Pharmacokinetics of propofol in adult patients undergoing coronary revascularization. The Multicenter Study of Perioperative Ischemia Research Group

Rapid Infusion of Hydroxyethyl Starch 70/0.5 but not Acetate Ringer’s Solution Decreases the Plasma Concentration of Propofol during Target-controlled Infusion

Background

Rapid fluid infusion resulting in increased hepatic blood flow may decrease the propofol plasma concentration (Cp) because propofol is a high hepatic extraction drug. The authors investigated the effects of rapid colloid and crystalloid infusions on the propofol Cp during target-controlled infusion.

Methods

Thirty-six patients were randomly assigned to 1 of 3 interventions (12 patients per group). At least 30 min after the start of propofol infusion, patients received either a 6% hydroxyethyl starch (HES) solution at 24 ml·kg−1·h−1 or acetated Ringer’s solution at 24 or 2 ml·kg−1·h−1 during the first 20 min. In all groups, acetated Ringer’s solution was infused at 2 ml·kg−1·h−1 during the next 20 min. The propofol Cp was measured every 2.5 min as the primary outcome. Cardiac output, blood volume, and indocyanine green disappearance rate were determined using a pulse dye densitogram analyzer before and after the start of fluid administration. Effective hepatic blood flow was calculated as the blood volume multiplied by the indocyanine green disappearance rate.

Results

The rapid HES infusion significantly decreased the propofol Cp by 22 to 37%, compared to the Cp at 0 min, whereas the rapid or maintenance infusion of acetate Ringer’s solution did not decrease the propofol Cp. Rapid HES infusion, but not acetate Ringer’s solution infusion, increased the effective hepatic blood flow.

Conclusions

Rapid HES infusion increased the effective hepatic blood flow, resulting in a decreased propofol Cp during target-controlled infusion. Rapid HES infusion should be used cautiously as it may decrease the depth of anesthesia.

A general purpose pharmacokinetic model for propofol

BACKGROUND

Pharmacokinetic (PK) models are used to predict drug concentrations for infusion regimens for intraoperative displays and to calculate infusion rates in target-controlled infusion systems. For propofol, the PK models available in the literature were mostly developed from particular patient groups or anesthetic techniques, and there is uncertainty of the accuracy of the models under differing patient and clinical conditions. Our goal was to determine a PK model with robust predictive performance for a wide range of patient groups and clinical conditions.

METHODS

We aggregated and analyzed 21 previously published propofol datasets containing data from young children, children, adults, elderly, and obese individuals. A 3-compartmental allometric model was estimated with NONMEM software using weight, age, sex, and patient status as covariates. A predictive performance metric focused on intraoperative conditions was devised and used along with the Akaike information criteria to guide model development.

RESULTS

The dataset contains 10,927 drug concentration observations from 660 individuals (age range 0.25-88 years; weight range 5.2-160 kg). The final model uses weight, age, sex, and patient versus healthy volunteer as covariates. Parameter estimates for a 35-year, 70-kg male patient were: 9.77, 29.0, 134 L, 1.53, 1.42, and 0.608 L/min for V1, V2, V3, CL, Q2, and Q3, respectively. Predictive performance is better than or similar to that of specialized models, even for the subpopulations on which those models were derived.

CONCLUSIONS

We have developed a single propofol PK model that performed well for a wide range of patient groups and clinical conditions. Further prospective evaluation of the model is needed.

Renal drug metabolism in humans: the potential for drug-endobiotic interactions involving cytochrome P450 (CYP) and UDP-glucuronosyltransferase (UGT)

