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Education  |   November 2002
Concentration–Effect Relation of Succinylcholine Chloride during Propofol Anesthesia
Author Affiliations & Notes
  • Julie J. Roy, B.Pharm., M.Sc.
    *
  • François Donati, PhD., M.D., F.R.C.P.C.
  • Daniel Boismenu, B.Sc., M.Sc., Ph.D.
  • France Varin, B.Pharm., Ph.D.
    §
  • * Ph.D. Student, § Professor, Faculté de Pharmacie, † Professor, Département d'anesthésiologie, Faculté de Médecine, Université de Montréal. ‡ Assistant Director, The Mass Spectrometry Unit, McGill University, Montréal, Québec, Canada.
  • Received from the Faculté de Pharmacie, Université de Montréal, Département d'Anesthésiologie, Centre Hospitalier de l'Université de Montréal, Montréal, Québec, Canada.
Article Information
Education
Education   |   November 2002
Concentration–Effect Relation of Succinylcholine Chloride during Propofol Anesthesia
Anesthesiology 11 2002, Vol.97, 1082-1092. doi:
Anesthesiology 11 2002, Vol.97, 1082-1092. doi:
SUCCINYLCHOLINE chloride, a depolarizing neuromuscular blocking agent, has been extensively used in anesthesia since 1951. In most patients with no qualitative or quantitative plasma cholinesterase deficiency, succinylcholine has the advantage of a rapid onset and short duration of action, two characteristics that are extremely valuable, especially in the full-stomach, emergency situation. It is generally recognized that the short time course of succinylcholine blockade is due to its rapid metabolism, but other factors could play a role. The search for the ideal nondepolarizing drug involves an understanding of the factors that makes succinylcholine blockade so evanescent. This entails simultaneous measurement of plasma concentrations (pharmacokinetics) and neuromuscular blockade (pharmacodynamics).
Succinylcholine is known to be hydrolyzed by plasma cholinesterase. 1 Genetic variants of the plasma cholinesterases, low plasma cholinesterase activity due to an acquired deficiency (liver disease, carcinoma, debilitating disease, and so forth), and other factors, among which some remain unknown, may lead to an episode of prolonged apnea after succinylcholine. 2 Because of this recognized variation in the activity of plasma cholinesterases, large interpatient differences in pharmacokinetic, pharmacodynamic, and pharmacokinetic–pharmacodynamic relations are expected.
For drugs with a fast onset of effect, the importance of obtaining frequent samples during the first minutes after a bolus injection has been demonstrated. 3,4 In addition, a pharmacokinetic–pharmacodynamic analysis must be performed by measuring the drug effect simultaneously. Pharmacokinetic studies of succinylcholine have been hindered by the lack of a suitable assay due to the analytical challenge involved with measuring its concentration in plasma. Only one human pharmacokinetic study has been reported, 5 and it did not include pharmacokinetic–pharmacodynamic relations. Recently, an electrospray tandem mass spectrometry method that is 80 times as sensitive was developed in our laboratory. 6 
The primary objective of this study was to provide a quantitative model to describe the kinetics and the dynamics of succinylcholine chloride after a 1-mg/kg bolus dose in anesthetized patients. The secondary objective was to determine if the in vitro  rate of degradation in plasma is of predictive value for the in vivo  elimination rate of succinylcholine in a given patient.
Materials and Methods
Patients
The study protocol was approved by the Research Ethics Committee at the Centre Hospitalier de l'Université de Montréal. Written informed consent was obtained from each subject. Patients included were adults, aged 18–65 yr, with American Society of Anesthesiologists status I or II who were scheduled for surgery for which an arterial cannula was indicated. Patients with cardiovascular, pulmonary, neuromuscular, hepatic, or renal disease were excluded. Other exclusion criteria were the concurrent administration of drugs known to be or suspected of interfering with the neuromuscular function, anemia, and body mass index less than 19 or greater than 29.9. Patients with a personal or familial history of an abnormal response to succinylcholine or mivacurium were excluded.
Once patients arrived in the operating room, a cannula was inserted into the left radial artery before induction of anesthesia. Monitoring included continuous electrocardiography, pulse oximetry, and an automatic noninvasive blood pressure device. Anesthesia was in-duced with remifentanil (0.5–1 μg/kg) and propofol (2–3 mg/kg) to provide sufficient depth of anesthesia for tracheal intubation without neuromuscular blocking agents. Mechanical ventilation was adjusted to keep end-tidal carbon dioxide in the range of 30–35 mmHg. Anesthesia was maintained with 100–200 μg · kg−1· min−1propofol, 0.1–0.2 μg · kg−1· min−1remifentanil, and oxygen. After stabilization of vital signs and neuromuscular function, a 1-mg/kg bolus dose of succinylcholine chloride was administered over 2 s. If a patient required maintenance of neuromuscular block after the first dose of succinylcholine, vecuronium was given after at least 75% of twitch recovery relative to presuccinylcholine control was reached. The study was terminated when the last plasma sample was collected and at least 75% of neuromuscular recovery from the baseline value was reached.
