Early Drug Distribution: A Generally Neglected Aspect of Pharmacokinetics of Particular Relevance to Intravenously Administered Anesthetic Agents

DETERMINATION OF INDUCTION TIME IN ADULT PATIENTS WITH VALVULAR HEART DISEASE

OBJECTIVE

Intravenous anaesthetics induce loss of consciousness in one arm-brain circulation time. As the circulatory transit time in patients with mitral stenosis (MS) and aortic stenosis (AS) is increased, the delivery of anaesthetics to the brain may be prolonged and consequently the onset of hypnosis. This study aimed to compare the induction time in patients with and without valvular heart disease (VHD).

DESIGN

Prospective, single-center, open-label analytical study.

SETTING

It was conducted in adult patients undergoing elective cardiac surgery.

PARTICIPANTS

The patients (n = 144) were segregated into three groups; Group 1 - Stenotic VHD (MS, AS), Group 2 - Regurgitant VHD (Mitral Regurgitation, Aortic Regurgitation), and Group 3 - Control (coronary artery disease).

METHOD

General anaesthesia was induced with intravenous thiopental 4mg kg-1 bolus over 20s. The time to induction was noted as loss of eyelash reflex. Bispectral Index (BIS) values were recorded over 2 minutes. Statistical analysis was performed using SPSS software version 25.0. A P value < 0.05 was considered significant.

MAIN RESULTS

Patients in Group 1 (n = 48) had significantly prolonged induction time (99.6 ± 12.9s; P < 0.001) compared to the other two groups (n = 48 each) (68.5 ± 9.6s in Group 2 and 70.4 ± 11.8s in Group 3). Time required for BIS to fall below 60 was significantly longer in Group 1 (139.4 ± 24.6s; P < 0.001) compared to Group 2 (90.4 ± 6.3s) and Group 3 (92.1 ± 12s).

CONCLUSION

The induction time was prolonged in patients with stenotic VHD compared to patients with regurgitant VHD or those without VHD.

A review of equine anesthetic induction: Are all equine anesthetic inductions "crash" inductions?

Horses are the most challenging of the common companion animals to anesthetize. Induction of anesthesia in the horse is complicated by the fact that it is accompanied by a transition from a conscious standing position to uncconconscious recumbency. The purpose of this article is to review the literature on induction of anesthesia with a focus on the behavioral and physiologic/pharmacodynamic responses and the actions and interactions of the drugs administered to induce anesthesia in the healthy adult horse with the goal of increasing consistency and predictability.

Distribution Clearance: Significance and Underlying Mechanisms

PURPOSE

Evaluation of distribution kinetics is a neglected aspect of pharmacokinetics. This study examines the utility of the model-independent parameter whole body distribution clearance (CLD) in this respect.

METHODS

Since mammillary compartmental models are widely used, CLD was calculated in terms of parameters of this model for 15 drugs. The underlying distribution processes were explored by assessment of relationships to pharmacokinetic parameters and covariates.

RESULTS

The model-independence of the definition of the parameter CLD allowed a comparison of distributional properties of different drugs and provided physiological insight. Significant changes in CLD were observed as a result of drug-drug interactions, transporter polymorphisms and a diseased state.

CONCLUSION

Total distribution clearance CLD is a useful parameter to evaluate distribution kinetics of drugs. Its estimation as an adjunct to the model-independent parameters clearance and steady-state volume of distribution is advocated.

The effect of posture and exercise on blood CO kinetics during the optimized carbon monoxide rebreathing procedure

An indispensable precondition for the determination of hemoglobin mass (Hbmass) and blood volume by CO rebreathing is complete mixing of CO in the blood. The aim of this study was to demonstrate the kinetics of CO in capillary and venous blood in different body positions and during moderate exercise. Six young subjects (4 male, 2 female) performed three 2-min CO rebreathing tests in seated (SEA) & supine (SUP) positions as well as during moderate exercise (EX) on a bicycle ergometer. Before, during, and until 15 min after CO rebreathing cubital venous and capillary blood samples were collected simultaneously and COHb% was determined. COHb% kinetics were significantly slower in SEA than in SUP or EX. Identical COHb% in capillary and venous blood were reached in SEA after 5.0 ± 2.3 min, in SUP after 3.2 ± 1.3 min and in EX after 1.9 ± 1.2 min (EX vs. SEA p < .01, SUP vs. SEA p < .05). After 7th min, Hbmass did not differ between the resting positions (capillary: SEA 766 ± 217 g, SUP 761 ± 227 g; venous: SEA 759 ± 224 g, SUP 744 ± 207 g). Under exercise, however, a higher Hbmass (p < .05) was determined (capillary: 823 ± 221 g, venous: 804 ± 226 g). In blood, the CO mixing time in the supine position is significantly shorter than in the seated position. By the 6th minute complete mixing is achieved in either position giving similar Hbmass determinations. CO-rebreathing under exercise conditions, however, leads to ∼7% higher Hbmass values.