Although knowledge of human renal cytochrome P450 (CYP) and UDP-glucuronosyltransferase (UGT) enzymes and their role in xenobiotic and endobiotic metabolism is limited compared with hepatic drug and chemical metabolism, accumulating evidence indicates that human kidney has significant metabolic capacity. Of the drug metabolizing P450s in families 1 to 3, there is definitive evidence for only CYP 2B6 and 3A5 expression in human kidney. CYP 1A1, 1A2, 1B1, 2A6, 2C19, 2D6 and 2E1 are not expressed in human kidney, while data for CYP 2C8, 2C9 and 3A4 expression are equivocal. It is further known that several P450 enzymes involved in the metabolism of arachidonic acid and eicosanoids are expressed in human kidney, CYP 4A11, 4F2, 4F8, 4F11 and 4F12. With the current limited evidence of drug substrates for human renal P450s drug-endobiotic interactions arising from inhibition of renal P450s, particularly effects on arachidonic acid metabolism, appear unlikely. With respect to the UGTs, 1A5, 1A6, 1A7, 1A9, 2B4, 2B7 and 2B17 are expressed in human kidney, whereas UGT 1A1, 1A3, 1A4, 1A8, 1A10, 2B10, 2B11 and 2B15 are not. The most abundantly expressed renal UGTs are 1A9 and 2B7, which play a significant role in the glucuronidation of drugs, arachidonic acid, prostaglandins, leukotrienes and P450 derived arachidonic acid metabolites. Modulation by drug substrates (e.g. NSAIDs) of the intrarenal activity of UGT1A9 and UGT2B7 has the potential to perturb the metabolism of renal mediators including aldosterone, prostaglandins and 20-hydroxyeicosatetraenoic acid, thus disrupting renal homeostasis.

Cross-validation for nonlinear mixed effects models

Cross-validation is frequently used for model selection in a variety of applications. However, it is difficult to apply cross-validation to mixed effects models (including nonlinear mixed effects models or NLME models) due to the fact that cross-validation requires "out-of-sample" predictions of the outcome variable, which cannot be easily calculated when random effects are present. We describe two novel variants of cross-validation that can be applied to NLME models. One variant, where out-of-sample predictions are based on post hoc estimates of the random effects, can be used to select the overall structural model. Another variant, where cross-validation seeks to minimize the estimated random effects rather than the estimated residuals, can be used to select covariates to include in the model. We show that these methods produce accurate results in a variety of simulated data sets and apply them to two publicly available population pharmacokinetic data sets.

The impact of extracorporeal life support and hypothermia on drug disposition in critically ill infants and children

Extracorporeal membrane oxygenation (ECMO) support is an established lifesaving therapy for potentially reversible respiratory or cardiac failure. In 10% of all pediatric patients receiving ECMO, ECMO therapy is initiated during or after cardiopulmonary resuscitation. Therapeutic hypothermia is frequently used in children after cardiac arrest, despite the lack of randomized controlled trials that show its efficacy. Hypothermia is frequently used in children and neonates during cardiopulmonary bypass (CPB). By combining data from pharmacokinetic studies in children on ECMO and CPB and during hypothermia, this review elucidates the possible effects of hypothermia during ECMO on drug disposition.

Extracorporeal circulation interference on emergence from anesthesia in patients submitted to myocardial revascularization

BACKGROUND AND OBJECTIVES

Extracorporeal circulation (ECC) may change drug pharmacokinetics as well as brain function. The objectives of this study are to compare emergence time and postoperative sedation intensity assessed by the bispectral index (BIS) and the Ramsay sedation scale in patients undergoing myocardial revascularization (MR) with or without ECC.

METHOD

Ten patients undergoing MR with ECC (ECC group) and 10 with no ECC (no-ECC group) were administered with sufentanyl, propofol 2.0 μg.mL(-1) and pancuronium target controlled infusion. After surgery, propofol infusion was reduced to 1 μg.mL(-1) and suspended when extubation was indicated. Patients BIS, Ramsay scale and time to wake up were assessed.

RESULTS

The ECC group showed lower BIS values beginning at 60 minutes after surgery (no-ECC = 66±13 and ECC = 53±14, p=0.01) until 120 minutes after infusion (no-ECC = 85±8 and ECC = 73±12, p=0.02). Sedation level measured by the Ramsay scale was higher in the ECC group at 30 minutes after the end of the surgery (no-ECC = 5±1 and ECC = 6±0, p=0.021), at the end of infusion (no-ECC = 5±1 and ECC = 6±1, p=0.012) and 5 minutes after the end of infusion (no-ECC = 4±1 and ECC = 5±0.42, p=0.039). Emergence from anesthesia time was higher in the ECC group (no-ECC = 217±81 and ECC = 319±118, p=0.038).

CONCLUSIONS

There was a higher intensity of sedation after the end of surgery and a longer wake up time in ECC group, suggesting changes in the pharmacokinetics of propofol or effects of ECC on central nervous system.

