot send signals = no pain sensation
Pharmacological effect of local anaesthetics
- Not all nerves are blocked at once: small and non-insulated nerves first, small and insulated second, and last large and insulated nerves
- Sympathetic transmission is blocked first: nerves controlling sweating and constriction ⭢ later touch, pressure and movement
- Nervous function disappears in a specific order: ability to sense pain ⭢ warmth ⭢ touch ⭢ deep pressure ⭢ ability to move
- Return of the nervous function returns in the opposite order:
Ability to move ⭢ deep pressure ⭢ touch ⭢ warmth ⭢ pain
Insulated or non-insulate = myelinated or non-myelinated
Factors influencing the activity
1. Inflammation and low oxygen: Local anaesthetics do not work well in such areas
2. Vasoconstriction: Adding a vasoconstrictor, local anaesthetics can last longer and reduce the risk of side effects. The vasoconstrictors keep the local anaesthetics in one specific area for a longer time.
Pharmacokinetics of local anaesthetics, absorption:
1. Site of application: A cream on the skin will absorb different than an injection
2. Dosage: More drug = more absorption
3. Parallel use of vasoconstriction
4. Type of local anaesthetic: They all have different properties affecting the absorption
5. How long it lasts: The drug will work until the body absorbs enough of it to lower the concentration below a certain level
Pharmacokinetics of local anaesthetics, metabolism:
- Esters: Broken down quickly in the blood and liver. They are generally safe, but there are exceptions ⭢ Tetracaine
- Amides: Mostly broken down in the liver.
- Metabolites (waste): Removed by kidneys
Side effects of local anaesthetics
Relatively free from harmful side effects
- Allergy
- Asthmatic seizure
- Local tissue irritation
- Systemic effect – Due to elevated plasma concentration
- Cardiovascular effect: myocardial depression and vasodilation decreased blood pressure
- CNS signs: confusion, tremors, convulsions, respiratory depression
Toxicity of local anaestetics :
In increasing order:
Mepivacaine ⭢ Procaine / lidocaine ⭢ tetracaine / bupivacaine
Indications for local anaesthetics
1. Topical anaesthesia
Applied to the cornea, mucous membranes or the skin to loose sensation by paralyzing the nerve endings.
Used for conditions ulcers, painful disease and examinations
2. Local infiltration
Injecting small volumes of anaesthetic either SC or IM.
Used for minor surgeries and dentistry
3. Perineural injection
Blocks the peripheral nerves.
Used in density and minor surgeries on head, legs, hoofs
4. Intraarticular use
Used to diagnose lameness and for operations on joints
5. Intravenous regional anaesthesia
Anaesthetic injected IV, distal to a tourniquet
Used for operations, to cause extreme isolation from the rest of the circulations
Frequently used for operations of the digits in cattle
6. Intravenous usage
Refers to TIVA = Total Intravenous Anaesthesia
7. Epidural and spinal block
Spinal blocks are quick and strong for lower body surgeries, while the epidurals are slower but can be used for longer pain relief.
The injection spot depends on the animal.
Epidural block
Needs a larger amount of drug, it takes more time to work, but can be given continuously.
Disadvantage: Might not block pain completely
Used for obstetric operations and surgery of perineal region, anal region
Spinal block
Small amount of drug is needed, and it is a quick and strong block
Places of injections for spinal block
1. Horse, Cattle:
First or second intercoccygeal spaces ⭢ Caudal anaesthesia
Between the last lumbar and first sacral vertebrae
2. Dog, sheep, pig:
At the lumbosacral space ⭢ Lumbar anaesthesia
*Between the sacral and first coccygeal vertebrae *
