The effect of halothane on the recirculatory pharmacokinetics of physiologic markers

Mathematical Modeling for the Physiological and Clinical Investigation of Glucose Homeostasis and Diabetes

Mathematical modeling in the field of glucose metabolism has a longstanding tradition. The use of models is motivated by several reasons. Models have been used for calculating parameters of physiological interest from experimental data indirectly, to provide an unambiguous quantitative representation of pathophysiological mechanisms, to determine indices of clinical usefulness from simple experimental tests. With the growing societal impact of type 2 diabetes, which involves the disturbance of the glucose homeostasis system, development and use of models in this area have increased. Following the approaches of physiological and clinical investigation, the focus of the models has spanned from representations of whole body processes to those of cells, i.e., from in vivo to in vitro research. Model-based approaches for linking in vivo to in vitro research have been proposed, as well as multiscale models merging the two areas. The success and impact of models has been variable. Two kinds of models have received remarkable interest: those widely used in clinical applications, e.g., for the assessment of insulin sensitivity and β-cell function and some models representing specific aspects of the glucose homeostasis system, which have become iconic for their efficacy in describing clearly and compactly key physiological processes, such as insulin secretion from the pancreatic β cells. Models are inevitably simplified and approximate representations of a physiological system. Key to their success is an appropriate balance between adherence to reality, comprehensibility, interpretative value and practical usefulness. This has been achieved with a variety of approaches. Although many models concerning the glucose homeostasis system have been proposed, research in this area still needs to address numerous issues and tackle new opportunities. The mathematical representation of the glucose homeostasis processes is only partial, also because some mechanisms are still only partially understood. For in vitro research, mathematical models still need to develop their potential. This review illustrates the problems, approaches and contribution of mathematical modeling to the physiological and clinical investigation of glucose homeostasis and diabetes, focusing on the most relevant and stimulating models.

Influence of sevoflurane anesthesia with mechanical ventilation and fluid-therapy on distribution of subcutaneously administered robenacoxib in dogs

Robenacoxib is a novel nonsteroidal anti-inflammatory drug approved for dogs. The present study aimed to evaluate influences of sevoflurane anesthesia on the distribution of robenacoxib in dogs. Ten healthy beagle dogs (1 to 11 years old, 9.3 to 14.3 kg body weight, 6 males and 4 females) were subcutaneously administered robenacoxib (2 mg/kg) under conscious condition or sevoflurane anesthesia inhaled a 1.3-fold predetermined individual minimum alveolar concentration of sevoflurane at a 28-day interval. The dogs under sevoflurane anesthesia were also mechanically ventilated and received fluid-therapy. On each occasion, serum samples were collected from the dogs before and at 5, 15, 30, 60, 120, 180, and 240 min after the robenacoxib administration. Serum robenacoxib concentration was measured by a liquid chromatography-tandem mass spectrometry. Maximum serum concentration of robenacoxib (C_max) was 2.2 µg/ml [range: 1.2–4.6] (median [range: minimum-maximum]) and time of C_max (T_max) was 90 min [range: 60–120] in the conscious dogs. In the sevoflurane-anesthetized dogs, the C_max significantly declined (1.3 µg/ml [range: 0.8–1.4], P=0.008) and T_max was delayed (120 min [range: 120–240], P=0.018) compared with those in the conscious dogs. The serum robenacoxib concentration at 240 min (C₂₄₀) decreased to 0.5 µg/ml [range: 0.2–0.9] in the conscious dogs, while it remained higher in the sevoflurane-anesthetized dogs (1.0 µg/ml [range: 0.3–1.4], P=0.011). In conclusion, the anesthetic procedure with sevoflurane, mechanically ventilated, and received fluid-therapy might affect the pharmacokinetics of subcutaneously administered robenacoxib in dogs.

Anesthetic recovery and hemodynamic effects of continuous thiopental infusion versus halothane for maintenance anesthesia in patients undergoing ocular surgery

PURPOSE

To investigate anesthesia recovery and hemodynamic status in patients under thiopental infusion or halothane maintenance anesthesia undergoing ocular surgery.