Neuromuscular Monitoring
After induction of anesthesia, the mechanomyographic response of the adductor pollicis muscle to single twitch (0.2 ms duration and frequency of 0.1 Hz) of the ulnar nerve at the right wrist was measured with a Grass FT10 force displacement transducer (Grass Instruments, Quincy, MA). A stabilization period of at least 3 min was allowed before injection of succinylcholine (reference value for onset). Muscle relaxation was monitored continuously until neuromuscular tension had returned to more than 75% of its baseline value. All patients but one (patient no. 1) recovered completely (reference value for recovery) before the administration of another neuromuscular blocking agent.
Blood Sampling Schedule
Before induction of anesthesia, an arterial blood sample (48 ml) was drawn from each patient into heparinized tubes to determine the patients’ phenotype and the in vitro  rate of degradation of succinylcholine in plasma. Another sample was drawn (1.5 ml) into EDTA and echothiophate (20 μl of a 40-mm solution per milliliter of plasma) enriched Vacutainer tube for the in vivo  pharmacokinetic profile (blank sample). After the 1-mg/kg intravenous bolus dose of succinylcholine chloride, arterial blood samples (1.5 or 3 ml) were drawn into tubes containing heparin (20 U) and echothiophate. Echothiophate was added to prevent the hydrolysis of succinylcholine by plasma cholinesterase. 7 For the first patient, arterial blood samples were drawn every 5 s for the first 2 min (1.5 ml each) and at 3, 4, 5, 7, 10, 15, 20, and 25 min (3 ml each). For this patient, succinylcholine was not detectable after 7 min. Therefore, for the other patients, the sampling schedule was the same for the first 2 min, then blood samples were drawn every minute until 10 min (3 ml each). During the first 2 min, sampling was performed by letting blood come out under arterial pressure and switching collecting tubes every 5 s. The midpoint of collection was used as the recorded time point for the pharmacokinetic analysis. To minimize the ex vivo  degradation of succinylcholine, blood samples were kept in an ice-water bath and centrifuged within 5 min. The in vivo  plasma samples were frozen on dry ice immediately thereafter and were stored at −70°C until analysis. The plasma samples obtained for in vitro  analysis was kept in an ice-water bath until processing.
Phenotyping of Patients’ Plasma
Each patient's phenotype was characterized using the conventional laboratory methods, by measurement of cholinesterase activity and biochemical inhibition reactions, such as dibucaine number, fluoride number, and chloride number. 8 These analyses were conducted in the biochemistry laboratory at Maisonneuve-Rosemont Hospital.
In Vitro  Degradation Study
The in vitro  study was performed within 4 h of blood collection to ensure the integrity of the medium. Incubations in fresh plasma were conducted at the patient's body temperature as measured in the operating room (36 ± 1°C). Succinylcholine was added to the patient's plasma to achieve a final concentration of 2 μg/ml and, immediately after, 1 ml of the incubation mixture was removed and stabilized with echothiophate. This was designated as time zero for the incubation. Subsequent plasma samples were treated similarly at 2, 3, 4, 5, 7, 10, 15, 20, and 25 min for the first patient and at 0.5, 1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, and 10 min for all other patients. Samples were flash frozen on dry ice and stored at −70°C until analysis.
