Remifentanil Versus Alfentanil: Comparative Pharmacokinetics and Pharmacodynamics in Healthy Adult Male Volunteers

After obtaining Institutional Review Board approval and informed consent, ten American Society of Anesthesiology (ASA) physical status 1 volunteers were enrolled in the study. Only English-speaking men between the ages of 18-40 yr without history of significant medical illness or medication requirements who were within 15% of their ideal body weight were eligible for participation. Prospective volunteers were ineligible if they had a history of alcohol abuse or illegal drug use, a habit of tobacco use greater than 10 cigarettes per day, a history of hypersensitivity to opioids, or a record of significant psychiatric disease. To confirm eligibility, each subject underwent a physical examination and a comprehensive battery of laboratory tests, including serum chemistries, liver and renal function tests, a complete blood count, a urinalysis, a urine drug screen, and an electrocardiogram.

Each subject was brought to the study site without premedication. An 18-G catheter was placed in a forearm vein for drug and fluid administration. A 20-G radial artery catheter was placed for blood sampling and continuous blood pressure monitoring. A solution of normal saline was infused intravenously at an approximate rate of 60 ml/h. Safety monitors included a continuous five-lead electrocardiogram, continuous pulse oximetry, and a precordial stethoscope.

Instrumentation for electroencephalographic (EEG) monitoring was performed in accordance with the International 10-20 system. [5]Four channels (F³-P³, F⁴-P⁴, C^z-P³, C^z-P⁴) of the EEG were amplified and recorded using a Nihon Kohden EEG machine (model 5210, Nihon Kohden, Irvine, CA).

After the instrumentation was completed, the volunteers received 0.2 mg glycopyrrolate intravenously to prevent opioid-induced bradycardia and 0.5 mg pancuronium intravenously to mitigate opioid-induced muscle rigidity. Volunteers breathed 100% Oxygen²by face mask delivered via a nonrebreathing circuit in preparation for drug administration.

Volunteers were randomized to receive either remifentanil or alfentanil during their initial visit and the other drug on their subsequent visit. Study sessions were separated by at least 2 weeks but no longer than 4 weeks. Both remifentanil and alfentanil were administered intravenously as a constant rate infusion by a laboratory syringe pump (Harvard Apparatus XG2000, South Natick, MA) for at least 10 min or until maximal changes were evident on the raw EEG. Remifentanil was administered at 3 micro gram *symbol* kg sup -1 *symbol* min¹(except the first subject, who received 2 micro gram *symbol* kg sup -1 *symbol* min sup -1), and alfentanil was administered at 1,500 micro gram *symbol* min sup -1.

During both visits, 3-ml arterial blood samples were obtained at preset intervals, with more rapid sampling during the infusion and immediately after termination of the infusion. After the infusion commenced, samples were collected every 30 s from 1 to 5 min, every minute from 6 to 10 min, and every 2 min from 12 to 20 min until the infusion was terminated. After the infusion was stopped, samples were collected every 30 s from 1 to 5 min, every minute from 6 to 10 min, and every 2 min from 12 to 20 min. Thereafter, samples were obtained at 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 120, 150, 180, 210, and 240 min after the infusion was stopped. Additional samples at 360, 480, and 600 min after infusion were obtained during the alfentanil sessions.

During drug infusion, ventilation was assisted by bag and mask with 100% Oxygen²as needed. A continuous infusion of succinylcholine was used as necessary to mitigate the effects of opioid-induced rigidity and to facilitate ventilation. Frequent arterial blood gas analysis confirmed the adequacy of ventilation and oxygenation.

Vital signs and subjective well-being were monitored after the end of the infusion for at least 3 h. Adverse events associated with drug administration were recorded as they occurred. Nausea and/or vomiting were treated as necessary by an intravenous injection of 10 mg metoclopramide.

Because of remifentanil's metabolic pathway, special processing was necessary to prevent continued metabolism of remifentanil after sample collection. The details of our sample-processing technique have been described previously. [2]Both the remifentanil and alfentanil samples were processed in this manner.

Remifentanil blood concentrations were measured by a high-resolution, gas chromatographic, mass spectrometry assay with a quantitation limit of 0.1 ng *symbol* ml sup -1 and an interassay coefficient of variation of less than 15% for concentrations greater than 0.1 ng *symbol* ml sup -1. Tetradeuterated remifentanil was included in the collection tubes as an internal standard to correct for variations in recovery among samples. [6].