Brain monitoring with electroencephalography and the electroencephalogram-derived bispectral index during cardiac surgery

Cardiac surgery presents particular challenges for the anesthesiologist. In addition to standard and advanced monitors typically used during cardiac surgery, anesthesiologists may consider monitoring the brain with raw or processed electroencephalography (EEG). There is strong evidence that a protocol incorporating the processed EEG bispectral index (BIS) decreases the incidence intraoperative awareness in comparison with standard practice. However, there is conflicting evidence that incorporating the BIS into cardiac anesthesia practice improves "fast-tracking," decreases anesthetic drug use, or detects cerebral ischemia. Recent research, including many cardiac surgical patients, shows that a protocol based on BIS monitoring is not superior to a protocol based on end-tidal anesthetic concentration monitoring in preventing awareness. There has been a resurgence of interest in the anesthesia literature in limited montage EEG monitoring, including nonproprietary processed indices. This has been accompanied by research showing that with structured training, anesthesiologists can glean useful information from the raw EEG trace. In this review, we discuss both the hypothesized benefits and limitations of BIS and frontal channel EEG monitoring in the cardiac surgical population.

Reinforcement learning versus proportional-integral-derivative control of hypnosis in a simulated intraoperative patient

BACKGROUND

Research has demonstrated the efficacy of closed-loop control of anesthesia using bispectral index (BIS) as the controlled variable. Model-based and proportional-integral-derivative (PID) controllers outperform manual control. We investigated the application of reinforcement learning (RL), an intelligent systems control method, to closed-loop BIS-guided, propofol-induced hypnosis in simulated intraoperative patients. We also compared the performance of the RL agent against that of a conventional PID controller.

METHODS

The RL and PID controllers were evaluated during propofol induction and maintenance of hypnosis. The patient-hypnotic episodes were designed to challenge both controllers with varying degrees of interindividual variation and noxious surgical stimulation. Each controller was tested in 1000 simulated patients, and control performance was assessed by calculating the median performance error (MDPE), median absolute performance error (MDAPE), Wobble, and Divergence for each controller group. A separate analysis was performed for the induction and maintenance phases of hypnosis.

RESULTS

During maintenance, RL control demonstrated an MDPE of -1% and an MDAPE of 3.75%, with 80% of the time at BIS(target) ± 5. The PID controller yielded a MDPE of -8.5% and an MDAPE of 8.6%, with 57% of the time at BIS(target) ± 5. In comparison, the MDAPE in the worst-controlled patient of the RL group was observed to be almost half that of the worst-controlled patient in the PID group.

CONCLUSIONS

When compared with the PID controller, RL control resulted in slower induction but less overshoot and faster attainment of steady state. No difference in interindividual patient variation and noxious destabilizing challenge on control performance was observed between the 2 patient groups.

Effects of cardiopulmonary bypass on propofol pharmacokinetics and bispectral index during coronary surgery

PURPOSE

Cardiopulmonary bypass is known to alter propofol pharmacokinetics in patients undergoing cardiac surgery. However, few studies have evaluated the impact of these alterations on postoperative pharmacodynamics. This study was designed to test the hypothesis that changes in propofol pharmacokinetics increase hypnotic effects after cardiopulmonary bypass.

METHODS

Twenty patients scheduled for on-pump coronary artery bypass graft (group, n=10) or off-pump coronary artery bypass graft (group, n=10) coronary artery bypass grafts were anesthetized with sufentanil and a propofol target controlled infusion (2.0 microg/mL). Depth of hypnosis was monitored using the bispectral index. Blood samples were collected from the induction of anesthesia up to 12 hours after the end of propofol infusion, at predetermined intervals. Plasma propofol concentrations were measured using high-performance liquid chromatography, followed by a non-compartmental propofol pharmacokinetic analysis. Data were analyzed using ANOVA, considering p<0.05 as significant.

RESULTS

After cardiopulmonary bypass, despite similar plasma propofol concentrations in both groups, bispectral index values were lower in the on-pump coronary artery bypass graft group. Time to extubation after the end of propofol infusion was greater in the on-pump coronary artery bypass graft group (334 +/- 117 vs. 216 +/- 85 min, p = 0.04). Patients undergoing cardiopulmonary bypass had shorter biological (1.82 +/- 0.5 vs. 3.67 +/- 1.15 h, p < 0.01) and terminal elimination (6.27 +/- 1.29 vs. 10.5h +/- 2.18, p < 0.01) half-life values, as well as higher total plasma clearance (28.36 +/- 11.40 vs.18.29 +/- 7.67 mL/kg/min, p = 0.03), compared to patients in the off-pump coronary artery bypass graft group.