Analysis of Samples
Succinylcholine was extracted from human plasma on C1 solid-phase cartridges and was analyzed using positive ion electrospray tandem mass spectrometry (ESI/MS/MS) after direct probe injection. 6 Briefly, plasma concentrations were determined with a stable isotope dilution assay using hexadeuterosuccinylcholine as the internal standard. The calibration curve was prepared using the ratio of intensities of the major product ions;m/z  = 115.5 for succinylcholine and m/z  = 117.0 for the internal standard. Calibration curves were linear from 25 to 4,000 ng/ml. Dilution of patient samples for concentrations exceeding 4,000 ng/ml was validated using dilution of quality control samples. For intraday precision, coefficient of variation was 6% or less, and accuracy ranged from 98 to 103%. For the interday precision, coefficient of variation was 10% or less, and accuracy ranged from 90 to 102%. The method was specific as each patient's blank plasma sample was free of interference in the spectral area of interest. 6 
Pharmacokinetic Analysis
In the design of our study, we had hypothesized that the in vitro  speed of degradation could be related to the in vivo  elimination half-life of succinylcholine. As the in vitro  rate constant is usually determined with a noncompartmental model, a similar approach was also conducted for the in vivo  study. The results of the noncompartmental model served also to validate the results obtained with the compartmental approach. All pharmacokinetic analyses were performed using raw data only. Plasma concentration versus  time profiles were analyzed using WINNONLIN (Professional Edition version 1.5; Pharsight, Mountain View, CA). Point estimates and pharmacokinetic parameters were optimized using a standard minimization method (a Levenberg and Hartley modification of the Gauss-Newton algorithm). 9,10 
Noncompartmental Approach
The area under the plasma concentration–time curve after intravenous administration of succinylcholine was calculated by the linear trapezoidal rule method. Lag time was defined as the time between injection of succinylcholine and the time when the last null concentration was observed. The in vitro  and the in vivo  rate constants (kin vitroand kel, respectively) were obtained from the least-square fitted log-linear terminal portion of the plasma concentration–time profile. The mean number of points, in the terminal portion of the curve, used for the determination of kin vitrowas 6 (range, 5–7 points) and for kelwas 7 (range, 4–14 points). Total body clearance and the volume of distribution at steady state (Vss) were calculated using standard formulae. 11 
Compartmental Approach
Pharmacokinetic parameters were also derived using a two-compartment model with central elimination only. 12,13 Traditional models assume an instantaneous input function after bolus administration. To account for the delay before mixing in the central compartment and to adequately characterize the intravascular mixing phase, the compartmental model included a lag time and a first-order input function (k01), similar to an absorption phase. According to the Akaike information criterion, the pharmacokinetics of succinylcholine was better described by a two-compartmental model than by a one-compartment model. 14 A weighting function of 1/(predicted y2) was applied. The following parameters were determined: the input rate constant to the central compartment (k01), the distribution (α) and elimination (β) rate constants and their corresponding A and B coefficients, the volume of the central compartment (V1), the intercompartmental clearances (Cl12and Cl21), the total clearance, the transfer rate constant from the central to peripheral compartment (k12), the transfer rate constant from peripheral to central compartment (k21), the exit rate constant from central compartment (k10), and the volume of distribution at steady state assuming central elimination only (Vss,c).
Succinylcholine undergoes the same metabolic pathway as mivacurium, and as much as 40% of the latter was recently found to be hydrolyzed during its passage through the forearm in humans. 15 Therefore, we could not exclude the possibility of an elimination from both the central and peripheral compartment (k20) for succinylcholine as this, most importantly, may lead to an underestimation of the Vss. Therefore, the Vsswhen assuming both central and peripheral elimination (Vss,c+p) was also calculated. In the absence of measured peripheral concentrations, k20cannot be derived independently without making some assumptions. 16 We have previously proposed two approaches to account for peripheral elimination: K20can either be assumed to be equal to kin vitroobtained from each patient's plasma or equal to β if it has already been shown that kin vitroequals β in a given patient. 13 The value that would be substituted for K20would be decided according to the results obtained in vivo  and in vitro  .
Pharmacokinetic–Pharmacodynamic Analysis
The pharmacokinetic–pharmacodynamic analysis were conducted using two approaches: a nonparametric and a two-stage parametric approach. For both approaches, the pharmacodynamic parameters were determined using each patient's data separately.
For the nonparametric approach, the effect compartment equilibration rate constants, ke0, was determined using the nonparametric method of Unadkat et al.  17 This approach makes no assumptions about the pharmacodynamic or the pharmacokinetic model while establishing a link between the plasma concentration and effect data. The optimal ke0was found by successive iterations and defined as the value that minimized the average of squared distances between the ascending and descending limbs of the hysteresis loop. During the first 2 min, collection of blood samples was more frequent than twitch data (every 5 vs.  10 s). As the nonparametric approach requires the input of both plasma concentration and neuromuscular twitch at a given time, linear interpolation had to be used to generate twitch results during onset. Also, because the effect persisted while the plasma concentrations were below the limit of quantification, corresponding plasma concentrations were extrapolated using the terminal slope. A sigmoid Emaxmodel was used to correlate the effect, with the effect–compartment concentration, thus providing values for the effect–compartment concentration at 50% blockade (EC50) and the slope factor (γ). 17–19 The goodness of fit was assessed by the Akaike information criterion.
For the parametric approach, a sequential method was used. The pharmacokinetic parameters obtained with the compartmental analysis were fixed for the pharmacokinetic–pharmacodynamic modeling. A parametric link model was used to derive the equilibrium rate constant (ke0) between the central compartment and the effect compartment using WINNONLIN (Professional Edition version 1.5; Pharsight). The EC50and γ were determined using the sigmoid Emaxmodel. 20 A weighting function of 1 was applied. Goodness of fit was assessed by the Akaike information criterion.
