SUCCINYLCHOLINE DOSE-RESPONSE IN OBESE ADOLESCENTS

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S415 Introduction: Obese adolescents are at increased risk of aspiration during general anesthesia [1]. A rapid sequence induction (RSI) may be used to reduce risk. Succinylcholine (sux) has the most rapid onset of all muscle relaxants currently in use. A dose response relationship for sux in obese adolescents has not been established, thus we undertook the present study. Methods: After IRB approval and informed parental consent, 30 children, 9-15 yrs old with a body mass index (BMI)>30 were enrolled. Subjects 1-20 were randomized to receive sux 100 mcg/kg (group A) or sux 250 mcg/kg (group B). The ED50 of sux was estimated. A third group (C) of 10 children received this dose, sux 150 mcg/kg. An IV catheter was placed and blood for dibucaine number (Db) and pseudocholinesterase level (Pch) obtained 30 minutes after midazolam 0.4 mg/kg PO (max=20mg). Routine monitors and a Datex relaxograph were placed. Patients were preoxygenated for 2 minutes before induction (atropine 0.01 mg/kg IV, thiopental 5 mg/kg IV, fentanyl 3 mcg/kg IV, cricoid pressure). A baseline response of the adductor pollicis to supramaximal train of four ulnar nerve stimuli (TOF) every 10 seconds was recorded. Then, the study dose of sux was administered. After maximum twitch depression occurred, a second dose of sux was given (total dose of sux 2 mg/kg), and the patient's trachea intubated. Demographic data were analyzed by ANOVA. A linear regression was done since the data satisfied criteria for linearity and no patient had a 0% or 100% response. The variables used included the log of the study dose of sux vs the probit transformation of the maximum twitch depression during TOF [2]. ED50, ED90, and ED95 were calculated with 95% confidence intervals. Results: No difference between groups exists in age (12.2 +/- 2.2 years), weight (98.6 +/- 31.3 kgs), BMI (36.5 +/- 10), Db (85.2 +/- 3.4), or Pch (18.8 +/- 3.61 mM/L). The estimated (see Figure 1) ED50, ED90, and ED95 (with lower and upper 95% confidence intervals) in mcg/kg are: 158 (79 and 316), 293 (133 and 644), and 348 (153 and 789), respectively. Pch was > the upper limit of normal (7-19[micro sign]mol/l) in 48% of subjects tested (n=25).Figure 1: Linear regression of the log of the study dose of sux versus the probit transformation of the maximum response (% twitch depression).Discussion: Intuition mandates that the dose of sux be based on lean body mass. Adult literature suggests that altered physiology (increased Pch and extracellular fluid volume) renders this dosing inappropriate [3]. We conclude: 1) The ED90 in obese adolescents is similar to that reported in adults, 290 mcg/kg [4], and less than that reported in children, 352 mcg/kg [5]; 2) Pch may be increased in obese adolescents; and 3) The recommended dose of sux in obese adolescents should be 1.5 mg/kg (4 x ED95) of actual body mass.