Local Anesthetics

Question 1

Name the 3 basic components of all local anesthetics.

Answer 1

1. A lipophilic group (usually a benzene ring).
2. A hydrophilic group (tertiary amine).
3. An intermediate bond (a hydrocarbon chain).

Question 2

What are the 2 types of local anesthetics, and how are they classified?

Answer 2

esters = procaine, cocaine, chloroprocaine, and tetracaine

amides = lidocaine, bupivacaine, ropivacaine, mepivacaine, and prilocaine
- contain an “i” before the “-caine”

Question 3

List the most common sites of injection in order of speed of systemic uptake (from fastest to slowest).

Answer 3

Fastest = highly vascular areas

Intravenous > tracheal > intercostal > caudal > paracervical > epidural > brachial plexus > sciatic > subcutaneous

Question 4

What is the definition of pKa?

Answer 4

the pH = amounts ionized and un-ionized forms

low pKa = higher proportion of the un-ionized, lipid soluble form at normal tissue pH

lipid solubility –> potency
pKa –> speed of onset = lower pKa = faster onset

Question 5

Which local anesthetics have the highest pKa, and the lowest pKa?

Answer 5

Procaine pKa 8.9

Chloroprocaine 8.7

Tetracaine 8.5

Bupivacaine 8.1

Ropivacaine 8.1

Lidocaine 7.9

Prilocaine 7.9

Mepivacaine 7.6

Question 6

How do local anesthetics block transmission of sensory and motor signals along the neuron?

Answer 6

bind and inhibit the voltage-gated Na channels –> blocking propagation of the action potential.

Question 7

How are ester and amide local anesthetics metabolized?

Answer 7

Esters = in the plasma by pseudocholinesterase enzymes

Amides = degraded in the liver by microsomal P-450 enzymes.

Question 8

How does epinephrine prolong the action of local anesthetics?

Answer 8

vasoconstricting nearby blood vessels –> decreasing blood flow and slowing systemic absorption of the local anesthetic

Question 9

Name the two organ systems most affected by local anesthetic toxicity.

Answer 9

neurological

cardiovascular

Question 10

Name three manifestations of lidocaine toxicity in the central nervous system.

Answer 10

perioral numbness and tinnitus –> seizures and coma

initial excitatory phase = shivering, muscle tremors, and tonic-clonic seizures

progresses into a depressant phase = coma, hypoventilation and respiratory arrest.

Question 11

Name the symptoms of cauda equina syndrome.

Answer 11

bowel and bladder dysfunction
perineal sensory loss
lower extremity motor weakness

Question 12

What is Transient Neurological Symptoms and name three risk factors associated with its development.

Answer 12

unilateral or bilateral radicular PAIN - buttocks, lower back, and sometimes the lower extremities.

Risk factors = use of lidocaine, the lithotomy position, obesity, knee arthroscopy, and outpatient surgery.

Question 13

How does local anesthetic toxicity affect cardiac function?

Answer 13

inhibition of sodium, potassium and calcium channels, which prolong myocardial action potentials

1. prolongation of PR and QRS intervals
2. depression of the SA and AV nodes
3. QT interval is prolonged–> ventricular tachycardia, torsade de pointes and ventricular fibrillation

bupivacaine is the most arrhythmogenic, most refractory to resuscitation, resistant to traditional resuscitation may require cardiopulmonary bypass

Question 14

What local anesthetic class is most associated with allergic reactions?

Answer 14

Esters - due to cross-reactivity to p-aminobenzoic acid (PABA).

Esters - metabolized to PABA and can cause reactions in patients sensitive to this compound

Question 15

How do you give intralipid?

Answer 15

20% Intralipid emulsion begins with a bolus of 1.5 mL/kg intravenously over one minute, followed by a continuous infusion of 0.25 mL/kg/min

Question 16

Name the local anesthetic and its metabolite associated with methemoglobinemia.

Answer 16

prilocaine?
Benzocaine spray

O-toluidine is the hepatic metabolite of prilocaine. It oxidizes the heme group’s ferrous (Fe2+) form to the ferric (Fe3+) form, resulting in methemoglobin, making it incapable of transporting oxygen.

Question 17

What does the addition of sodium bicarbonate do local anesthetics solution do?

Answer 17

raises pH –> more un-ionized molecules = faster onset

Question 18

What is the time of onset and duration of Transient Neurological Symptoms?

Answer 18

onset = 12 to 24 hours after spinal&raquo_space;> epidural anesthesia

duration = 6 hours to 4 days, rarely 7 or more days

Question 19

Max dose of lidocaine?

Answer 19

without epi = 300mg

WITH EPI = 500mg