Statistical Analysis
During the model discrimination process, the predicted data were compared with the observed data by visual inspection of predicted concentration–time profile, of weighted residuals, and by comparing the values of the Akaike information criterion obtained with either model.
Data are presented as mean values ± SD. For each patient, a paired t  test or a Wilcoxon signed rank test when normality test failed was used to compare pharmacokinetic and pharmacokinetic–pharmacodynamic parameter estimates obtained with different models. The threshold for statistical significance (α) was set at 0.05.
Results
Demographic data are presented in table 1. A mean age of 48 ± 7 yr and mean weight of 74 ± 10 kg were observed for the seven patients recruited. In all but one patient, the phenotype was considered normal (table 1). Results for patient no. 3 are not available because the in vitro  blood sample was lost. This precluded from obtaining the patient's phenotype results and also the in vitro  degradation half-life.
Table 1. Demographic Data
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Table 1. Demographic Data
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Pharmacodynamic parameters are presented in table 2. All patients reached 100% neuromuscular block, and the onset time ranged from 0.8 to 1.8 min. Excluding 0% and 100% blockade, we observed 3–5 pharmacodynamic data points during the onset phase. The mean 25–75% recovery index ranged from 1.7 to 3.6 min.
Table 2. Pharmacodynamic Parameters
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Table 2. Pharmacodynamic Parameters
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The in vivo  concentration–time profiles of succinylcholine in each patient, as well as the effect as a function of time, are presented in figure 1. The first detectable concentration was observed between 17.5 and 27.5 s. Peak concentrations were reached between 22.5 and 47.5 s and ranged from 30,378 to 56,906 ng/ml. Then, a rapid decrease in concentrations was observed. Recovery occurred after plasma concentrations of succinylcholine were no longer detectable.
Fig. 1. (A  ) Individual succinylcholine in vivo  plasma concentration-versus  -time curves for the seven patients who received 1 mg/kg succinylcholine chloride. (B  ) Individual neuromuscular effect of succinylcholine for the seven patients.
Fig. 1. (A 
	) Individual succinylcholine in vivo 
	plasma concentration-versus 
	-time curves for the seven patients who received 1 mg/kg succinylcholine chloride. (B 
	) Individual neuromuscular effect of succinylcholine for the seven patients.
Fig. 1. (A  ) Individual succinylcholine in vivo  plasma concentration-versus  -time curves for the seven patients who received 1 mg/kg succinylcholine chloride. (B  ) Individual neuromuscular effect of succinylcholine for the seven patients.
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In figure 2, the plasma concentration–time profiles for the in vitro  incubation of succinylcholine are presented for six of seven patients. Using pairwise comparisons, the in vitro  degradation rate constant (kin vitro) and the in vivo  elimination rate constant (kel) obtained for each patient were not significantly different. The mean ± SD values for kin vitroand kelwere 1.07 ± 0.49 min1(corresponding to a half-life of 39 s) and 0.97 ± 0.30 min1(corresponding to a half-life of 43 s), respectively. When corresponding keland kin vitrovalues were plotted against each other, an excellent linear correlation (r2= 0.94) was observed (fig. 3).
Fig. 2. Individual succinylcholine in vitro  speed of degradation in plasma for six patients.
Fig. 2. Individual succinylcholine in vitro 
	speed of degradation in plasma for six patients.
Fig. 2. Individual succinylcholine in vitro  speed of degradation in plasma for six patients.
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Fig. 3. Linear relation between the in vitro  speed of degradation (kin vitro) and the in vivo  elimination constant (kel) for six patients.
Fig. 3. Linear relation between the in vitro 
	speed of degradation (kin vitro) and the in vivo 
	elimination constant (kel) for six patients.
Fig. 3. Linear relation between the in vitro  speed of degradation (kin vitro) and the in vivo  elimination constant (kel) for six patients.
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The results from the compartmental pharmacokinetic analysis are presented in table 3. First, there was no significant difference between the maximum concentration (Cmax), time when Cmaxoccurred (Tmax), and lag time observed and those estimated by the compartmental analysis. The mean observed Cmax, Tmax, and lag time were 49,182 ± 11,236 ng/ml, 0.57 ± 0.15 min, and 0.26 ± 0.07 min, respectively. A distribution phase with a mean half-life of 0.08 min (5 s) followed by a fast elimination phase with a mean half-life of 0.69 min (41 s) was observed. Mean total body clearance was similar for the compartmental (37 ± 9 ml · min−1· kg−1;table 3) and the noncompartmental (37 ± 7 ml · min−1· kg−1) approaches. Likewise, the mean elimination rate constants (kelfor the noncompartmental approach and β for the compartmental approach) were not different from each other, the mean noncompartmental elimination rate constant being 1.01 ± 0.29 min−1. The Vsscalculated with the noncompartmental approach (20 ± 7 ml/kg) also gave a similar value to that obtained with the compartmental approach (Vss,c). However, a twofold increase in Vsswas observed for the compartmental approach assuming both central and peripheral elimination (Vss,c+p). As no statistical difference was observed between kin vitroand β for succinylcholine, Vss,c+pwas calculated assuming that k20=β.