CONCLUSION

Aside from the increased sensitivity of the brain to anesthetics after cardiopulmonary bypass, changes in propofol pharmacokinetics may contribute to its central nervous system effects.

Impact of bispectral index monitoring on propofol administration in patients undergoing cardiopulmonary bypass

Propofol anaesthesia using target control infusion during cardiac surgery has become more popular recently. However, without depth of anaesthesia monitoring, the standard target concentration used may be higher than necessary to maintain adequate hypnosis during hypothermic cardiopulmonary bypass. The purpose of this study was to evaluate the effect of bispectral index monitoring on propofol administration during hypothermic cardiopulmonary bypass. After ethics committee approval and written informed consent, 20 New York Heart Association class I-III patients scheduled for elective cardiac surgery requiring hypothermic cardiopulmonary bypass were studied in this prospective randomised controlled trial. In group C, routine anaesthesia was practised, where patients received propofol at target concentration between 1.5 to 2.5 microg/ml during cardiopulmonary bypass. In group B, the target concentration was titrated to a bispectral index value of 40 to 50. Mean arterial pressure and bispectral index were recorded at various time intervals. The use of propofol, phenylephrine, sodium nitroprusside and adrenaline were recorded. The median propofol administration in group B was significantly less than that in group C (2.9 mg/kg/h compared to 6.0 mg/kg/h). The bispectral index value during bypass was significantly lower in group C than in group B, reflecting a deeper state of anaesthesia. There was no difference in the use of inotropes, vasoconstrictors or vasodilators. Bispectral index monitoring enables a 50% reduction in propofol administration at this standard dose during hypothermic cardiopulmonary bypass.

The performance of a target-controlled infusion of propofol in combination with remifentanil: a clinical investigation with two propofol formulations

Target-controlled infusion (TCI) incorporates the pharmacokinetic variables of an IV drug to facilitate safe and reliable administration. In this clinical study we investigated the performance of propofol TCI in combination with remifentanil. Fifty-four adult patients scheduled for general surgery lasting longer than 1 h received a combined TCI of propofol (Marsh parameter set; propofol randomly either dissolved with long- or middle-/long-chain triglycerides) and remifentanil. Arterial propofol plasma concentrations and hemodynamic and derived electroencephalogram variables were determined at various stages before, during, and after surgery. Measured propofol plasma concentrations exceeded the predicted values by 59%, and 48% when recalculated with the Schnider parameter set. Pharmacokinetic population analysis showed a small central volume of distribution (3.55 L) and reduced clearance (1.31 L/min) for propofol. ASA status and sex were the only variables that had a significant influence on propofol pharmacokinetics. In a second step, a new pharmacokinetic variable set for propofol was determined in the first 27 patients. Post hoc performance analysis of the remaining 27 patients showed improved accuracy using the new variable set. Our results show that when remifentanil and propofol are combined, the Marsh and Schnider parameter sets systematically underestimate propofol plasma concentrations. Presented, in part, at the Annual Meeting of the European Society of Anesthesiologists, Amsterdam, The Netherlands, June 1, 1999, and the Annual Meeting of the American Society of Anesthesiologists, Dallas, Texas, October 12, 1999.

Kidneys contribute to the extrahepatic clearance of propofol in humans, but not lungs and brain

AIMS

The principal site for the metabolism of propofol is the liver. However, the total body clearance of propofol is greater than the generally accepted hepatic blood flow. In this study, we determined the elimination of propofol in the liver, lungs, brain and kidneys by measuring the arterial-venous blood concentration at steady state in patients undergoing cardiac surgery.

METHODS

After induction of anaesthesia, propofol was infused continuously during surgery. For measurement of propofol concentration, blood samples were collected from the radial and pulmonary artery at predetermined intervals. In addition, blood samples from hepatic and internal jugular vein were collected at the same times in 19 patients in whom a hepatic venous catheter was fitted and the other six in whom an internal jugular venous catheter was fitted, respectively. In six out of 19 patients fitted with a hepatic venous catheter, blood samples from the radial artery and the renal vein were also collected at the same time, when the catheter was inserted into the right renal vein before insertion into the hepatic vein.

RESULTS

Hepatic clearance of propofol was approximately 60% of total body clearance. The hepatic extraction ratio of propofol was 0.87 +/- 0.09. There was no significant difference in the concentration of propofol between the radial, pulmonary arteries and internal jugular vein. However, a high level of propofol extraction in the kidneys was observed--the renal extraction ratio being 0.70 +/- 0.13.