Theoretical Examination Seeking Tangible Physical Meanings of Slopes and Intercepts of Plasma Concentration-Time Relationships in Minimal Physiologically Based Pharmacokinetic Models

In minimal physiologically based pharmacokinetic (mPBPK) models, physiological (e.g., cardiac output) and anatomical (e.g., blood/tissue volumes) variables are utilized in the domain of differential equations (DEs) for mechanistic understanding of the plasma concentration-time relationships [Formula: see text]. Although fundamental biopharmaceutical variables in terms of distribution (e.g., [Formula: see text] and [Formula: see text]) and elimination kinetics (e.g., [Formula: see text]) in mPBPK provide greater insights in comparison to classical compartment models, an absence of kinetic elucidation of slopes and intercepts in light of such DE model parameters hinders more intuitive appreciation of [Formula: see text]. Therefore, this study seeks the tangible physical meanings of slopes and intercepts of the plasma concentration-time relationships in one- and two-tissue mPBPK models (i.e., m2CM and m3CM), with respect to time parameters that are readily understandable in PK analyses, i.e., the mean residence ([Formula: see text]) and transit ([Formula: see text]) times. Utilizing the explicit equations (EEs) for the slopes, intercepts, and areas of each exponential phase in the m2CM and m3CM, we theoretically and numerically examined the limiting/boundary conditions of such kinetic properties, based on the ratio of the longest tissue [Formula: see text] to the [Formula: see text] in the body (i.e., [Formula: see text]) that is useful for dissecting complex PBPK systems. The kinetic contribution of the area of each exponential phase to the total drug exposure was assessed to identify the elimination phase between the terminal and non-terminal phases of the [Formula: see text] in the m2CM and m3CM. This assessment provides improved understanding of the complexities inherent in all PBPK profiles and models.

Influence of Sampling Site and Fluid Flow on the Accuracy of Total Body Clearance Calculation

Studies have showed that by assuming arteriovenous drug concentrations are homogenous after intravenous injection, the determination of total body clearance based on venous drug concentrations is often inaccurate. This study considers the use of a fluidic pharmacokinetic profile generator where 28 different profile types were generated corresponding to a physiological model with varying sampling sites, administration locations, and fluid flow rates. Clearance was calculated using established equations, commercial software, and recently proposed models. The results show large differences in clearance values calculated with published equations and commercial software relative to the actual value of clearance. Alterations in sampling site, administration location, and fluid flow rates each influence the extent of calculation errors. The data show that a significant drug concentration gradient exists within the central circulatory system. The results show that the best way to address this issue would be to inject the drug at a peripheral location to allow for sufficient mixing and then sample from a large vein. Extrapolating for missing data can also lead to large errors in clearance calculation; this can be addressed by collecting more samples early after IV bolus administration or by collecting data during steady state conditions for an IV infusion.

Pharmacokinetics of vatinoxan in male neutered cats anesthetized with isoflurane

OBJECTIVE

To characterize the pharmacokinetics of vatinoxan in isoflurane-anesthetized cats.

STUDY DESIGN

Prospective experimental study.

ANIMALS

A group of six adult healthy male neutered cats.

METHODS

Cats were anesthetized using isoflurane in oxygen. Venous catheters were placed to administer the drug and sample blood. Vatinoxan, 1 mg kg^-1, was administered intravenously over 5 minutes. Blood was sampled before and at various times during and up to 8 hours after vatinoxan administration. Plasma vatinoxan concentration was measured using liquid chromatography/tandem mass spectrometry. Compartment models were fitted to the time-concentration data using population methods and nonlinear mixed effect modeling.

RESULTS

A three-compartment model best fitted the data. Typical value (% interindividual variability) for the three volumes (mL kg^-1), the metabolic clearance and two distribution clearances (mL minute^-1 kg^-1) were 34 (55), 151 (35), 306 (18), 2.3 (34), 42.6 (25) and 5.6 (0), respectively. Hypotension increased the second distribution clearance to 10.6.