METHODS

Fifty-nine voluntary patients undergoing ocular surgery in Farabi hospital were allocated to one of two maintenance anesthesia groups: inhaled halothane, 0.8 to 1 per cent, (group I, n=37) and thiopental infusion, 10 to 12 mg/kg/hour, (group II, n=22). Hemodynamic parameters were recorded at the time of patient entrance to the operation room and at the 1, 2, 5, 10, 15, 20, 25, 30, 35, and 40 minutes following anesthesia. Anesthesia recovery variables were also compared between the two groups.

RESULTS

In group I, arterial blood pressure at 10 to 40 min and heart rate at 1 and 25 min after the administration of anesthetics were significantly lower when compared with group II (W ²= 25.10, p= 0.005). Arterial oxygen saturation was similar in the two groups over the whole points of time. The time intervals between the end of surgery and beginning of the first body movements and respiratory efforts were significantly longer in group received halothane (p<0.001).

CONCLUSION

Continuous infusion of thiopental can be applied effectively and safely for maintenance of anesthesia. In comparison with halothane, it is associated with lower changes of intraoperative hemodynamics and faster anesthesia recovery.

Contributions of PK/PD modeling to intravenous anesthesia

Pharmacokinetic (PK)/pharmacodynamic (PD) modeling has made an enormous contribution to intravenous anesthesia. PK/PD models have provided us with insight into the factors affecting the onset and offset of drug effect. For example, we are now able to describe the influence of cardiac output on the disposition of intravenous drugs within the first few minutes after administration of the drug. We are able to calculate intravenous loading doses that allow for the delay between the concentration of the drug in the plasma and the rising concentration at the site of drug effect. We are able to achieve and maintain a stable level of anesthetic effect using computerized infusion pumps that target the site of drug effect rather than the plasma. Importantly, on the basis of models of drug interaction and an understanding of how drug offset varies with duration of administration, we are now able to rationally combine hypnotics and opioids.

Recirculatory Pharmacokinetic Model of the Uptake, Distribution, and Bioavailability of Prochlorperazine Administered as a Thermally Generated Aerosol in a Single Breath to Dogs

A thermal aerosol generation process is capable of delivering pure drug reliably to the alveoli where it is absorbed systemically. Although deep lung absorption of drugs administered as an aerosol has been shown to be rapid, detailed characterization of their absorption and distribution has not been reported. The present study describes the pharmacokinetics of prochlorperazine from the moment of administration as either a rapid intravenous infusion or a thermally generated aerosol and determines the bioavailability of the aerosol by two independent methods. Prochlorperazine disposition was determined in four anesthetized dogs after a 5-s intravenous infusion and after thermally generated aerosol administration in one breath. Venous blood samples were collected frequently from the time of drug administration to 24 h and left ventricular blood samples were drawn more often until 10 min after drug administration. Prochlorperazine disposition after intravenous and aerosol administration was characterized by fitting a recirculatory model to left ventricular and venous drug concentration data simultaneously. Prochlorperazine aerosol administration produced plasma drug concentrations similar to those after rapid intravenous administration of the same nominal dose, with peak left ventricular concentrations achieved in less than 30 s. Plasma concentration profiles of prochlorperazine administered by both routes were well described by the recirculatory model. Bioavailability of the thermally generated aerosol was consistent and averaged more than 80% of emitted dose. Pulmonary administration of a thermally generated drug aerosol in one breath may be a viable alternative to rapid intravenous administration of drugs requiring rapid and predictable production of effective plasma concentrations.

β-Adrenergic Blockade Affects Initial Drug Distribution Due to Decreased Cardiac Output and Altered Blood Flow Distribution

