Pharmacokinetics and pharmacodynamics of cisatracurium after a short infusion in patients under propofol anesthesia

Synergism between rocuronium and cisatracurium: comparison of the Minto and Greco interaction models

Background

This study was conducted to investigate the pharmacodynamic interaction between rocuronium and cisatracurium using the response surface model, which is not subject to the limitations of traditional isobolographic analysis.

Methods

One hundred and twenty patients were randomly allocated to receive one of the fifteen predefined combinations of rocuronium and cisatracurium. To study single drugs, cisatracurium 0.2, 0.15, or 0.1 mg/kg or rocuronium 0.8, 0.6 or 0.4 mg/kg doses were administered alone. To study the pharmacodynamic interaction, drugs were applied in three types of combination ratio, i.e., half dose of each drug alone, 75% of each single dose of rocuronium and 25% of each single dose of cisatracurium, and vice versa. Train-of-four (TOF) ratio and T1% (first twitch of the TOF presented as percentage compared to the initial T1) were used as pharmacodynamic endpoints, and the Greco and Minto models were used as surface interaction models.

Results

The interaction term α of the Greco model for TOF ratio and T1% measurements showed synergism with values of 0.977 and 1.12, respectively. Application of the Minto model resulted in U₅₀ (θ) values (normalized unit of concentration that produces 50% of the maximal effect in the 0 < θ < 1 region) less than 1 for both TOF ratio and T1% measurements, indicating that rocuronium and cisatracurium exhibit synergism.

Conclusions

Response surface modeling of the interaction between rocuronium and cisatracurium, based on considerations of their effects on muscle relaxation as measured by TOF ratio and T1%, indicated that the two drugs show considerable synergism.

Simulation of the kinetics of neuromuscular block: implications for speed of onset

BACKGROUND

The onset time for paralysis varies 3-fold among nondepolarizing muscle relaxants. Possible explanations include: (a) pharmacokinetic differences among drugs and (b) buffering of drug molecules by acetylcholine receptors as they diffuse into the neuromuscular junction. Although some pharmacokinetic models consider buffered diffusion, these models do not account for either the high density of receptors or synapse geometry. Here, I used computer simulations to calculate the kinetics of buffered diffusion. The goal was to determine the conditions under which buffered diffusion could account for differences in onset time among nondepolarizing muscle relaxants.

METHODS

Monte Carlo simulation was used along with a realistic 3-dimensional model of the rat neuromuscular junction. Simulations determined the time dependence of the number of drug-bound receptors. A 1000-fold range of drug potency was examined. In some simulations, the drug concentration outside the junction was changed instantaneously. In other simulations, the concentration changed according to predictions of pharmacokinetic models assuming time-dependent changes in plasma drug concentration. The rate constant for equilibration of drug between plasma and muscle, keo, was varied between 0.15 and 0.6 min(-1). Twitch amplitude was calculated from receptor occupancy assuming a high safety margin for neuromuscular transmission. Some simulations used a synaptic model with an increased nerve-muscle contact width.

RESULTS

Simulations with instantaneous changes in drug concentration at the synapse, indicated that the time to 50% twitch depression (onset time) was 0.1 to 30 seconds and was proportional to drug potency. This corresponds to iontophoretic application of drug to isolated neuromuscular junctions, but is too fast to explain onset times in humans. When pharmacokinetic models were used to calculate the drug concentration outside the synapse, buffered diffusion increased onset times of potent drugs (drugs for which the effective concentration at 50% twitch height is <600 nM). Simulations using keo = 0.6 min(-1) and a model with a 2- to 3-fold wider nerve-muscle contact width indicated that buffered diffusion could account for the differences in clinical onset times among the nondepolarizing muscle relaxants.

CONCLUSION

Monte Carlo simulation provides a biophysically appropriate way to incorporate buffered diffusion into pharmacokinetic modeling. The simulations indicated that buffered diffusion could account for differences in onset time among drugs. However, a better understanding of the geometry of the human neuromuscular junction is needed before the magnitude of the effect of buffered diffusion can be quantified.

Effect of isoflurane versus propofol-remifentanil anesthesia on neuromuscular blockade and hemodynamic responses by cisatracurium bolus injection

BACKGROUND

Inhalational anesthetics potentiate nondepolarizing muscle relaxants. Cisatracurium is a recently introduced neuromuscular blocker in Korea. We studied the effect of inhalational anesthesia and total intravenous anesthesia (TIVA) on neuromuscular blockades and hemodynamic responses by cisatracurium bolus injection.

METHODS

Forty patients undergoing elective surgery were randomly divided into isoflurane and propofol-remifentanil groups. A bolus dose of cisatracurium of 0.15 mg/kg (3 × ED(95)) was administered after induction and the onset time and clinical duration of action were recorded. The nueromuscular blockade was monitored using train-of-four (TOF) stimulation. Hemodynamic parameters were also recorded.