Table 3. Compartmental Pharmacokinetic Results
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Table 3. Compartmental Pharmacokinetic Results
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Table 3. (continued) 
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Table 3. (continued) 
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Figure 4shows the in vivo  plasma concentration–time curves in three patients having the best, worst, and median fit as well as the respective plots showing the distribution of the weighted residuals. Likewise, the observed and predicted effects for these three patients are shown in figure 5.
Fig. 4. (A  ) Observed versus  predicted plasma concentration for patients no. 1, 5, and 7 using compartmental approach (central elimination only). (B  ) Their respective plots of weighted residuals.
Fig. 4. (A 
	) Observed versus 
	predicted plasma concentration for patients no. 1, 5, and 7 using compartmental approach (central elimination only). (B 
	) Their respective plots of weighted residuals.
Fig. 4. (A  ) Observed versus  predicted plasma concentration for patients no. 1, 5, and 7 using compartmental approach (central elimination only). (B  ) Their respective plots of weighted residuals.
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Fig. 5. Observed versus  predicted neuromuscular effect for patients no. 1, 5, and 7 using the compartmental approach.
Fig. 5. Observed versus 
	predicted neuromuscular effect for patients no. 1, 5, and 7 using the compartmental approach.
Fig. 5. Observed versus  predicted neuromuscular effect for patients no. 1, 5, and 7 using the compartmental approach.
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An anticlockwise hysteresis was observed in the plasma concentration–effect relation of succinylcholine, and a link model had to be used to simulate effect compartment concentrations before a sigmoid Emaxmodel could be applied. Both the ke0and EC50values were not significantly different when derived by either the nonparametric or parametric approach (table 4). However, γ was twice as large with the parametric approach. The standard deviations of the EC50, ke0, and γ parameter estimates for individual patients ranged between 1 and 11%. When the onset times were plotted against their respective ke0values, a strong linear correlation (r2= 0.9) was found (fig. 6).
Table 4. Pharmacokinetic–Pharmacodynamic Analysis
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Table 4. Pharmacokinetic–Pharmacodynamic Analysis
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Fig. 6. Linear relation between the effect compartment equilibration rate constant (ke0) and onset for seven patients using the noncompartmental pharmacokinetic–pharmacodynamic approach.
Fig. 6. Linear relation between the effect compartment equilibration rate constant (ke0) and onset for seven patients using the noncompartmental pharmacokinetic–pharmacodynamic approach.
Fig. 6. Linear relation between the effect compartment equilibration rate constant (ke0) and onset for seven patients using the noncompartmental pharmacokinetic–pharmacodynamic approach.
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Discussion
This study confirms the fast elimination half-life of succinylcholine and shows that, for each patient, the in vivo  elimination half-life correlates strongly with the in vitro  speed of degradation. The pharmacokinetic–pharmacodynamic analysis reveals that succinylcholine has a ke0value comparable with that of most nondepolarizing agents and an EC50denoting a low potency. Thus, our results suggest that the rapid onset and offset of succinylcholine neuromuscular block are due to a short plasma half-life, not to a fast access to muscle.
To our knowledge, only one pharmacokinetic study has been reported for succinylcholine in humans. 5 In that study, a bolus dose of 1 or 2 mg/kg of succinylcholine chloride was used. The mean values for clearance for both doses (700 and 250 ml · min−1· kg−1, respectively), were 7–20 times as large as those observed in the current study (37 ml · min−1· kg−1). In addition, the elimination half-life of succinylcholine (16.6 and 11.7 s, respectively) was only 30% of that observed in our study. The discrepancy is probably the result of a lack of sensitivity of the analytical method in the previous study as succinylcholine could not be quantified more than 2 min after its administration. This would also explain the faster elimination half-life reported in that study.