Beta-adrenergic receptor blockers decrease intravenous anesthetic dose requirements. The present study determined the effect of propranolol on indocyanine green and antipyrine disposition from the moment of rapid intravenous injection. Anti-pyrine is a physiological marker that distributes to a volume as large as total body water in a blood flow-dependent manner and is a pharmacokinetic surrogate for many lipophilic drugs, including intravenous anesthetics. Antipyrine and indocyanine green disposition were determined twice in five healthy adult males in this Institutional Review Board-approved study, once during propranolol infusion. After rapid indocyanine green and antipyrine injection, arterial blood samples were collected frequently for 2 min and less frequently thereafter. Plasma indocyanine green and antipyrine concentrations were measured by high-performance liquid chromatography. Indocyanine green and antipyrine disposition were characterized, using SAAM II, by a recirculatory pharmacokinetic model that describes drug disposition from the moment of injection. Parameters were compared using the paired t test. The disposition of indocyanine green demonstrated that propranolol decreased cardiac output at the expense of the fast peripheral (nonsplanchnic) intravascular circuit. The area under the antipyrine concentration versus time relationship was doubled for at least the first 3 min after injection due to both decreased cardiac output and maintenance of nondistributive blood flow at the expense of a two-thirds reduction of blood flow (intercompartmental clearance) to the rapidly equilibrating (fast, splanchnic) tissue volume. The increase in antipyrine area under the curve due to propranolol-induced alteration of initial antipyrine disposition could explain decreased intravenous anesthetic dose requirements in the presence of beta-adrenergic receptor blockade.

The concordance of early antipyrine and thiopental distribution kinetics

Studies of factors affecting the initial disposition of drugs with a rapid onset of effect following i.v. administration have used antipyrine as a surrogate for lipophilic drugs because it lacks cardiovascular effects. The present study tested the assumption that antipyrine is a useful surrogate for the flow-dependent tissue distribution of the lipophilic drug thiopental by comparing the recirculatory pharmacokinetic models of antipyrine and thiopental disposition after concomitant administration to five dogs anesthetized with 1.5% halothane. The pharmacokinetics of indocyanine green, a marker of the intravascular behavior of antipyrine and thiopental, and antipyrine in these dogs was nearly identical to that described previously in dogs anesthetized with 1.5% halothane but not given thiopental. The total volume of distribution of the highly lipophilic drug thiopental was more than 60% larger than that of antipyrine, 53 versus 33 liters, respectively. Nonetheless, the initial distribution kinetics of the two drugs, including the pulmonary tissue volume and the volume of the nondistributive pathway as well as the clearance to it, were nearly identical. As a result, the fraction of cardiac output involved in distribution of the two drugs to peripheral tissues was similarly identical, although the distribution of cardiac output between clearance to the rapidly equilibrating tissues and clearance to the slowly equilibrating tissues differed slightly. This study validates the assumption that antipyrine is a useful surrogate for lipophilic drugs in pharmacokinetic studies in which physiologic stability is desirable to meet the assumption of system stationarity.

Evaluation of the splanchnic circulation with indocyanine green pharmacokinetics in liver transplant patients

Although indocyanine green (ICG) can be used to estimate cardiac output (CO) and blood volume independently, a recirculatory multicompartmental ICG model enables description of these and additional intravascular factors. This model was used to describe the effect of end-stage liver disease (ESLD) on systemic and splanchnic hemodynamics in patients undergoing orthotopic liver transplantation. ICG disposition was determined during the dissection phase in six patients with ESLD undergoing orthotopic liver transplantation and six healthy adult living liver donors. After injecting ICG, plasma concentrations were obtained for approximately 10 to 12 minutes by noninvasive pulse dye densitometry. The recirculatory model characterizes three distinct intravascular circuits: lumped parallel fast (presumably nonsplanchnic circulation) and slow peripheral (splanchnic) circuits and a central circuit (central blood volume). Mean transit time (MTT) in the fast peripheral circuit was not different in patients with ESLD and controls. However, ESLD resulted in a significant decrease in MTT in the central (0.11 +/- 0.028 [SD] v 0.24 +/- 0.094 minutes in controls; P <.001) and slow peripheral circuit (0.67 +/- 0.41 v 1.37 +/- 0.37 minutes in controls; P <.001) because of increased flows to the central and slow peripheral circuits. These findings are consistent with the described hyperdynamic systemic and splanchnic circulations in patients with ESLD. In conclusion, the ICG model is able to derive estimates of not only blood volume and CO, but also splanchnic hemodynamics under different physiological conditions. This model can be a useful tool to evaluate the effect of pharmacological manipulation of splanchnic hemodynamics.