CONCLUSION AND CLINICAL RELEVANCE

The pharmacokinetics of vatinoxan in anesthetized cats were characterized by a small volume of distribution and a low clearance. An intravenous bolus of 100 μg kg^-1 of vatinoxan followed by constant rate infusions of 55 μg kg^-1 minute^-1 for 20 minutes, then 22 μg kg^-1 minute^-1 for 60 minutes and finally 10 μg kg^-1 minute^-1 for the remainder of the infusion time is expected to maintain the plasma concentration within 90%-110% of the plasma vatinoxan concentration previously shown to attenuate the cardiovascular effects of dexmedetomidine (25 μg kg^-1) in conscious cats.

Combined Recirculatory-compartmental Population Pharmacokinetic Modeling of Arterial and Venous Plasma S(+) and R(–) Ketamine Concentrations

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

The pharmacokinetics of infused drugs have been modeled without regard for recirculatory or mixing kinetics. We used a unique ketamine dataset with simultaneous arterial and venous blood sampling, during and after separate S(+) and R(–) ketamine infusions, to develop a simplified recirculatory model of arterial and venous plasma drug concentrations.

Methods

S(+) or R(–) ketamine was infused over 30 min on two occasions to 10 healthy male volunteers. Frequent, simultaneous arterial and forearm venous blood samples were obtained for up to 11 h. A multicompartmental pharmacokinetic model with front-end arterial mixing and venous blood components was developed using nonlinear mixed effects analyses.

Results

A three-compartment base pharmacokinetic model with additional arterial mixing and arm venous compartments and with shared S(+)/R(–) distribution kinetics proved superior to standard compartmental modeling approaches. Total pharmacokinetic flow was estimated to be 7.59 ± 0.36 l/min (mean ± standard error of the estimate), and S(+) and R(–) elimination clearances were 1.23 ± 0.04 and 1.06 ± 0.03 l/min, respectively. The arm-tissue link rate constant was 0.18 ± 0.01 min–1, and the fraction of arm blood flow estimated to exchange with arm tissue was 0.04 ± 0.01.

Conclusions

Arterial drug concentrations measured during drug infusion have two kinetically distinct components: partially or lung-mixed drug and fully mixed-recirculated drug. Front-end kinetics suggest the partially mixed concentration is proportional to the ratio of infusion rate and total pharmacokinetic flow. This simplified modeling approach could lead to more generalizable models for target-controlled infusions and improved methods for analyzing pharmacokinetic-pharmacodynamic data.

Pharmacokinetic-pharmacodynamic model for propofol for broad application in anaesthesia and sedation

BACKGROUND

Pharmacokinetic (PK) and pharmacodynamic (PD) models are used in target-controlled-infusion (TCI) systems to determine the optimal drug administration to achieve a desired target concentration in a central or effect-site compartment. Our aim was to develop a PK-PD model for propofol that can predict the bispectral index (BIS) for a broad population, suitable for TCI applications.

METHODS

Propofol PK data were obtained from 30 previously published studies, five of which also contained BIS observations. A PK-PD model was developed using NONMEM. Weight, age, post-menstrual age (PMA), height, sex, BMI, and presence/absence of concomitant anaesthetic drugs were explored as covariates. The predictive performance was measured across young children, children, adults, elderly, and high-BMI individuals, and in simulated TCI applications.

RESULTS

Overall, 15 433 propofol concentration and 28 639 BIS observations from 1033 individuals (672 males and 361 females) were analysed. The age range was from 27 weeks PMA to 88 yr, and the weight range was 0.68-160 kg. The final model uses age, PMA, weight, height, sex, and presence/absence of concomitant anaesthetic drugs as covariates. A 35-yr-old, 170 cm, 70 kg male (without concomitant anaesthetic drugs) has a V₁, V₂, V₃, CL, Q₂, Q₃, and k_e0 of 6.28, 25.5, 273 litres, 1.79, 1.75, 1.11 litres min^-1, and 0.146 min^-1, respectively. The propofol TCI administration using the model matches well with recommendations for all age groups considered for both anaesthesia and sedation.

CONCLUSIONS

We developed a PK-PD model to predict the propofol concentrations and BIS for broad, diverse population. This should be useful for TCI in anaesthesia and sedation.