Many methodologic issues had to be considered in the design of our study. To follow adequately the concentration–time profile immediately after the bolus injection, our study design included arterial sampling every 5 s for the first 2 min. The possibility of a random error in timing cannot be excluded but can be substantially reduced by strict adherence to the experimental protocol and by having the same persons responsible for blood sampling (experienced anesthetist) and time recording (experienced investigator). Any deviation from schedule was taken into account during the analysis. To minimize the error in the estimation of clearance, sampling from the arterial site was conducted from time of injection as frequently as possible. 21 In contrast to previous studies conducted by this group with neuromuscular blocking agents, 3,22,23 it was felt necessary to increase the frequency of blood sampling from every 10 s to every 5 s in view of the very fast onset of action of succinylcholine. A collection period of 10 min was judged adequate as the extrapolated terminal portion of the area under the curve was less than 2% of the total area under the curve. Neuromuscular function, however, was monitored every 10 s only, in agreement with the recommendations for studies on neuromuscular blocking agents. 24 Therefore, effect was not measured as often as concentration during onset. The in vitro  samples were incubated at the same temperature as that of the patient because temperature is known to influence the enzyme activity. 25 
As reported previously, the phenotyping results using conventional biochemical methods do not always allow correct genotyping or prediction of the response to neuromuscular blocking agents, and many patients cannot be classified using the traditional methods. 26,27 It was previously found that in 28.1% of 225 patients who presented an episode of prolonged apnea after succinylcholine administration, both the type and quantity of plasma cholinesterase were normal. 2 Subjects with genotypically normal enzyme but with low cholinesterase activity showed an increase in the duration of action of succinylcholine with decreasing cholinesterase activity. 28 In this study, the two patients with the lowest and highest cholinesterase activity (patients no. 1 and 4, respectively) also showed, respectively, the longest and the shortest recovery index, which is in agreement with previous findings. Two patients (no. 1 and 2) showed a relatively low plasma cholinesterase activity and had longer duration of action and plasma elimination half-life than the other subjects. In these patients, a lower cholinesterase activity was associated with EC50values within the range observed in our patients (table 2). These results suggest that increased plasma concentrations, and not increased sensitivity, are probably responsible for the changes in pharmacodynamic response and the lower dose required in one case report of a homozygous patient having a atypical plasma cholinesterase. 29 
The advantage of measuring the in vitro  rate of degradation of succinylcholine is its strong correlation with the in vivo  rate of elimination in the same patient. Therefore, it would be interesting to investigate if the in vitro  rate of degradation of succinylcholine could be a more robust method to detect, prior to anesthesia, the patients susceptible to have a prolonged apnea after succinylcholine administration, instead of conventional biochemical tests, as the latter do not seem to be good predictors. The determination of the in vitro  rate of degradation of succinylcholine would be of clinical value in patients with a family history of prolonged apnea. However, quantitative determination of succinylcholine is not readily feasible in clinical practice.
As mentioned in Methods, we also believed that accounting for both central and peripheral elimination was appropriate for a more accurate estimation of the volume of distribution for succinylcholine (Vss,c+p). The use of a kin vitroas a substitute for K20was first proposed by Fisher et al.  30 for atracurium. For cisatracurium, the kin vitrovalue obtained by Welch et al.  31 had to be used, in the absence of the patient's in vitro  data. 32–34 In our study, kin vitrovalues were obtained for six patients, but a large interpatient variability was observed. The use of this group of patient's mean kin vitrovalue would lead to significant error in the estimation of Vss,c+pin many of our patients. Recently, we proposed an alternative approach where k20is assumed to be equal to β. 13 This approach would be only indicated when the in vivo  elimination half-life and the in vitro  rate of degradation in plasma of a drug are similar in the same patient. We have therefore tested that assumption for succinylcholine and found that the kin vitro(1.07 ± 0.49 min−1), kel(1.01 ± 0.29 min−1), and β (1.01 ± 0.28 min−1) values were not statistically different for succinylcholine. In a model assuming both central and peripheral elimination, 13 it mathematically follows that the elimination rates in both compartments are identical. Although it is impossible to give a physiologic meaning to these compartments, our results suggest that the organ independent clearance (e.g.  , hydrolysis by plasma cholinesterase) accounts for all of the total body clearance. Therefore, k20could be fixed to β without the necessity of performing individual in vitro  incubations in future studies involving succinylcholine.
When using compartmental models, it is assumed that the sampling site (the intravascular space) is part of a homogenous compartment (V1), which may not be the case immediately after an intravenous bolus injection. Indeed, in one cycle of circulation, it would be unrealistic to presume that the central compartment is well stirred. Using physiologic markers, such as indocyanine green, allows full characterization of the intravascular mixing of muscle relaxants and has been shown to include not only the central circulation but also two peripheral circuits that return drug to the central compartment without equilibrating with tissues. 35 In our patients, it was possible to identify, by visual inspection of the initial front-end kinetics, a data point that could be associated with a small recirculation peak. This increase occurred after the initial mixing phase at approximately 65 ± 13 s, but its magnitude was within the analytical error range (< 15%) in five of seven patients. As the initial intravascular mixing phase of succinylcholine was well characterized by the input function (k01), we opted for a two-compartment pharmacokinetic model. To obtain a reasonable fit with experimental data, a lag time and an exponential input function k01(analogous to an oral absorption function) had to be added to the two-compartment model to account for the intravascular mixing phase. In our study, the noncompartmental approach was used a model-independent reference to validate the compartmental model. The close concordance between both analyses provides additional support to our modeling approach.
