A 35-year-old woman presents to general surgery resident clinic with a 3 × 4 cm soft tissue mass on her upper back that she would like to have removed. She states that the mass has been there for several years, but recently started growing after she hit that portion of her back on a door. The mass is soft and mobile and pictured in Figure 52-1. Her lymph nodes are all normal and the mass feels like a lipoma.
A picture of a lipoma that was removed in clinic under local anesthetic.
1. What are the 2 main classes of local anesthetics?
2. What is the mechanism of action of local anesthetics?
Local anesthetics can be separated into 2 distinct groups called amides or esters based on their chemical structures. All anesthetics that end in “-caine” and contain the letter i in the prefix are amide agents. An easy way to remember this for the ABSITE is that the word amide also has an “i” in it. So, for example, lidocaine is an amide and cetacaine is an ester. Esters have the advantage of being faster acting. The disadvantages of ester anesthetics are that they have a shorter shelf life and cannot be combined with epinephrine.
Remember from your preclinical years that pain signals are transmitted via action potentials along the peripheral nerves. Local anesthetics act by reversibly blocking these nerve impulses by disrupting cell membrane permeability to sodium during an action potential. The basic pharmacokinetics of common local anesthetics is illustrated in Table 58-1. The 2 most common local anesthetics general surgery residents use are lidocaine (Xylocaine®) and bupivacaine (Marcaine®). Both take several minutes to take effect after injection and last anywhere from several hours in the case of lidocaine to many hours for bupivacaine.
The most important detail to remember when using these agents is to keep track of the total amount of anesthetic given. This is especially important when you may be suturing by yourself in the emergency department on a patient with multiple lacerations. If you think you will need a lot of local anesthetic due to multiple lacerations, then the 1% concentration of lidocaine (10 mg/mL) or 0.5% bupivacaine (5 mg/mL) is a convenient choice. For an average patient with no hepatic impairment, up to 4.5 mg/kg of lidocaine or 2.5 mg/kg of bupivacaine can be given during the procedure.
Epinephrine is combined with these anesthetics in various concentrations and helps to promote vasoconstriction of nearby vessels. This provides several benefits including lower required dosages of anesthetic, prolonged duration of action, and improved control of local bleeding. Local anesthetic with epinephrine is contraindicated in areas of the body with end ...