Beta blockade increases pulmonary and systemic transit time heterogeneity: evaluation based on indocyanine green kinetics in healthy volunteers

Knowledge of factors influencing the heterogeneity of blood transit times is important in cardiovascular physiology. The aim of the study was to investigate the effect of beta-adrenergic blockade on blood transit time dispersion in awake, anxious volunteers. Recirculatory modelling of the disposition of intravascular markers using parametric forms for transit time distributions, such as the inverse Gaussian distribution, provides the opportunity to estimate the systemic and pulmonary transit time dispersion in vivo. The latter is determined by the flow heterogeneity in the microcirculatory network. Using this approach, we have analysed indocyanine green (ICG) disposition data obtained in four subjects by frequent early arterial blood sampling before and after beta-adrenergic blockade by propranolol. Propranolol decreased cardiac output from 9·3 ± 2·8 l min^-1 to 3·5 ± 0·47 l min^-1 (P<0·05). This reduction was accompanied by a 4·5 ± 0·6-fold and 2·1 ± 0·3-fold increase (P<0·001) in the relative dispersion (dimensionless variance) of blood transit times through the systemic and pulmonary circulation, respectively.

Diazepam or midazolam for orotracheal intubation in the ICU?

OBJECTIVE

to compare clinical and cost effectiveness of midazolam and diazepam for urgent intubation.

METHODS

patients admitted to the Central ICU of the Santa Casa Hospital Complex in Porto Alegre, over the age of 18 years, undergoing urgent intubation during 6 months were eligible. Patients were randomized in a single-blinded manner to either intravenous diazepam or midazolam. Diazepam was given as a 5 mg intravenous bolus followed by aliquots of 5 mg each minute. Midazolam was given as an intravenous bolus of 5 mg with further aliquots of 2.5 mg each minute. Ramsay sedation scale 5-6 was considered adequate sedation. We recorded time and required doses to reach adequate sedation and duration of sedation.

RESULTS

thirty four patients were randomized, but one patient in the diazepam group was excluded because data were lost. Both groups were similar in terms of illness severity and demographics. Time for adequate sedation was shorter (132 ± 87 sec vs. 224 ± 117 sec, p = 0.016) but duration of sedation was similar (86 ± 67 min vs. 88 ± 50 min, p = 0.936) for diazepam in comparison to midazolam. Total drug dose to reach adequate sedation after either drugs was similar (10.0 [10.0-12.5] mg vs. 15.0 [10.0-17.5] mg, p = 0.248). Arterial pressure and sedation intensity reduced similarly overtime with both drugs. Cost of sedation was lower for diazepam than for midazolam (1.4[1.4-1.8] vs. 13.9[9.4-16.2] reais, p <0.001).

CONCLUSIONS

intubation using intravenous diazepam and midazolam is effective and well tolerated. Sedation with diazepam is associated to a quicker sedation time and to lower costs.

Population pharmacokinetic-pharmacodynamic analysis for sugammadex-mediated reversal of rocuronium-induced neuromuscular blockade

AIMS

An integrated population pharmacokinetic-pharmacodynamic model was developed with the following aims: to simultaneously describe pharmacokinetic behaviour of sugammadex and rocuronium; to establish the pharmacokinetic-pharmacodynamic model for rocuronium-induced neuromuscular blockade and reversal by sugammadex; to evaluate covariate effects; and to explore, by simulation, typical covariate effects on reversal time.

METHODS

Data (n= 446) from eight sugammadex clinical studies covering men, women, non-Asians, Asians, paediatrics, adults and the elderly, with various degrees of renal impairment, were used. Modelling and simulation techniques based on physiological principles were applied to capture rocuronium and sugammadex pharmacokinetics and pharmacodynamics and to identify and quantify covariate effects.

RESULTS

Sugammadex pharmacokinetics were affected by renal function, bodyweight and race, and rocuronium pharmacokinetics were affected by age, renal function and race. Sevoflurane potentiated rocuronium-induced neuromuscular blockade. Posterior predictive checks and bootstrapping illustrated the accuracy and robustness of the model. External validation showed concordance between observed and predicted reversal times, but interindividual variability in reversal time was pronounced. Simulated reversal times in typical adults were 0.8, 1.5 and 1.4 min upon reversal with sugammadex 16 mg kg(-1) 3 min after rocuronium, sugammadex 4 mg kg(-1) during deep neuromuscular blockade and sugammadex 2 mg kg(-1) during moderate blockade, respectively. Simulations indicated that reversal times were faster in paediatric patients and slightly slower in elderly patients compared with adults. Renal function did not affect reversal time.