RESULTS

Onset time was 194.0 ± 39.1 sec in the isoflurane group and 226.5 ± 62.2 sec in the propofol-remifentanil group. Clinical duration of action was 49.2 ± 9.0 min in the isoflurane group and 43.0 ± 9.2 min in the propofol-remifentanil group. Systolic blood pressure (SBP) and diastolic blood pressure (DBP) immediately before intubation decreased in the propofol-remifentanil group. Heart rate (HR), SBP and DBP 1 and 3 min after tracheal intubation increased in the isoflurane group.

CONCLUSIONS

Onset time was similar between isoflurane and propofol-remifentanil anesthesia. Clinical duration of action was significantly longer in isoflurane anesthesia. SBP and DBP immediately before intubation and HR, SBP and DBP 1 and 3 min after tracheal intubation were significantly different between the two groups.

Peripheral link model as an alternative for pharmacokinetic-pharmacodynamic modeling of drugs having a very short elimination half-life

Attempts to obtain estimates of pharmacokinetic-pharmacodynamic (PK-PD) parameters for mivacurium with traditional central link models were unsuccessful in many patients. We hypothesized that a link model with the peripheral compartment would be more appropriate for mivacurium in view of its extremely rapid plasma clearance and its potential elimination by tissue pseudocholinesterases. For validation purposes, the peripheral link model was applied to other neuromuscular blocking agents (NMBA), i.e., atracurium and doxacurium which have respectively an intermediate and a long elimination half-life. Assuming peripheral elimination in PK-PD modeling was investigated but found to have no impact on the estimation of PK-PD parameters. Our results indicate that, for drugs having intermediate and long elimination half-lives, EC50 values are similar with either the central or peripheral link model. For mivacurium, a peripheral link model enables PK-PD modeling in all subjects, with more precision in the PK-PD parameter estimates and a better fitting of the effect data when compared to the central link model. For these reasons, a peripheral link model should be preferred for mivacurium.

Recovery from neuromuscular blockade after either bolus and prolonged infusions of cisatracurium or rocuronium using either isoflurane or propofol-based anesthetics

We examined the recovery characteristics of cisatracurium or rocuronium after bolus or prolonged infusion under either isoflurane or propofol anesthesia. Sixty patients undergoing neurosurgical procedures of at least 5 h were randomized to receive either isoflurane with fentanyl (Groups 1 and 2) or propofol and fentanyl (Groups 3 and 4) as their anesthetic. Groups 1 and 3 received cisatracurium 0.2 mg/kg IV bolus, spontaneously recovered, after which time an infusion was begun. Groups 2 and 4 received rocuronium 0.6 mg/kg IV, spontaneously recovered, and an infusion was begun. Before the end of surgery, the infusion was stopped and recovery of first twitch (T(1)), recovery index, clinical duration, and train-of-four (TOF) recovery was recorded and compared among groups by using appropriate statistical methods. Clinical duration was shorter for rocuronium compared with cisatracurium using either anesthetic. Cisatracurium T(1) 75% recovery after the infusion was shorter with propofol compared with isoflurane. Cisatracurium TOF 75% recovery was similar after either bolus or infusion, but rocuronium TOF 75% recovery after the infusion was delayed. Infusion rates decreased for cisatracurium but remained relatively constant for rocuronium regardless of the anesthetic used. Isoflurane enhances the effect of both muscle relaxants but prolonged cisatracurium recovery more than rocuronium. Of the two muscle relaxants studied, rocuronium's recovery was most affected by length of the infusion. Cisatracurium may be a more desired muscle relaxant for prolonged procedures because recovery was least affected by prolonged infusion.

IMPLICATIONS

This study describes the effect of different anesthetic techniques on the recovery of two different muscle relaxants, cisatracurium and rocuronium, when administered as either a single bolus or prolonged infusion during neurosurgery. This study demonstrates the feasibility of using these relaxants for these prolonged procedures.

Assuming peripheral elimination: its impact on the estimation of pharmacokinetic parameters of muscle relaxants

For anesthetic drugs undergoing nonorgan-based elimination, there is a definite trend towards using pharmacokinetic (PK) models in which elimination can occur from both central (k10) and peripheral compartments (k20). As the latter cannot be assessed directly, assumptions have to be made regarding its value. The primary purpose of this paper is to evaluate the impact of assuming various degrees of peripheral elimination on the estimation of PK parameters. For doing so, an explanatory model is presented where previously published data from our laboratory on three muscle relaxants, i.e., atracurium, doxacurium, and mivacurium, are used for simulations. The mathematical aspects for this explanatory model as well as for two specific applications are detailed. Our simulations show that muscle relaxants having a short elimination half-life are more affected by the presence of peripheral elimination as their distribution phase occupies the major proportion of their total area under the curve. Changes in the exit site dependent PK parameters (Vdss) are also mostly significant when k20 is smaller than k10. Although the physiological processes that determine drug distribution and those affecting peripheral elimination are independent, the two are mathematically tied together in the two-compartment model with both central and peripheral elimination. It follows that, as greater importance is given to k20, the rate of transfer from the central compartment (k12) increases. However, as a result of a proportional increase in the volume of the peripheral compartment, peripheral concentrations remain unchanged whether or not peripheral elimination is assumed. These findings point out the limitations of compartmental analysis when peripheral elimination cannot be measured directly.