Succinylcholine is expected to be very hydrosoluble and thus to have a small volume of distribution. The Vss,cderived with either the noncompartmental or compartmental analyses are similar to that reported by Hoshi et al.  , 5 19 or 20 ml/kg versus  16.4 ml/kg, respectively. Thus, the body distribution of succinylcholine would appear to be smaller than the intravascular space. However, it is generally recognized that the apparent Vssof a drug will be underestimated if peripheral elimination is not taken into account. 13,30 After correction for peripheral elimination, the body distribution of succinylcholine appears to correspond to the intravascular space (Vss,c+p= 39 ml/kg). In view of the extremely rapid clearance of succinylcholine, the hypothesis that a significant amount of the intravenous dose would be metabolized before its first detection in the arterial blood was investigated. The initial central volume of distribution for a series of muscle relaxants, 3,22,23 where extensive blood sampling was conducted during the first 2 min, were grossly estimated by dividing the dose by the maximum plasma concentrations. To our surprise, the volume obtained for succinylcholine was not that different from those having a slower plasma clearance. This observation would therefore exclude the possibility of a major contribution of the cardiopulmonary first-pass effect to the underestimation of the apparent volume of distribution of succinylcholine.
Everyone now agrees that it is of prime importance to characterize the front-end kinetics for a drug with a very rapid onset of action. Recently, a complete recirculatory model was used to describe the front-end kinetics of rocuronium in patients. In this study, blood samples were drawn at 3-s intervals during the first 2 min, and indocyanine green was used as a physiologic marker. 36 In their study, a smaller V1was reported for rocuronium when compared with that observed by our group using a 10-s sampling interval and noncompartmental analysis. 37 However, pharmacokinetic–pharmacodynamic estimates (keoand EC50values) were similar in both studies. This suggests that the recirculation peaks have no significant impact on the estimation of pharmacokinetic–pharmacodynamic parameters.
When conducting nonparametric pharmacokinetic– pharmacodynamic modeling, some linear interpolation of the effect data had to be made during the first 2 min after succinylcholine administration. Also, because the effect lasted longer than plasma concentrations were quantifiable, the terminal portion of the concentration–time profile had to be extrapolated to get the corresponding concentration. The advantage of the parametric pharmacokinetic–pharmacodynamic analysis is that these interpolations or extrapolations are done automatically. Although the γ differed in both analysis, the estimates of EC50and ke0were similar.
Transient muscular fasciculations were observed in two patients, but no significant twitch increase was seen on the twitch tracing. Thus, onset of action could be considered as unimodal and was treated as such. Recovery occurred much earlier than the time required for phase II block to develop (30–45 min). Thus, the changing nature of succinylcholine action did not have to be taken into consideration in our pharmacodynamic model, and the traditional sigmoid Emaxmodel was applied to our data. Our results indicate that the potency of succinylcholine is low. Indeed, its molar EC50(2.6 μm) is almost 5-fold that of atracurium (0.49 μm), 21-fold that of doxacurium (0.12 μm), and 47-fold that reported for mivacurium (0.055 μm). 22,38,39 
The ke0is thought to be governed by several factors, such as perfusion to the effect site, diffusion from the capillary lumen to the effect site (molecular weight, pKa, lipid solubility, protein binding), and occupancy of receptors to induce drug effect (potency). 40,41 Interestingly, a strong correlation between ke0and onset was observed in our patients (r2= 0.9) (fig. 6). Despite its fast onset of action, the equilibration half-life between succinylcholine plasma concentrations and its effect was relatively slow, being similar to that obtained for various nondepolarizing neuromuscular blocking agents; as such, mean ke0values of 0.043 ± 0.004 min−1were reported for atracurium, 0.053 ± 0.006 min−1for doxacurium, and 0.058 ± 0.005 min−1for vecuronium. 3 22,42 Recently, Torda et al.  43 used a pharmacodynamic model to derive the pharmacokinetic–pharmacodynamic parameters for succinylcholine from effect data alone and found a keoof 0.27 ± 0.15 min−1, which is almost five times as fast as what we obtained. The fact that plasma concentrations of succinylcholine were not measured in their patients may well explain these discrepancies. In a study involving rapacuronium, a nondepolarizing blocking agent with a fast onset of action, a ke0ranging from 0.377 to 0.405 min−1was reported at the adductor pollicis. 44 It was concluded that the more rapid onset of rapacuronium compared with the other nondepolarizing muscle relaxants resulted from a more rapid equilibration between plasma and effect site concentrations. 44 The same conclusion cannot be applied to succinylcholine. Pharmacokinetic factors other than a fast transfer to the site of action are probably responsible for the rapid onset of action of succinylcholine, namely, the combination of a low potency and high plasma clearance .  45,46 
In conclusion, succinylcholine is a low potency drug with very fast clearance that equilibrates relatively slowly with the effect compartment. Its very fast disappearance from plasma has been characterized in vivo  and in vitro  , and the in vitro  speed of degradation is a strong predictor of the in vivo  elimination rate constant.
