Local Anesthetic Calculations: Avoiding Trouble with Pediatric Patients
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Anesthesia and Pain Control Local anesthetic calculations: avoiding trouble with pediatric patients Mana Saraghi, DMD n Paul A. Moore, DMD, PhD, MPH n Elliot V. Hersh, DMD, MS, PhD Local anesthetic systemic toxicity (LAST) is a rare but avoidable will also be reviewed, as well as the appropriate treatment procedures consequence of local anesthetic overdose. This article will review for a local anesthetic overdose. the mechanism of action of local anesthetic toxicity and the signs Received: July 17, 2013 and symptoms of LAST. Due to physiologic and anatomic differences Revised: October 30, 2013 between children and adults, LAST occurs more frequently in Accepted: February 10, 2014 children; particularly when 3% mepivacaine is administered. The calculation of the maximum recommended dose based on mg/lb body Key words: local anesthetic toxicity, systemic toxicity, maximum weight, Clark’s rule, and the Rule of 25 in order to prevent LAST recommended dose, Clark’s rule, lidocaine, mepivacaine pproximately 1 million cartridges shorter duration of soft tissue anesthesia neurons are blocked to such a profound of local anesthetic are used each and prevent postoperative self-inflicted level that CNS and respiratory depression, Aday in the United States.1 Local lip and cheek trauma.10,11 unconsciousness, and respiratory arrest anesthetic systemic toxicity (LAST) is can occur. At even higher plasma concen- dose-related and although rare, occurs Local anesthesia: mechanism trations, systemic vasodilation results in more frequently in small children than of action and toxicity significant hypotension and cardiovascular adults. LAST occurs more frequently Local anesthetics are essential for intra- depression. Local anesthetics also block when the patient is administered con- operative dental analgesia; they work by sodium channels in the myocardium, comitant central nervous system (CNS) blocking sodium channels in neurons resulting in bradycardia. Bradycardia is a depressants, such as opioid/sedative so that pain signals from the periphery major cause of concern when bupivacaine medications.2-9 cannot be transmitted to the CNS. LAST is used, as it can induce a use-dependent The following case serves as a reminder is mediated by the same mechanism when blockade at normal heart rates.12,13 Because to proceed cautiously when administering the maximum recommended dose (MRD) of its extended duration of action, bupiva- routine local anesthetic, always keeping is exceeded. This dose-related toxicity caine is rarely indicated for children. The weight-based dosing in mind. A 50-lb, is especially important as the sodium sequelae of depressed cardiac conduction 8-year-old girl with a history of extensive channels in the cardiovascular system are include atrioventricular block, ventricular caries and dental fear—but otherwise blocked along with those in the CNS.12,13 arrhythmias, cardiac arrest, and ultimately, no medical problems, diseases, or aller- Regardless of which local anesthetic death. A local anesthetic overdose can gies—presented for multiple extractions. is administered, the same progression of result in significant morbidity and mor- For the initial sedation, the patient effects on the CNS and cardiovascular tality unless life support interventions received oral promethazine, as well as system occur with increasing plasma levels can be initiated following standard basic nitrous oxide-oxygen inhalational seda- of local anesthetic.12,13 Symptoms of early and advanced cardiac life support guide- tion. A half hour later, the sedation was toxicity consist of numbness and tingling lines.12,13 Concomitant opioid sedative supplemented with an intramuscular dose of the mouth and lips, metallic taste, administration will augment respiratory of meperidine. After another half hour diplopia, tinnitus, nausea, dizziness, and depression and decrease the seizure thresh- had elapsed, the child received injections drowsiness.12,13 These reactions are usually old of local anesthetics.3,5,6,12-15 of 6 cartridges of 3% mepivacaine plain self-limiting and often are due to a mild (without a vasoconstrictor). Seizures and overdose or an inadvertent intravascular LAST: a greater tendency in respiratory distress followed 5 minutes injection. As the plasma concentrations pediatric patients later. Resuscitation efforts followed, but of local anesthetic increase, the inhibitory There are some important physiological were unsuccessful, and the patient died of neurons in the CNS are blocked, leaving differences between children and adults anoxic encephalopathy.5 excitatory neurons unopposed. Clinically, that play a role in the greater tendency The most common cause of morbidity this manifests as tremors and tonic-clonic for LAST to be reported in the pediatric and mortality due to LAST is respira- (also known as grand mal) seizures. CNS population. Seated in the dental chair, a tory depression or apnea.10 LAST occurs arousal may stimulate