CONCLUSIONS

We have demonstrated substantial renal extraction of propofol in human. Metabolic clearance of propofol by the kidneys accounts for almost one-third of total body clearance and may be the major contributor to the extrahepatic elimination of this drug.

Perioperative Management of Special Populations: The Geriatric Patient

Americans over age 65 represent the fastest growing segment of the United States population. As a result, the demographic landscape of America is changing. Knowledge of aged physiology is necessary to construct a risk-benefit analysis tailored for each patient to improve perioperative outcomes and lower the morbidity and mortality rates among the elderly. Benefit estimates should account for a patient's life expectancy and quality of life before and after surgery. With aging, baseline functions of almost every organ system undergo progressive decline resulting in a decreased physiologic reserve and ability to compensate for stress. Pain control, postoperative cognitive dysfunction, end-of-life issues, and realistic expectations after surgery are paramount issues throughout the perioperative period.

Rapid fluid infusion therapy decreases the plasma concentration of continuously infused propofol

BACKGROUND

Rapid fluid infusion therapy to treat hypovolemia in anesthetized patients is a common practical regimen in daily clinical settings. This study investigated the effect of large volume loading on the plasma concentration of propofol (Cp), hemodynamic parameters, hemoglobin concentration (Hb), hematocrit value (Ht) and the bispectral index (BIS).

METHODS

Sixty patients were administered propofol using a target-controlled infusion technique. We studied two independent groups. Half of the patients (group F, n = 30) were administered fentanyl, and the other half (group E, n = 30) epidural administration of mepivacaine for analgesia. After achieving a pseudo-steady state of propofol anesthesia, baseline values of blood pressure, heart rate, Hb, Ht, cardiac output, Cp and BIS were measured, and 10 ml/kg Ringer's solution was infused over 15 min.

RESULTS

In group F, Cp was significantly decreased from 2.24 (0.69) [mean (SD)] to 2.07 (0.61) microg/ml and in group E from 2.02 (0.98) to 1.75 (0.51) microg/ml immediately after infusion (P < 0.05). The significant reduction lasted until 30 min in group F, whereas, Cp quickly recovered in group E. Cardiac output was increased only in group F. The dilution ratio demonstrated the prolonged diluting effect in group E and the significant correlation with the rate of decrease in Cp (P < 0.0003, R = 0.21). The BIS value showed no significant change immediately after infusion.

CONCLUSION

Large volume loading decreased Cp without a significant change in BIS values. The effect of infusion therapy on the depth of anesthesia might be small and usually negligible during propofol anesthesia.

Cardiac and thoracic vascular surgery

The number of aged individuals is growing, and consequently the demands on resources for cardiac surgery will increase in the elderly. Even in the absence of obvious coexisting diseases, advanced age is always accompanied by a general decline in organ functions, and specifically by changes in structure and function of the heart and vasculature that will ultimately affect cardiovascular performance (e.g. hypertension, ischaemic heart disease, etc.). These alterations have to be taken into account when older patients require anaesthesia for cardiac surgery. Pre-operative examination must be performed carefully to estimate cardiac function as well as dysfunction of other organ systems. Benzodiazepines as well as alpha2-adrenoceptor agonists can be used for pre-medication; induction and maintenance of anaesthesia can be performed as balanced anaesthesia or total intravenous anaesthesia. Essential monitoring should include pulmonary artery catheterization and/or transoesophageal echocardiography. During cardiac surgery the risk for elderly individuals is increased; in particular, the central nervous system is more often compromised in the elderly than in younger patients. However, elderly patients without significant co-morbidity have a mortality rate comparable to that of younger patients.

Context-sensitive half-times: what are they and how valuable are they in anaesthesiology?

The context-sensitive half-time is the time required for blood or plasma concentrations of a drug to decrease by 50% after discontinuation of drug administration. The context-sensitive half-time often cannot be predicted by the elimination half-life (a measure of the time needed for actual drug metabolism or elimination) since it also depends on drug distribution. The context-sensitive half-time is a function of the duration of drug administration and may only be estimated by computer simulation. It is more relevant than other isolated pharmacokinetic parameters to understanding the kinetics of drug concentrations. However, understanding the kinetics of drug effect requires concomitant consideration of pharmacodynamics.