The determinants of propofol induction of anesthesia dose

Recently it was reported that the pharmacokinetics of propofol are modified by changes in cardiac output. The objective of this study was to evaluate the effects of cardiac output and other factors on the hypnotic dose of propofol. One-hundred surgical patients were administered indocyanine green immediately before the induction of anesthesia to measure their cardiac outputs and blood volumes. Propofol (250 microg. kg(-1). min(-1)) was infused IV for 8 min, and the hypnotic dose of propofol and the time to hypnosis were recorded. The plasma concentration of propofol immediately after 2 mg/kg infusion was measured. Multiple regression analysis showed that, in addition to age and weight, cardiac output was a small but significant factor for predicting the hypnotic dose of propofol (R(2) = 0.468, P < 0.001), the time to hypnosis (R(2) = 0.454, P < 0.001), and the plasma concentration of propofol (R(2) = 0.248, P < 0.01). Cardiac output, age, and weight showed similar partial coefficients for the hypnotic dose (0.128, 0.137, and 0.140, respectively).

IMPLICATIONS

This study demonstrates a significant relationship between cardiac output and the hypnotic dose of propofol. We suggest that anesthesiologists should include cardiac output, as well as age and weight, in calculating the induction dose of propofol.

[Pharmacodynamics and safety of mivacurium in infants and children under halothane-nitrous oxide anesthesia]

OBJECTIVES

To determine pharmacodynamic effects and safety of mivacurium in paediatric patients.

STUDY DESIGN

Multicentric, prospective, open, non-randomized study.

PATIENTS

Forty-eight three-month-old to eight-year-old physical class ASA I or II children.

METHOD

Anaesthesia was induced and maintained with halothane and nitrous oxide. Tracheal intubation was performed without a neuromuscular blocking agent. Neuromuscular blockade was measured with a strain force transducer after train-of-four stimulation of the ulnar nerve at the wrist every ten seconds. A single bolus dose of mivacurium (0.2 mg.kg-1) was injected during 15 seconds in patients allocated into three groups. Group 1: three to 12-month-old infants (n = 15), group 2: one- to three-year-old children (n = 16) and group 3: three- to eight-year-old children (n = 17). Onset and recovery parameters were measured in each patient. Heart rate and noninvasive arterial blood pressure were recorded every minute for five minutes after mivacurium injection.

RESULTS

Following halothane administration for 29 and 32 min, and a FEThalothane = 1 vol%, mivacurium (0.2 mg.kg-1) determined a 100% neuromusmcular blockade in all patients. The onset time was 71 +/- 34 s (mean +/- SD) in all patients and did not differ between groups. Time to 25% and 95% recovery of the first twitch and recovery index for all the patients were 12 +/- 3 min, 19 +/- 5 min and 4 +/- 2 min respectively and did not differ between groups. No prolonged paralysis was observed. No significant changes of HR and BP occurred.

CONCLUSIONS

Following 0.2 mg.kg-1 of mivacurium in patients aged between three months to eight years, a complete blockade occurs with a rapid onset time and a short duration of action, without significant cardiovascular effect.

Cardiac output is a determinant of the initial concentrations of propofol after short-infusion administration

Indicator dilution theory predicts that the first-pass pulmonary and systemic arterial concentrations of a drug will be inversely related to the cardiac output. For high-clearance drugs, these first-pass concentrations may contribute significantly to the measured arterial concentrations, which would therefore also be inversely related to cardiac output. We examined the cardiac output dependence of the initial kinetics of propofol in two separate studies using chronically instrumented sheep in which propofol (100 mg) was infused IV over 2 min. In the first study, steady-state periods of low, medium, and high cardiac output were achieved by altering carbon dioxide tension in six halothane-anesthetized sheep. The initial area under the curve and peak value of the pulmonary artery propofol concentrations were inversely related to cardiac output (R2 = 0.57 and 0.66, respectively). For the systemic arterial concentrations, these R2 values were 0.68 and 0.71, respectively. In our second study, transient reductions in cardiac output were achieved in five conscious sheep by administering a short infusion of metaraminol concurrently with propofol. Cardiac output was lowered by 2.2 L/min, and the area under the curve to 10 min of the arterial concentrations increased to 143% of control.

IMPLICATIONS

The initial arterial concentrations of propofol after IV administration were shown to be inversely related to cardiac output. This implies that cardiac output may be a determinant of the induction of anesthesia with propofol.