the cardiovascular child may appear deceptively large. The more frequently in children when 3% system, possibly resulting in hyperten- reason that the child appears to be larger mepivacaine is administered, with the sion, tachycardia, and increased cardiac is that in the dental chair—with a bib, false presumption that a local anesthetic output.12,13 At higher plasma levels of local napkin, or blanket—only the child’s dis- without a vasoconstrictor will have a anesthetic, both excitatory and inhibitory proportionately large head is visible.4 This 48 January/February 2015 General Dentistry www.agd.org 6’ 6’ 3’ than a local anesthetic with a vasoconstric- 1.00 Head size 0.75 tor, such as 2% lidocaine with 1:100,000 epinephrine.11,12 Mepivacaine does offer 5’ shorter pulpal anesthesia (20-40 minutes) as compared to lidocaine with epinephrine (60-90 minutes), but soft tissue anesthe- 4’ sia is similar between the 2 anesthetics: 120-180 minutes and 120-240 minutes for mepivacaine plain and lidocaine with 11 3’ epinephrine, respectively. Hersh et al Height found that “the onset of soft tissue numb- ness, peak numbness effects, and numb- 2’ ness duration were quite similar” when comparing 3% mepivacaine plain and 2% lidocaine with epinephrine.11 1’ Using 3% mepivacaine plain instead of 2% lidocaine with epinephrine does not provide any benefit with respect to the prevention of postoperative lip/mouth trauma, but the higher concentration of Figure. Diagram comparing the relative proportions in height vs head size between a 3.5-year-old child local anesthetic in the 3% mepivacaine and an adult. solution makes it easier to reach or exceed the MRD.3,4,11,16,17 A brief review of local anesthetic calculations illustrates this point: a 2% formulation of a drug means Table 1. Local anesthetic calculation: amount of local anesthetic in cartridges. that there is 2 grams of drug in 100 ml volume. If 2 grams are in 100 ml, then 2% anesthetic = 2 grams/100 ml in volume = 2000 mg/100 ml = 20 mg/ml 2000 mg are in 100 ml, which means 3% anesthetic = 3 grams/100 ml in volume = 3000 mg/100 ml = 30 mg/ml that 20 mg are in each ml. Since a dental 1 cartridge of local anesthetic is 1.8 ml in volume (exception: 4% articaine has 1.7 ml) cartridge contains approximately 1.8 ml volume, then there are 36 mg drug per Therefore… cartridge (Table 1). Similarly, when a drug 2% cartridge: 20 mg/ml x 1.8 ml/cartridge = 36 mg/cartridge is in a 3% formulation, there are 30 mg 3% cartridge: 30 mg/ml x 1.8 ml/cartridge = 54 mg/cartridge per ml, thus there are 54 mg per 1.8 ml dental cartridge. Therefore, a cartridge of 3% mepivacaine contains 50% more local anesthetic than a cartridge of 2% makes it more critical to determine the tongues, tonsils, and adenoids than adults. lidocaine; thus it would take less volume maximum dose and number of cartridges These anatomic differences—coupled with (or fewer cartridges) of the more concen- based on the child’s actual weight. the heightened susceptibility to CNS and trated drug (3% mepivacaine) to reach its The following example of a 3.5-year-old respiratory depressants—render children respective MRD.11 child illustrates the point that children more vulnerable to losing airway patency.4 often appear deceptively large and how Preventing local anesthetic this may prompt the dentist to overes- Local anesthetic selection: toxicity: calculating appropriate timate the child’s size based solely on misconceptions about prolonged weight-based dose appearances.4 Because the head develops soft tissue numbness Respecting weight-based dosing limits quickly during early childhood, children When treating children, it is important is essential, as previous cases of LAST have disproportionately large heads; at the to inform parents or caregivers that close have resulted in significant morbidity age of 3.5, a child’s head is nearly 75% postoperative supervision is needed to and mortality when dosing limits were of the size of his/her adult counterpart.4,5 prevent the child from biting their lips, exceeded.3,4,12,16,17 In a 1983 retrospec- However, the same child has only 50% cheeks, and tongue. While the soft tissues tive study, pediatric dental patients that of the height, 25% of the blood volume, are still numb, significant trauma from received local anesthesia and opioid and 20% of the weight compared to lip and cheek biting can occur. There sedation—either local alone or local plus his/her adult counterpart (Figure).4 The is a misconception that using a local narcotic dose—exceeded their combined child’s airway is also different, with nar- anesthetic without a vasoconstrictor, such MRDs by a factor of ≥3; the result was rower nasal passages, larynx, and trachea. as mepivacaine 3% plain, will provide a either permanent brain damage or death.6 Meanwhile, children have relatively larger shorter duration of soft tissue anesthesia In a 1992 survey of local anesthetic use www.agd.org General Dentistry January/February 2015 49 Anesthesia and Pain Control Local anesthetic calculations: avoiding trouble with pediatric patients Table 3.