Validation of fentanyl pharmacokinetics in patients undergoing coronary artery bypass grafting

PURPOSE

The current emphasis on more rapid recovery and earlier tracheal extubation after cardiac surgery requires greater precision in administering opioids to reap their benefits while minimizing the duration of postoperative respiratory depression. Therefore, we aimed to define a pharmacokinetic model that accurately predicts fentanyl concentrations before, during, and after cardiopulmonary bypass (CPB) in patients undergoing coronary artery bypass grafting (CABG).

METHODS

Parameters for two-compartment and three-compartment models were estimated by applying population pharmacokinetic modelling to fentanyl concentration vs time data measured in 29 patients undergoing elective, primary CABG. The ability of these models to predict fentanyl concentrations in a second series of ten patients undergoing CABG was then assessed.

RESULTS

A simple, three-compartment model had excellent predictive ability, with a median prediction error (PE = ([Fentanyl]meas - [Fentanyl]pred)/[Fentanyl]pred x 100%) of -0.5%, and a median absolute PE (APE = /PE/) of 14.0%. In comparison to the two-compartment models, linear regression of measured:predicted concentration ratios indicated that the three-compartment model was free of systematic and time-related changes in bias (P < 0.05). The parameters of this three-compartment model are: V1 15.0 l, V2 20.0 l, V3 86.1 l, Cl1 1.08 L x min(-1), Cl2 4.90 L x min(-1), and Cl3 2.60 L x min(-1).

CONCLUSIONS

Our pharmacokinetic model provides a rational foundation for designing fentanyl dose regimens for patients undergoing CABG. When combined with previously published information regarding intraoperative fentanyl pharmacodynamics, dose regimens that reliably achieve and maintain desired fentanyl concentrations throughout the intraoperative period can be designed to achieve specific therapeutic goals.

Target-controlled infusion or manually controlled infusion of propofol in high-risk patients with severely reduced left ventricular function

OBJECTIVE

To compare hemodynamics, time to extubation, and costs of target-controlled infusion (TCI) with manually controlled infusion (MCI) of propofol in high-risk cardiac surgery patients.

DESIGN

Prospective, randomized.

SETTING

Major community university-affiliated hospital.

PARTICIPANTS

Twenty patients undergoing first-time implantation of a cardioverter-defibrillator with severely reduced left ventricular function (left ventricular ejection fraction <30%).

INTERVENTIONS

Anesthesia was performed using remifentanil, 0.2 to 0.3 microg/kg/min, and propofol. Propofol was used as TCI (plasma target concentration, 2 to 3 microg x mL; n = 10) or MCI (2.5 to 3.5 mg/kg/hr; n = 10).

MEASUREMENTS AND MAIN RESULTS

Hemodynamics were measured at 6 data points: T1, before anesthesia; T2, after intubation; T3, after skin incision; T4, after first defibrillation; T5, after third defibrillation; and T6, after extubation. There were no significant hemodynamic differences between the 2 groups. Dobutamine was required to maintain cardiac index >2 L/min/m(2) in significantly more patients of the TCI group than of the MCI group. Mean dose of propofol was higher in the TCI patients (6.0 +/- 1.0 mg/kg/hr) than in the MCI patients (3.0 +/- 0.4 mg/kg/hr) (p < 0.05), whereas doses of remifentanil did not differ. Time to extubation was significantly shorter in the MCI (11.9 +/- 2.4 min) versus the TCI group (15.6 +/- 6.8 min). Costs were significantly lower in MCI patients (34.73 dollars) than in TCI patients (44.76 dollars).

CONCLUSIONS

In patients with severely reduced left ventricular function, TCI and MCI of propofol in combination with remifentanil showed similar hemodynamics. TCI patients needed inotropic support more often than MCI-treated patients. Although extubation time was longer in TCI patients and costs were higher, both anesthesia techniques can be recommended for early extubation after implantation of a cardioverter-defibrillator.

