Inservice Exam - 2005
Local Anesethetics

A 46-year-old woman undergoes suction lipectomy of the abdomen using tumescent anesthesia containing lidocaine. Despite doses of lidocaine higher than recommended for local infiltration, toxicity to the drug does not develop in this patient. Which of the following is the most likely explanation?

(A) Chemical structure of lidocaine
(B) pH of the tissue
(C) Rate of lidocaine absorption
(D) Removal of lidocaine by suction
(E) Toxic threshold for plasma lidocaine concentration

The correct response is Option C.

Lidocaine for local infiltration is available in 0.5%, 1%, 1.5%, and 2% solutions that contain epinephrine at a concentration of 1:100,000 or 1:200,000. In contrast, lidocaine for tumescent anesthesia commonly is available in a 0.05% solution that contains epinephrine at a concentration of 1:1,000,000. This dilute solution results in a very slow rate of lidocaine absorption from subcutaneous tissue, which prevents high peak plasma concentrations of the drug as well as toxicity. Although a standard 1% solution yields a maximum plasma concentration of lidocaine in 1 hour, a tumescent solution provides a maximal plasma concentration in 8 to 12 hours.

The chemical structure of lidocaine can vary with the pH of the tissue. However, this is not a primary factor in the pharmacokinetics of tumescent lidocaine. Suction lipectomy removes some lidocaine, which reduces the peak plasma concentration by approximately 25% (compared with a nonBsuction-lipectomy control). However, this is not the primary mechanism that allows a fivefold increase in dose without toxicity. In human studies of the toxic threshold for plasma lidocaine, central nervous system symptoms occurred at concentrations above 4 _g/mL; this was equally true for standard and tumescent solutions.

References:
1. Scott DB, Jebson PJR, Braid P, et al. Factors affecting plasma levels of lignocaine and prilocaine. Br J Anaesth. 1972;44:1040-1048.
2. Klein JA. Tumescent technique for regional anesthesia permits lidocaine doses of 35 mg/kg for liposuction. J Dermatol Surg Oncol. 1990;16:248-263.
3. Coleman WP III. Tumescent anesthesia with a lidocaine dose of 55 mg/kg is safe for liposuction. Dermatol Surg. 1996;22:919.


A 27-year-old woman has a true allergy to a local anesthetic agent used in the past. Use of which of the following local anesthetic agents is most appropriate for this patient?

(A) Benzocaine
(B) Chloroprocaine
(C) Lidocaine
(D) Procaine
(E) Tetracaine

The correct response is Option C.

True allergic reactions to local anesthetics, although rare, can occur with the ester-linked agents chloroprocaine, tetracaine, procaine, and benzocaine. The degradation product of ester-linked anesthetics is para-aminobenzoic acid (PABA), which is an antigenic substance. The amide-type anesthetics lidocaine, bupivacaine, mepivacaine, and prilocaine do not cause true allergic reactions. Allergic reactions to amide-type anesthetics may occur but are usually related to the preservative methylparaben, which is structurally similar to PABA.

References:
1. Strombergh BV. Anesthesia. In: McCarthy JG, May JW, Littler JW, eds. Plastic Surgery. Vol 1. Philadelphia: WB Saunders; 1990:143-144.
2. Rizzuti RP, McArthur A, Riefkohl R. Commonly used drugs and their interactions. In: Georgiade GS, Riefkohl R, Levin LS, eds. Georgiade Plastic, Maxillofacial, and Reconstructive Surgery. 3rd ed. Baltimore: Williams & Wilkins; 1997:1327-1330.
3. Dentz ME, Grichnick KP, Silbert KS, et al. Anesthesia and postoperative analgesia. In: Sabiston DC, Lyerly HK, eds. Textbook of Surgery: The Biological Basis of Modern Surgical Practice. 15th ed. Philadelphia: WB Saunders; 1997:186-206.


Which of the following is LEAST sensitive to increasing plasma levels of lidocaine?

(A) Blood pressure
(B) Central nervous system activity
(C) Heart rate
(D) Muscle tone


The correct response is Option A.


Blood pressure is typically insensitive to increasing plasma levels of lidocaine and other local anesthetics because a compensatory increase in systemic vascular resistance prevents the blood pressure from increasing.


Adverse reactions in the central nervous system are much more common and are biphasic. Initially, an excitatory phase occurs, which may be due to inhibition of the amygdala. This phase may produce muscle twitching in the face and extremities followed by tremors that can progress to seizures. As the amount of local anesthetic increases, a depressive phase occurs and is characterized by drowsiness, unconsciousness, and respiratory arrest.


The cardiovascular system is thought to be more resistant than the central nervous system to the effects of local anesthetics. However, it can sustain dangerous reactions, usually at higher plasma levels. With toxic doses of local anesthetics, cardiovascular reactions may include arrhythmias, cardiovascular depression, and shock. Cardiovascular depression tends to be serious and difficult to treat. The more lipid-soluble local anesthetics, such as bupivacaine, tend to have a higher toxicity than the less lipid-soluble drugs, such as lidocaine.


Toxic effects of local anesthetics result from inappropriately high dosage or unintentional intravascular injection. Management of lidocaine toxicity consists of ECG monitoring and oxygen administration. If seizures occur, they are typically controlled with diazepam or midazolam. If mechanical ventilation is required, paralytic agents may be administered. Because of the seriousness of the toxic effects, appropriate monitoring and personnel trained in advanced cardiac life support are required during local anesthetic use, regardless of the magnitude of the procedure.


References:
1. Matarasso A. Lidocaine in ultrasound-assisted lipoplasty. Clin Plast Surg. 1999;26(3):431-439.
2. Trott SA, Beran SJ, Rohrich RJ, et al. Safety considerations and fluid resuscitation in liposuction: an analysis of 53 consecutive patients. Plast Reconstr Surg. 1998;102(6):2220-2229.
3. Vaughan TM, Burt J. Local anesthetics. Selected Readings in Plastic Surgery. 1999;9(4).
4. Cousins MJ, Bridenbaugh PO. Neural Blockade in Clinical Anesthesia and Management of Pain. 2nd ed. Philadelphia: LB Lippincott; 1988.
5. Mehra P, Caiazzo A, Maloney P. Lidocaine toxicity. Anesth Prog. 1999;45:38.
6 [No authors listed]: Cardiotoxicity of local anesthetic drugs [Editorial]. Lancet. 1986;2(8517):1192-1194.