Alfentanil blood concentrations were determined by a gas chromatographic, mass spectrometry technique with an interassay coefficient of variation of less than 10% and a quantitation limit of 1 ng *symbol* ml¹. [7]Fentanyl was included as an internal standard.

The digitized raw EEG data were processed by computer to obtain the spectral edge parameter, a univariate summary descriptor identifying the frequency below which 95% of the EEG power is located. [8]After filtering with a 50-Hz low-pass filter, the raw signal was analyzed in epochs of 2 s by Fourier transformation to separate it into frequency bins between 0.5 and 30 Hz. The power spectrum was calculated by squaring the amplitudes of the individual frequency components. Finally, the spectral edge was determined by calculating the area under the power versus frequency histogram and identifying the frequency below which 95% of the total area is found.

The pharmacokinetics were analyzed using three techniques. A classic moment analysis (area under the curve analysis) was done to facilitate comparison of each drug's model independent parameters in the same individual. A mixed-effect population approach based on the computer program NONMEM* was completed to estimate the population compartmental pharmacokinetic parameters. Finally, to address the clinical aspects of the mathematically based pharmacokinetic analysis, computer simulations of the context-sensitive half-times based on the NONMEM population pharmacokinetic parameters were performed.

Moment Analysis. Applying the theory of statistical moments to pharmacokinetics, [9]a model independent moment analysis was performed to calculate the clearance (CL), mean residence time (MRT), and apparent volume of distribution at steady-state (VD^ss) for both drugs in each subject. The area under the concentration versus time curve (AUC) was calculated for each blood concentration (C^b) versus time (t) plot using the trapezoidal method with linear interpolation when concentrations were increasing and log-linear interpolation when concentrations were decreasing. [10]The terminal slope for each data set was estimated by log-linear regression after visually identifying the terminal portion of each curve.

CL, MRT, and VD^sswere calculated using standard equations. Individual moment analysis parameters for both drugs in each subject were contrasted in graph form.

Nonlinear Mixed Effects Model Compartmental Analysis. The population compartmental pharmacokinetic parameters for both drugs were estimated using the NONMEM approach. Because it had been previously demonstrated for the remifentanil dose range studied, linear pharmacokinetics was assumed for the purposes of this analysis. [2,3]In contrast with the two-stage approach, wherein the population pharmacokinetic model is obtained by averaging the parameters estimated from individuals, NONMEM simultaneously analyzes an entire population's data and provides estimates of typical values for the pharmacokinetic parameters with an estimate of the parameter's interindividual variability within the population studied.

A three-compartment mamillary model was fit to the remifentanil and alfentanil concentration versus time data. Interindividual error on each parameter was modeled using a log-normal error model: Equation 1where Theta^individualis the true value in the individual, Theta^typicalis the population mean estimate, and eta^individualis a random variable whose distribution is estimated by NONMEM with a mean of zero and a variance of omega². Residual error was modeled assuming a log-normal distribution.

After obtaining estimates for the population volumes and clearances from NONMEM, the other three-compartment mamillary model parameters (micro and macro rate constants) were calculated using standard equations. [11].

The performance of the population models constructed by NONMEM for both drugs was assessed in terms of the ability to predict the measured blood concentrations. The models were quantitatively assessed in terms of weighted residuals (WRs), the difference between a measured blood concentration (C^m) and the model-predicted concentration (C^p) in terms of C^p. Thus, WR can be defined as: Equation 2.

Using this definition, the WRs for the NONMEM population models were computed at every measured data point. Using the WR data, the overall inaccuracy of the model was determined by computing the median absolute weighted residual (MDAWR), defined as: Equation 3where n is the total number of samples in the study population. Using this formula, the MDAWRs for the population models constructed by NONMEM were computed for each drug.

The performance of the models was visually assessed by plotting the C^m/C^pversus time and examining the plots for accuracy and bias.

Computer Simulations. Computer simulations using the pharmacokinetic parameters obtained from the NONMEM compartmental analysis were performed to provide an illustration of the predicted decline in blood concentrations when remifentanil and alfentanil are administered by infusion. These simulations predict the time necessary to achieve a 50% or 80% decrease in drug concentration in the blood after termination of a variable-length infusion targeted to a constant drug concentration. The simulations are based on Euler's solution to the three-compartment model with a step size of 1 s.