CONCLUSIONS

Simulations of the therapeutic dosing regimens demonstrated limited impact of age, renal function and sevoflurane use, as predicted reversal time in typical subjects was always <2 min.

Performance evaluation of paediatric propofol pharmacokinetic models in healthy young children

BACKGROUND

The performance of eight currently available paediatric propofol pharmacokinetic models in target-controlled infusions (TCIs) was assessed, in healthy children from 3 to 26 months of age.

METHODS

Forty-one, ASA I-II children, aged 3-26 months were studied. After the induction of general anaesthesia with sevoflurane and remifentanil, a propofol bolus dose of 2.5 mg kg(-1) followed by an infusion of 8 mg kg(-1) h(-1) was given. Arterial blood samples were collected at 1, 2, 3, 5, 10, 20, 40, and 60 min post-bolus, at the end of surgery, and at 1, 3, 5, 30, 60, and 120 min after stopping the infusion. Model performance was visually inspected with measured/predicted plots. Median performance error (MDPE) and the median absolute performance error (MDAPE) were calculated to measure bias and accuracy of each model.

RESULTS

Performance of the eight models tested differed markedly during the different stages of propofol administration. Most models underestimated propofol concentration 1 min after the bolus dose, suggesting an overestimation of the initial volume of distribution. Six of the eight models tested were within the accepted limits of performance (MDPE<20% and MDAPE<30%). The model derived by Short and colleagues performed best.

CONCLUSIONS

Our results suggest that six of the eight models tested perform well in young children. Since most models overestimate the initial volume of distribution, the use for TCI might result in the administration of larger bolus doses than necessary.

Predictive ability of propofol effect-site concentrations during fast and slow infusion rates

BACKGROUND

The performance of propofol effect-site pharmacokinetic models during target-controlled infusion (TCI) might be affected by propofol administration rate. This study compares the predictive ability of three effect-site pharmacokinetic models during fast and slow infusion rates, utilizing the cerebral state index (CSI) as a monitor of consciousness.

METHODS

Sixteen healthy volunteers, 21-45 years of age, were randomly assigned to receive either a bolus dose of propofol 1.8 mg/kg at a rate of 1200 ml/h or an infusion of 12 mg/kg/h until 3-5 min after loss of consciousness (LOC). After spontaneous recovery of the CSI, the bolus was administered to patients who had first received the infusion and vice versa. The study was completed after spontaneous recovery of CSI following the second dose scheme. LOC was assessed and recorded when it occurred. Adequacies of model predictions during both administration schemes were assessed by comparing the effect-site concentrations estimated at the time of LOC during the bolus dose and during the infusion scheme.

RESULTS

LOC occurred 0.97 +/- 0.29 min after the bolus dose and 6.77 +/- 3.82 min after beginning the infusion scheme (P<0.05). The Ce estimated with Schnider (ke0=0.45/min), Marsh (ke0=1.21/min) and Marsh (ke0=0.26/min) at LOC were 4.40 +/- 1.45, 3.55 +/- 0.64 and 1.28 +/- 0.44 microg/ml during the bolus dose and 2.81 +/- 0.61, 2.50 +/- 0.39 and 1.72 +/- 0.41 microg/ml, during the infusion scheme (P<0.05). The CSI values observed at LOC were 70 +/- 4 during the bolus dose and 71 +/- 2 during the infusion scheme (NS).

CONCLUSION

Speed of infusion, within the ranges allowed by TCI pumps, significantly affects the accuracy of Ce predictions. The CSI monitor was shown to be a useful tool to predict LOC in both rapid and slow infusion schemes.

Cardiac output and systemic transit time dispersion as determinants of circulatory mixing time: a simulation study

A new approach to characterize the kinetics of intravascular mixing process is presented. The mixing time, defined as the time required for achieving 95% homogeneity, is calculated by numerical simulations using a circulatory model applied to the intravascular marker indocyanine green (ICG). The results suggest that the mixing time is determined by cardiac output and the relative dispersion of transit time distribution across the systemic circulation, whereby the rate of mixing increases with increasing cardiac output and decreasing transit time dispersion, and vice versa. The estimation of plasma volume from simulated ICG dilution data using the backextrapolation method shows that slow mixing is accompanied by an overestimation of blood volume. This error may be negligible for mixing times of less than approximately 3 min but high in disease states characterized by low cardiac output and/or high transit time dispersion. In view of the role of transit time dispersion as determinant of intravascular mixing, it would be interesting to know more about the effect of disease states on systemic transit time dispersion.