The pharmacokinetics of anesthetic drugs and adjuvants during cardiopulmonary bypass

The institution of cardiopulmonary bypass during cardiac surgery has profound effects on the plasma concentration of drugs and thus their therapeutic effectiveness. These changes occur through acute hemodilution, altered plasma protein binding, hypotension, as well as the use of hypothermia and heparin administration. Isolation of the lungs from the circulation and the possible sequestration of drugs in the bypass circuit also affect drug plasma concentrations on bypass. The individual characteristics of the drug in question are also important in determining the final plasma concentration: Lipid soluble drugs with a high volume of distribution may be more readily taken up by bypass equipment, but the initial fall in concentration at the start of cardiopulmonary bypass may be more readily counteracted by back diffusion into plasma, if large tissue stores have accumulated. The extent of the drug's plasma protein binding is of importance as the effective free fraction in plasma for highly bound drugs will be sensitive to changes in plasma protein binding brought on by factors such as hemodilution, heparin administration as well as alpha, acid-glycoprotein binding. Clearly the fate of drugs administered before or on bypass is complex and can only be accurately determined by specific studies evaluating drug plasma concentrations. This review updates the available data on anesthetics and drugs used during cardiac surgery in order that anesthetists may predict better the likely effect of drugs administered before or during cardiopulmonary bypass.

Binding of propofol to blood components: implications for pharmacokinetics and for pharmacodynamics

AIMS

Propofol is a widely used i.v. anaesthetic agent. However, its binding properties to blood components have not been fully studied.

METHODS

We studied the binding of propofol to erythrocytes, to human serum and to isolated serum proteins. Because propofol bound to ultrafiltration and equilibrium dialysis membranes, we used a co-binding technique with dextran coated charcoal and with erythrocytes.

RESULTS

Propofol free fraction in blood was 1.2-1.7% at total concentrations ranging from 2.80 to 179 microM (0.5 to 32 microg ml(-1)). Fifty percent was bound to erythrocytes and 48% to serum proteins, almost exclusively to human serum albumin. In the clinical range of concentrations (0.5-16 microg ml(-1)) 40% of the molecules bound to erythrocytes are on the red blood cells membranes. No binding to lipoproteins occurred and binding to alpha1-acid glycoprotein was less than 1.5%

CONCLUSIONS

We conclude that hypoalbuminaemia may increase propofol free fraction particularly during prolonged administration. Since propofol is non-restrictively cleared, no change in clearance is expected to occur, and the increase in free fraction will not be compensated by a parallel increase in clearance. It is also noted that many in vitro studies used concentrations 50 to 500 times the concentration expected to be encountered in the immediate cellular environment.

Early extubation after mitral valve surgery: a target-controlled infusion of propofol and low-dose sufentanil

OBJECTIVE

In the current study, the use of a target-controlled infusion of low-dose propofol was combined with a continuous infusion of sufentanil for patients undergoing mitral valve surgery. The purpose of the study was to evaluate the hemodynamic stability, the time to awakening and spontaneous ventilation, and the feasibility in an early extubation setting of a total intravenous anesthetic technique.

DESIGN

Prospective study.

SETTING

University hospital.

PARTICIPANTS

Fifteen patients scheduled for elective mitral valve surgery.

INTERVENTIONS

Induction of anesthesia consisted of sufentanil (1 microgram/kg), propofol (1 microgram/mL) target plasma concentration achieved over 3 minutes, and atracurium (0.5 mg/kg). The propofol target-controlled infusion was maintained at 1 microgram/mL throughout surgery and stopped at skin closure. A continuous infusion of sufentanil at 1.8 micrograms/kg/hr was started after induction and reduced to 0.9 microgram/kg/hr at the start of cardiopulmonary bypass and stopped at the end of bypass. Atracurium was infused at a rate of 0.5 mg/kg/hr up to sternal closure. No inhalation agents were used.

MEASUREMENTS AND MAIN RESULTS

Hemodynamic data were within normal limits. Six patients (40%) responded to verbal commands within 15 minutes postoperatively, 10 (67%) within the first hour, and all patients recovered within 2 hours. Four patients (27%) resumed spontaneous ventilation within the first 15 postoperative minutes. The time to successful spontaneous ventilation was 169 +/- 42 minutes. Spontaneous ventilation was associated with a 21% increase in cardiac index. Total sufentanil dose was 328 +/- 28 micrograms (4.6 +/- 0.2 microgram/kg), whereas total propofol dose was 862 +/- 44 mg (13.1 +/- 1.2 mg/kg). No patient required reintubation.

CONCLUSION

The simplicity of the method with only one change in infusion rate is a major advantage. The technique permits predictable recovery and return to spontaneous ventilation in all patients. Its use in patients entering early extubation protocols is appealing for its reproducibility, simplicity, and safety.