The pharmacodynamics were described using an effect compartment model in which k^e0, a first-order elimination rate constant characterizing effect-site equivalent, is used to estimate the apparent effect-site concentrations. [12]The pharmacodynamic analysis proceeded in three steps for each data set. First, k^e0was estimated using a hysteresis loop minimization technique. Second, the apparent concentration-effect relationship resulting from the hysteresis loop minimization technique was parametrically modeled assuming a sigmoidal shape for the relationship. Finally, making use of the k^e0and other pharmacokinetic and pharmacodynamic parameters estimated from the study, computer simulations were performed to contrast the onset, magnitude, and duration of effect resulting from equipotent doses of remifentanil and alfentanil.

K^e0Estimation Procedure. K^e0was estimated using a hysteresis loop minimization technique. [13]The theoretical foundation of this technique is that, in the effect site, there should be no delay or hysteresis between changes in drug concentration and changes in pharmacologic effect. In summary, this k^e0-estimating technique performs a numeric convolution of the measured drug concentrations with a candidate k^e0value to calculate the apparent effect-site concentrations. The optimal k^e0value minimizes the area of the hysteresis loop formed by plotting the apparent effect-site concentration versus effect. Potential k^e0values are sequentially tested until the optimal estimate of k^e0is obtained. The algorithm is thus an iterative process in which the hysteresis loops determined by numeric convolution of the measured drug concentrations with a potential k^e0are successively "collapsed" until a k^e0that results in minimal hysteresis is found. The optimal k^e0is used to calculate the apparent effect-site concentrations and thus identify the "pseudosteady-state" concentration-effect relationship.

Parametric Modeling of the Concentration-Effect Relationship. Because plots of the concentration-effect relationship determined by the estimation of k^e0were sigmoid in shape, an inhibitory "sigmoid E^max" equation (i.e., Hill equation) was used to model the relationship parametrically. [14]Using extended least-squares nonlinear regression implemented on an Excel spreadsheet (Microsoft, Redmond, WA), the equation: Equation 1where E is the predicted effect, E⁰is the baseline effect level, E^maxis maximal effect, C^eis effect-site concentration, gamma is a measure of curve steepness, and EC⁵⁰is the effect-site concentration that produces 50% of maximal effect, was fit to the effect versus effect-site concentration data. Having estimated the pharmacodynamic parameters, the model was used to calculate a predicted effect at each measured concentration of remifentanil and alfentanil.

Computer Simulations. Computer simulations were performed using the full pharmacokinetic/pharmacodynamic model from the study to illustrate the clinical application of the estimated parameters. The first simulation, intended to illustrate time to peak concentration, was a simulation of effect-site levels of remifentanil and alfentanil after bolus administration of equipotent doses (185 micro gram remifentanil, 3,500 micro gram alfentanil). The second simulation, intended to illustrate magnitude and duration of effect, was a simulation of effect-site concentrations that result from a 2-h computer-controlled infusion using Stanpump (Stanford University, Palo Alto, CA) targeted to an EC⁵⁰level for both remifentanil and alfentanil.

All ten volunteers originally enrolled completed the study. The volunteers were comparable in terms of age, lean body mass, and ASA physical status. Demographic means included an age of 28.5+/-4.8 yr, weight of 83.5+/-11.2 kg, and height of 183.7+/-5.8 cm (+/-SD).

All subjects received at least a 10-min infusion of remifentanil and alfentanil. One remifentanil subject required 14 min to exhibit maximal EEG changes, whereas two alfentanil subjects required infusions of 16 and 14 min, respectively, to reach maximal EEG changes. No infusion was terminated early because of an adverse event.

With the exception of adverse events such as muscular rigidity and nausea/vomiting that were anticipated as part of this protocol, no significant or unexpected complications were associated with remifentanil or alfentanil administration. In particular, there were no untoward hemodynamic events such as severe bradycardia, tachycardia, or hypotension requiring therapy or termination of the infusion. One subject experienced a brief (31 s) period of asymptomatic supraventricular tachycardia during remifentanil administration that spontaneously resolved.

The infusion schemes applied in this protocol resulted in concentration versus time curves characteristic of brief intravenous infusions. The raw data plots for each drug are contrasted in Figure 1.

Moment Analysis. The moment analysis reveals substantial differences in the pharmacokinetics of remifentanil and alfentanil. Remifentanil CL is approximately 8 times greater than alfentanil's. Similarly, remifentanil's MRT is roughly 13-fold shorter than alfentanil's. With respect to distribution, the drugs are more alike. Alfentanil's VD^ssis approximately 1.6 times greater than remifentanil's.

