Local Anesthesia Flashcards
What are the 2 main groups of local anesthetics?
- Esters
- Amides
What is the issue associated with Ester local anesthetics?
- They elicit an allergic response
What is the trick in remembering which local anesthetics are esters, and which are amides?
If the drug has an ‘i’ before ‘-caine’ it is an Amide
- e.g. Bup-i-vicaine
If the drug doesnt, then it is an ester
What is the mechanism of action of local anesthetics?
- Produce a reversible loss of sensation
- No loss of consciousness or change in CNS function occurs
- Can help avoid general anesthesia in some cases
note: they inhibit BOTH the initiation and conduction of action potentials, by interefering with both Na+ and K+
What are the 3 types of nerve fibres?
A: myelinated, largest and conduct pressure and motor sensations along with fast pain
B: myelinated, moderate size
C: transmit slow, dull and burning pain and temp sensations are the smallest and unmyelinated
The more lipid soluble an anesthetic is, the _______
the easier it can diffuse across the cell membrane, thus it is more potent, and has an increased duration of action
All local anesthetics are all weak ______
Are all weak bases, and thus need a HCl salt to make them water-soluble, and thus injectable
Most local anesthetics have a pKa between ___ and ___
7.7 and 8.9
the lower the pKa, the closer it is to the physiological pH (7.4) and therefore a higher proportion of the drug will be un-ionized and have a faster onset of action
Only the _____ portion of an anesthetic drug is able to exert its effects
Unbound
note: the more protein bound a drug is, the longer the duration of action, because it takes longer for the body to metabolize and excrete the drug
The perfect local anesthetic has what 3 properties?
1) Highly lipid soluble = potent + increased duration of action
2) Low pKa = fast onset
3) High protein-binding = longer duration of action, slower to be metabolized and less likely to cause toxicity
The higher the free drug, the higher the risk of ____
Toxicity
note: this is why patients with hypoalbuminemia have a higher risk of toxicity from local anesthetics
What are the toxicity signs associated with local anesthetic?
- CNS and CVS: tongue numbness, light-headedness, involuntary muscle twitches, seizures, respiratory depression and collapse
- Methemoglobinemia: oxidization of hemoglobin to Fe3+, unable to transport and deliver oxygen to tissues resulting in poor perfusion and cyonosis
- Allergic reactions: to the preservatives or to ester agents due to the metabolism to PABA
Which of the local anesthetics is known to have a higher chance of causing toxicity and why?
- Bupivicaine
- It binds to cardiac cells 10x more than lidocaine, which can cause arrythmias which are difficult to treat, and can lead to cardiac arrest
Which drugs are similar to Bupivicaine but differ slightly in structure and have a markedly reduced chance of causing cardiac arrhythmias ?
- ropivicaine
- levobupivacaine
What is the #1 greatest risk factor in causing toxicity from local anesthetic?
Accidentally administering local anesthetic IV
- This is why you must always draw back on the syringe to ensure you are NOT in a vessel
- If you are, then remove the needle/syringe and readjust
note: there is only one drug that can be adminstered IV, which is lidocaine (in dogs, horses and farm animals)
Cats have a very low threshold for IV lidocaine
What is the treatment for Methemoglobinemia ?
Antioxidants!
- Methylene blue
- Ascorbic acid
- N-acetylcysteine
What is the treatment for local anesthetic systemic toxicity?
1) Call for help
2) Stop all anesthesia
3) Maintain airways and provide positive pressure ventilation if needed
4) Seizures can be stopped with Midazolam or Diazepam
5) IV administration of Lipid Emulsion Therapy to help soak up the local anesthetic
6) Fluid therapy to maintain blood pressure
7) positive ionotrope and chronotrope drugs can be given
8) Ventricular arrhythmias are treated with amiodarone
What is the onset, duration and use for Procaine/ “Adrenocaine”?
Use: Large animal nerve blocks e.g. dehorning, along the line of incision and also prolongs the effect of penicillin
Onset: rapid
Duration: 30 - 60 mins
What is the use, onset, and duration for Lidocaine?
Use: ventricular tachycardia arrhythmia, GI ileus (prokinetic), anti-endotoxic and analgesic for visceral pain, and infusion during GA, testicular block
Onset: Fast (2-5 mins)
Duration: moderate, up to 90mins
What is the pKa and %protein bound of Lidocaine?
pKa: 7.7= fast onset
65% protein bound = metabolized relatively quickly and therefore only lasts 90mins
Can Lidocaine with adrenaline be used on the extremities of animals?
No!
The adrenaline in the mixture causes profound vasoconstriction and blood supply is cut off, leading to necrosis of the extremities
What is EMLA?
A Eutectic Mixture of Local Anesthetics, of lidocaine and prilocaine.
It is to be used on the intact skin for 60mins to allow for local anesthesia prior to venous catheter placement
caution: it should NOT be used on mucous membranes or ingested
What is the use, onset, duration, pKa and %protein binding of Prilocaine?
- Use: as mixture in EMLA cream
- Onset: Fast
- Duration: Moderate
- pKa: 7.7
- 55% protein binding
What is the use, onset, duration, pKa and %protein binding of Mepivacaine?
- Use: Diagnostic limb blocks in horses
- Onset: Fast (11 mins)
- Duration: Good (130mins)
- pKa: 7.7
- 80% protein binding
What is the use, onset, duration, pKa and %protein binding of Bupivacaine?
- Use: local analgesia and epidurals
- Onset: Good (30 mins)
- Duration: Excellent (Up to 8hrs)
- pKa: 8.1
- 95% protein binding
Caution: it should NEVER be administered IV
What is the use, onset, duration, pKa and %protein binding of Ropicavaine?
- Use: local analgesia and epidurals
- Onset: Moderate
- Duration: Excellent (Up to 8hrs)
- pKa: 8.1
- 94% protein binding
What is the mechanism of toxicity of bupivacaine after accidental intraveouns adminstration?
Cardiac conduction and excitability is depressed leading to AV block, ventricular arrhythmias and cardiac arrest
Myocardial contraction is depressed, and peripheral vasodilation occurs, leading to decreased cardiac output and blood pressure
