Local Anesthesia

Question 1

What are the 2 main groups of local anesthetics?

Answer 1

• Esters

- Amides

Question 2

What is the issue associated with Ester local anesthetics?

Answer 2

• They elicit an allergic response

Question 3

What is the trick in remembering which local anesthetics are esters, and which are amides?

Answer 3

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

Question 4

What is the mechanism of action of local anesthetics?

Answer 4

• 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+

Question 5

What are the 3 types of nerve fibres?

Answer 5

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

Question 6

The more lipid soluble an anesthetic is, the _______

Answer 6

the easier it can diffuse across the cell membrane, thus it is more potent, and has an increased duration of action

Question 7

All local anesthetics are all weak ______

Answer 7

Are all weak bases, and thus need a HCl salt to make them water-soluble, and thus injectable

Question 8

Most local anesthetics have a pKa between ___ and ___

Answer 8

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

Question 9

Only the _____ portion of an anesthetic drug is able to exert its effects

Answer 9

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

Question 10

The perfect local anesthetic has what 3 properties?

Answer 10

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

Question 11

The higher the free drug, the higher the risk of ____

Answer 11

Toxicity

note: this is why patients with hypoalbuminemia have a higher risk of toxicity from local anesthetics

Question 12

What are the toxicity signs associated with local anesthetic?

Answer 12

• 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

Question 13

Which of the local anesthetics is known to have a higher chance of causing toxicity and why?

Answer 13

• 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

Question 14

Which drugs are similar to Bupivicaine but differ slightly in structure and have a markedly reduced chance of causing cardiac arrhythmias ?

Answer 14

• ropivicaine

- levobupivacaine

Question 15

What is the #1 greatest risk factor in causing toxicity from local anesthetic?

Answer 15

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

Question 16

What is the treatment for Methemoglobinemia ?

Answer 16

Antioxidants!

• Methylene blue
• Ascorbic acid
• N-acetylcysteine

Question 17

What is the treatment for local anesthetic systemic toxicity?

Answer 17

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

Question 18

What is the onset, duration and use for Procaine/ “Adrenocaine”?

Answer 18

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

Question 19

What is the use, onset, and duration for Lidocaine?

Answer 19

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

Question 20

What is the pKa and %protein bound of Lidocaine?

Answer 20

pKa: 7.7= fast onset

65% protein bound = metabolized relatively quickly and therefore only lasts 90mins

Question 21

Can Lidocaine with adrenaline be used on the extremities of animals?

Answer 21

No!

The adrenaline in the mixture causes profound vasoconstriction and blood supply is cut off, leading to necrosis of the extremities

Question 22

What is EMLA?

Answer 22

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

Question 23

What is the use, onset, duration, pKa and %protein binding of Prilocaine?

Answer 23

• Use: as mixture in EMLA cream
• Onset: Fast
• Duration: Moderate
• pKa: 7.7
• 55% protein binding

Question 24

What is the use, onset, duration, pKa and %protein binding of Mepivacaine?

Answer 24

• Use: Diagnostic limb blocks in horses
• Onset: Fast (11 mins)
• Duration: Good (130mins)
• pKa: 7.7
• 80% protein binding

Question 25

What is the use, onset, duration, pKa and %protein binding of Bupivacaine?

Answer 25

• 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

Question 26

What is the use, onset, duration, pKa and %protein binding of Ropicavaine?

Answer 26

• Use: local analgesia and epidurals
• Onset: Moderate
• Duration: Excellent (Up to 8hrs)
• pKa: 8.1
• 94% protein binding

Question 27

What is the mechanism of toxicity of bupivacaine after accidental intraveouns adminstration?

Answer 27

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