The authors thank Orval A. Mamer, Ph.D. (Mass Spectrometry Unit, McGill University), for his sustained support, and Denis Babin, M.Sc. (Centre Hospi-talier de l'Université de Montréal), Johanne Couture, R.T., and Nathalie Rivest, B.Pharm. (Faculté de Pharmacie, Université de Montréal) for their collaboration in this study.
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Fig. 1. (A  ) Individual succinylcholine in vivo  plasma concentration-versus  -time curves for the seven patients who received 1 mg/kg succinylcholine chloride. (B  ) Individual neuromuscular effect of succinylcholine for the seven patients.
Fig. 1. (A 
	) Individual succinylcholine in vivo 
	plasma concentration-versus 
	-time curves for the seven patients who received 1 mg/kg succinylcholine chloride. (B 
	) Individual neuromuscular effect of succinylcholine for the seven patients.
Fig. 1. (A  ) Individual succinylcholine in vivo  plasma concentration-versus  -time curves for the seven patients who received 1 mg/kg succinylcholine chloride. (B  ) Individual neuromuscular effect of succinylcholine for the seven patients.
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Fig. 2. Individual succinylcholine in vitro  speed of degradation in plasma for six patients.
Fig. 2. Individual succinylcholine in vitro 
	speed of degradation in plasma for six patients.
Fig. 2. Individual succinylcholine in vitro  speed of degradation in plasma for six patients.
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Fig. 3. Linear relation between the in vitro  speed of degradation (kin vitro) and the in vivo  elimination constant (kel) for six patients.
Fig. 3. Linear relation between the in vitro 
	speed of degradation (kin vitro) and the in vivo 
	elimination constant (kel) for six patients.
Fig. 3. Linear relation between the in vitro  speed of degradation (kin vitro) and the in vivo  elimination constant (kel) for six patients.
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Fig. 4. (A  ) Observed versus  predicted plasma concentration for patients no. 1, 5, and 7 using compartmental approach (central elimination only). (B  ) Their respective plots of weighted residuals.
Fig. 4. (A 
	) Observed versus 
	predicted plasma concentration for patients no. 1, 5, and 7 using compartmental approach (central elimination only). (B 
	) Their respective plots of weighted residuals.
Fig. 4. (A  ) Observed versus  predicted plasma concentration for patients no. 1, 5, and 7 using compartmental approach (central elimination only). (B  ) Their respective plots of weighted residuals.
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Fig. 5. Observed versus  predicted neuromuscular effect for patients no. 1, 5, and 7 using the compartmental approach.
Fig. 5. Observed versus 
	predicted neuromuscular effect for patients no. 1, 5, and 7 using the compartmental approach.
Fig. 5. Observed versus  predicted neuromuscular effect for patients no. 1, 5, and 7 using the compartmental approach.
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Fig. 6. Linear relation between the effect compartment equilibration rate constant (ke0) and onset for seven patients using the noncompartmental pharmacokinetic–pharmacodynamic approach.
Fig. 6. Linear relation between the effect compartment equilibration rate constant (ke0) and onset for seven patients using the noncompartmental pharmacokinetic–pharmacodynamic approach.
Fig. 6. Linear relation between the effect compartment equilibration rate constant (ke0) and onset for seven patients using the noncompartmental pharmacokinetic–pharmacodynamic approach.
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Table 1. Demographic Data
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Table 1. Demographic Data
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Table 2. Pharmacodynamic Parameters
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Table 2. Pharmacodynamic Parameters
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Table 3. Compartmental Pharmacokinetic Results
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Table 3. Compartmental Pharmacokinetic Results
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Table 3. (continued) 
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Table 3. (continued) 
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Table 4. Pharmacokinetic–Pharmacodynamic Analysis
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Table 4. Pharmacokinetic–Pharmacodynamic Analysis
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