The individual and mean CL, MRT, and VD^ssvalues are shown in Table 1. Figure 2contrasts the model independent parameters for each drug in the same individual.

Nonlinear Mixed Effects Model Compartmental Analysis. As with the moment analysis, the NONMEM compartmental analysis reveals significant contrast between the pharmacokinetic parameters of remifentanil and alfentanil. Remifentanil's steady-state distribution volume is moderately smaller than alfentanil's. Remifentanil's clearance, however, is significantly greater than alfentanil's, a difference that is nearly an order of magnitude. The three-compartment model parameters for each drug estimated using the NONMEM approach are shown in Table 2.

The performance of the NONMEM models for both drugs are representative of what is generally expected for compartmental population models, with an MDAWR of approximately 15%. The MDAWRs for both models, along with the 10th and 90th percentiles, are shown in Table 3. Figure 3is a plot of the C^m/C^pversus time of these NONMEM models. Although there is no gross systematic bias, the models are obviously less accurate during the first few minutes of the infusion, during the first few minutes after stopping the infusion, and near the end of sampling.

Computer Simulations. The context-sensitive half-time simulations indicate that the decrease in blood concentration after drug administration is stopped is substantially more rapid for remifentanil than for alfentanil. Only 3 min is necessary to achieve a 50% decrease in remifentanil blood drug concentration after termination of an infusion despite lengthy infusions. Similarly, an 80% decrease in remifentanil blood concentration (i.e., 80% decrement time) can be achieved in 11 min. This is in contrast to the simulations for alfentanil; the time to achieve a decrease in blood concentration by 50% or 80% eventually plateaus at 46 and 161 min, respectively, exhibiting a marked dependence on the infusion duration. The results of these computer simulations are depicted in Figure 4.

Both remifentanil and alfentanil produce EEG changes characteristic of potent micro-receptor agonists. These changes consist of decreasing frequency and increasing amplitude in the raw EEG waveform, culminating eventually in pronounced delta-wave activity at maximal drug effect. An example of raw EEG signal from a remifentanil subject that is representative of the EEG changes observed in all subjects for both drugs is shown in Figure 5.

When processed by computer, the profound delta-wave activity produced by both drugs translates into a significant downward shift in the spectral edge parameter of the EEG. Figure 6shows a typical set of spectral edge parameter versus time data from a remifentanil subject.

K^e0Estimation. Remifentanil and alfentanil are similar with respect to the time required for equilibration of peak blood concentration and peak effect, as evidenced by their roughly equivalent k sub e0 values. Remifentanil's T¹/2 k^e0is 1.6 min; alfentanil's is 0.96 min. The k^e0and T¹/2 k^e0values for each subject are shown in Table 4. The upper panel of Figure 7contrasts the T¹/2 k^e0values for each drug in the same subject. Raw and collapsed remifentanil hysteresis loops representative of the entire data set for both drugs are shown in the upper and middle panels of Figure 8. Figure 8illustrates how the k^e0-optimizing procedure results in "collapsing" the raw hysteresis loop, thus identifying the pseudosteady-state concentration-effect relationship that then can be subjected to parametric modeling.

Parametric Modeling of the Concentration-Effect Relationship. Except for potency, the parametric modeling fails to reveal any significant differences in the EEG pharmacodynamics of remifentanil and alfentanil. The lower panel of Figure 8depicts a typical fit of the parametric model to the raw concentration-effect relationship. Remifentanil is 18.9 times more potent than alfentanil, as determined by comparison of the EC⁵⁰values. Remifentanil's mean EC⁵⁰is 19.9 ng *symbol* ml sup -1 compared with 375.9 ng *symbol* ml sup -1 for alfentanil. EC⁵⁰values for each subject, along with the other pharmacodynamic parameters, are displayed in Table 4. The lower panel of Figure 7contrasts the EC⁵⁰values for each drug in the same subject. Figure 9depicts the concentration-effect relationships for both drugs in each individual, as determined by the pharmacodynamic parameters estimated.

Computer Simulations. The computer simulations using the full pharmacokinetic/pharmacodynamic model reveal important similarities and differences in the clinical pharmacology of remifentanil and alfentanil. The simulation of equipotent bolus doses (Figure 10, upper panel) illustrates a nearly identical time to peak effect-site concentration for the two drugs. The simulation of a 2-h infusion targeted to equivalent effect-site concentrations (Figure 10, lower panel) illustrates the expected rapid decrease in remifentanil concentration after termination of the infusion compared to the more prolonged decline in alfentanil concentration.

