Pharmacology - Local Anesthetics (Practice)

The largest category of LA questions focuses on your ability to distinguish amide LAs from esters: (This I hope is deemphasized, since amide local anesthetics are used almost exclusively now)

esters = procaine, tetracaine, cocaine. All the rest are amides: lidocaine, mepivacaine, bupivacaine, prilocaine, dibucaine. They also require you to know that amides are metabolized in the liver, esters mainly by esterases in plasma. An infrequent question asks which class of drugs has the most consistency in structure . LAs are the drug group most consistent in drug structure, because LAs are either amides or esters, differing only in their structure in the intermediate chain (its either an amide or an ester) that connects the aromatic group to the secondary or tertiary amino terminus.

II. The next category of questions has to do with toxic reactions to local anesthetics, either due to high systemic levels of local anesthetics in general (cardiovascular collapse due to myocardial depression, hypotensive shock) or to a specific agent such as prilocaine, which causes methemoglobinemia.

III. A 3rd class of questions are aimed at your knowledge of the mechanism of action of local anesthetics: they prevent the generation of nerve impulses by interfering with sodium transport into the neuron.

IV. The last most frequent type of question regarding local anesthetics has to do with issues regarding absorption of local anesthetics. Remember, only the non-ionized (or free base form) form can penetrate tissue membranes. Inflamed tissue has a lower than normal pH, which decreases the amount of non-ionized form available to penetrate.

V. Usually at least one question comes up asking you to calculate how many mg of local anesthetic a patient has received, e.g. how many mg of lidocaine in 1.8 ml of a 2% lidocaine solution? 2% lidocaine is 20 gm/100 ml or 20 mg/1 ml, so 36 in 1.8 ml.