This study has contrasted the pharmacokinetics and pharmacodynamics of remifentanil and alfentanil in a population of healthy adult male volunteers using a randomized, open-label, crossover design. In summary, although differing in potency, remifentanil exhibits alfentanil-like pharmacodynamics with a shorter-acting pharmacokinetic profile.

Each of the three data analysis techniques confirmed the pharmacokinetic differences between remifentanil and alfentanil. Inspection of the raw data (Figure 1) provides perhaps the most compelling and assumption-free evidence of how remifentanil's pharmacokinetics differ from alfentanil's. The slope of remifentanil's concentration decline after termination of the infusion is markedly steeper than alfentanil's.

(Figure 2) illustrates that remifentanil CL is nearly an order of magnitude greater than alfentanil's. This is reflected in the short MRT. With regard to tissue distribution, the differences are not as marked, with alfentanil's VD^ssbeing slightly greater than remifentanil's.

The NONMEM analysis, although encumbered with the assumptions of compartmental analysis, illustrates the pharmacokinetic differences between remifentanil and alfentanil. The most striking difference is remifentanil's rapid clearance, a difference that, as in the moment analysis, approaches an order of magnitude. As with the moment analysis, the differences in tissue distribution were not nearly as marked.

The context-sensitive half-time simulations based on the NONMEM population parameters are perhaps the most clinically interpretable way of illustrating the pharmacokinetic differences between remifentanil and alfentanil. Using concepts developed by Shafer and Varvel, [15]these simulations are an attempt to provide context-sensitive half-times, as proposed by Hughes et al. [16]In this case, the "context" is the duration of a continuous infusion. Defined as the time required to achieve a 50% decrease in concentration after termination of a continuous infusion targeting a constant concentration, context-sensitive half-times are a method of providing some clinically interpretable meaning to what can be a confusing table of pharmacokinetic parameters. [17]The simulations depicted in Figure 4contrast the short, time-independent context-sensitive half-time of remifentanil with the longer, time-dependent context-sensitive half-time of alfentanil (80% decrement times are also included in Figure 5).

The pharmacokinetic parameters and the related conclusions published herein for both remifentanil and alfentanil are consistent with the existing literature. [2-4,18,19]Compared to prior reports, however, a unique feature of this study is the crossover design, enabling prospective comparison of each drug's pharmacokinetics in a single group of subjects. Prior comparisons of remifentanil and alfentanil pharmacokinetics have relied on literature values for alfentanil. [2,3]The only other prospective comparison study involved smaller doses given to conscious volunteers and employed a parallel group design in which neither group received both drugs. [4]This study is thus the first prospective, crossover confirmation of the previously reported pharmacokinetic differences between remifentanil and alfentanil.

The pharmacokinetic parameters estimated for remifentanil in prior studies and confirmed in this study closely approximate those theoretically required when a rapid decline in blood concentration is desired after termination of drug administration. Recent pharmacokinetic simulations by Youngs and Shafer indicate that, when a rapid decrease in blood concentration is the goal, it is beneficial to have a small central volume and a large central clearance. [20]From a clinical viewpoint, remifentanil possesses these desirable pharmacokinetic parameters to ensure a rapid decline in concentration after termination of an infusion. [21].

An important limitation of our pharmacokinetic analysis is that our estimates of VD^ssassume that all clearance occurs in the central compartment. This assumption may not be fully applicable to remifentanil. Thus, our estimates of remifentanil's VD^ssmay be low.

The EEG pharmacodynamic data for each drug were analyzed using a hysteresis minimization technique followed by parametric modeling of the apparent concentration-effect relationship. With the exception of a 19-times greater potency, this analysis confirms remifentanil's pharmacodynamic similarity with alfentanil and, by extrapolation, the other fentanyl congeners.

Inspection of the raw pharmacodynamic data (Figure 6) reveals the nearly identical pharmacodynamic profile for the two drugs. Both exhibit identical changes in the raw EEG waveform and spectral edge parameter in terms of onset speed and magnitude of effect. Recovery to a baseline EEG pattern, however, is more rapid with remifentanil, as judged by the raw data plots. The nature and magnitude of these EEG changes are classic for the potent micro agonists and can be viewed as the EEG fingerprint of this drug class. [22].

With regard to the latency to peak effect, the hysteresis loop minimization technique results suggest that both drugs will exhibit rapid onset when administered in sufficient doses. The t¹/2k^e0parameter, a factor known to be important in determining onset of peak drug effect, [23]is similar for the two drugs. This is in contrast to fentanyl and sufentanil, both of which exhibit slower equilibration between plasma and effect-site concentrations and thus are known to be drugs of relatively longer latency to peak effect unless administered in high doses. [24,25].

The parametric modeling of the concentration-effect relationship estimated an 18.9-times greater potency of remifentanil compared to alfentanil. This finding confirms that remifentanil is moderately less potent than fentanyl. [24]However, in comparing the potencies of remifentanil and the other fentanyl congeners, it is important to note that the EC⁵⁰values for the other congeners have traditionally been reported in terms of plasma concentration. [18,24,25]Thus, correction for the partitioning of alfentanil between whole blood and plasma using a ratio of 0.63 is necessary when making extrapolations from previously published alfentanil literature. [26]When comparing a remifentanil whole blood EC⁵⁰value from this study with a corrected plasma alfentanil EC⁵⁰value (596.7 ng *symbol* ml sup -1), remifentanil is 30 times more potent than alfentanil.

The pharmacodynamic simulations are perhaps the most important means of contrasting the clinical pharmacology of the two drugs. Because they make use of the full pharmacokinetic/pharmacodynamic model, these simulations illustrate the complex interaction of all the kinetic-dynamic parameters in a clinically comprehensible way. [27]The simulation depicted in the upper panel of Figure 10confirms that, when administered in equipotent bolus doses, remifentanil will exhibit a short latency to peak effect that is comparable to alfentanil. The second simulation (Figure 10, lower panel) suggests that, after a 2-h equipotent infusion is terminated, remifentanil will exhibit an obviously more rapid decline in effect-site concentration. Based on these simulations, remifentanil can be expected to be a rapid-onset, rapid-offset opioid.

The implication of the first simulation may not appear to be consistent with the fact that alfentanil's T¹/2k^e0is shorter than remifentanil's. Because T¹/2k e0 is only one of many factors that contribute to drug onset time, the finding is not surprising. Drugs that manifest an extremely rapid decline in plasma concentration after termination of drug administration inevitably exhibit a short time to peak effect, because effect-site concentrations are driven by the central compartment concentration gradient. If central compartment concentrations decline rapidly, peak effect-site concentration will be reached quickly, albeit at a lower peak. Thus, remifentanil's pharmacokinetic profile, in addition to its T¹/2k^e0, contributes to its rapid latency to peak effect. For practical purposes, the results of this study would suggest that remifentanil and alfentanil are essentially equivalent in terms of latency to peak effect; that is, both drugs should be regarded as rapid-onset agents. It should be emphasized that simulation of effect-site concentrations that result from bolus dosing are potentially limited by the fact that compartmental models do not consider the effect of recirculatory peaks. [28].

For infusions of short duration (e.g., 10 min in this case) the pharmacokinetic differences between remifentanil and alfentanil are not readily apparent. Only after infusions of longer duration (Figure 4) do the pharmacokinetic differences become more obviously evident. The context-sensitive half-times for the currently marketed fentanyl congeners are not grossly divergent until infusions of longer than 20-30 min. [15].

The results of the pharmacodynamic modeling for both remifentanil and alfentanil reported here are consistent with the existing literature. A previous report of remifentanil pharmacodynamics employing an experimental pain model revealed a t¹/2k^e0of 1.3 min and a 20-30-times greater potency of remifentanil compared to alfentanil. [4]Similarly, previous reports of alfentanil pharmacodynamic parameters included t¹/2k^e0values of 0.9 and 1.1 min and EC⁰values of 479 and 520 ng *symbol* ml sup -1. [18,24].

Anesthesiologists have long recognized the need for a short-acting opioid with predictable pharmacokinetics. Because the lengths of surgical procedures often are unpredictable, and because the level of surgical stimulation against which the depth of anesthesia must be balanced is highly variable and dynamic, the advantages of predictably short-acting agents are obvious. Recent advances in drug development for anesthesia have trended toward shorter-acting agents of all types, including muscle relaxants, sedative hypnotics, and inhalation gases. Remifentanil represents an example of this direction toward shorter-acting agents. Future clinical use will determine whether the theoretical advantages associated with a short-acting opioid are realized.

*Beal SL, Sheiner LB: NONMEM User's Guide. San Francisco, University of California, San Francisco, 1979.