Morphine pharmacokinetics after pulmonary administration from a novel aerosol delivery system

BACKGROUND

Successful pharmacotherapy of pain often depends on the mode of drug delivery. A novel, unit dose, aqueous aerosol delivery system (AERx Pulmonary Drug Delivery System) was used to examine the feasibility of the pulmonary route for the noninvasive systemic administration of morphine.

METHODS

The study had two parts: (1) a dose-ranging study in four subjects with three consecutive aerosolized doses of 2.2, 4.4, and 8.8 mg (nominal) morphine sulfate pentahydrate at 40-minute intervals, and (2) a crossover study, on separate days, in six subjects with 4.4 mg (nominal) aerosolized morphine sulfate administered over 2.1 minutes on three occasions and intravenous infusions of 2 and 4 mg over 3 minutes. Subjects were healthy volunteers from 19 to 34 years old. Arterial blood was sampled for a total of 6 hours and plasma morphine concentrations were measured by gas chromatography-mass spectrometry.

RESULTS

In part 1, plasma morphine concentrations were proportional to dose. In part 2, the mean +/- SD peak plasma concentration (Cmax) occurred at 2.7 +/- 0.8 minutes after the aerosol dose, with mean values for Cmax of 109 +/- 85, 165 +/- 22, and 273 +/- 114 ng/ml for the aerosol and 2 and 4 mg intravenous doses, respectively. The bioavailability [AUC(0-360 min)] of aerosol-delivered morphine was approximately 100% relative to intravenous infusion, with similar intersubject variability in AUC for both routes (coefficient of variation < 30%).

CONCLUSION

The time courses of plasma morphine concentrations after pulmonary delivery by the AERx system and by intravenous infusions were similar. This shows the utility of the pulmonary route in providing a noninvasive method for the rapid and reproducible systemic administration of morphine if an appropriate aerosol drug delivery system is used.

The influence of age upon opioid analgesic use in the patient-controlled analgesia (PCA) environment

It is often asserted that older patients are more sensitive to opioid analgesics than younger patients but experimental evidence for this assertion remains sparse. Two studies were conducted investigating the relationship between age and opioid analgesic use in the patient-controlled analgesia environment. In study I, the relationship was analysed subsequent to our publication of a study investigating patients' responses to opioid use with patient-controlled analgesia. Fifty-five postoperative patients, stratified into 'older' and 'younger' patients by median age, received morphine or pethidine or fentanyl patient-controlled analgesia. A strong inverse relationship was found between age and fentanyl and morphine use but not between age and pethidine use. Study II was a retrospective study of the medical records of 199 patient-controlled analgesia patients who had received morphine or pethidine patient-controlled analgesia; there were insufficient patients who had used fentanyl for a reasonable sample. There was a difference in morphine use with the younger patients using significantly more morphine than the older patients (< 60 years). Findings were less clear for patients receiving pethidine but there was an inverse correlation between age and pethidine use as well. Overall, the findings of these two studies supported the common clinical belief that older patients require less opioids than younger patients.

Lack of effect of high doses of inhaled morphine on exercise endurance in chronic obstructive pulmonary disease

Systemic opiates may relieve dyspnoea and improve exercise tolerance in patients with chronic obstructive pulmonary disease (COPD). Small doses of inhaled opiates may have similar effects; however, recent studies have shown no benefit. We studied higher doses of inhaled morphine and measured systemic absorption to determine whether any beneficial actions are local or systemic. Twenty and 40 mg doses and 0.9% saline were nebulized in a randomized, double-blind study of 16 patients with stable COPD. Patients performed 6 min walk tests immediately after the nebulized test solution (Walk 1) and again 60 min later (Walk 2). Arterial oxygen saturation (Sa,O2), modified Borg dyspnoea score and cardiac frequency were recorded during each walk. There was no difference between placebo and either dose of nebulized morphine on these measurements. The higher dose of nebulized morphine achieved a higher plasma concentration. The highest plasma concentration was measured immediately after nebulization, and this decreased steadily in the hour thereafter (p<0.002). There was no correlation between the change in walk distance and the change in plasma morphine concentration after either dose of nebulized morphine. We conclude that higher doses of nebulized morphine do not improve exercise endurance or relieve dyspnoea in patients with chronic obstructive pulmonary disease, and that morphine is rapidly absorbed systemically after inhalation.

Influence of renal failure on the disposition of morphine, morphine-3-glucuronide and morphine-6-glucuronide in sheep during intravenous infusion with morphine

The influence of experimentally induced renal failure on the disposition of morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) was examined in seven sheep infused intravenously with morphine for 6 hr. Between 5 and 6 hr, blood was collected from the aorta, pulmonary artery, hepatic, hepatic portal and renal veins and posterior vena cava. Additional samples from the aorta and urine were collected up to 144 hr. Morphine, M3G and M6G were determined in plasma and urine by high-performance liquid chromatography. Constant concentrations of morphine, but not of M3G and M6G, were achieved in plasma between 5 and 6 hr. Significant (P < .001) extraction of morphine by the liver (0.72 +/- 0.05) and kidney (0.42 +/- 0.15) occurred. Compared with sheep with normal kidneys (Milne et al., 1995), renal failure did not alter (P = .11) the mean total clearance of morphine (1.5 +/- 0.3 liters/min); clearance by the kidney was less (P < .001). However, a paired comparison using sheep common to this study and from the study when their kidneys were normal revealed a significant reduction in mean total clearance of 25%. The renal extraction of M3G and M6G and urinary recovery of the dose as summed morphine, M3G and M6G were reduced by renal failure. The kidney metabolized morphine to M3G. The data suggest that nonrenal elimination of M3G becomes more important during renal failure.

Nausea and vomiting in the postoperative patient-controlled analgesia environment

Despite common clinical opinion that patient-controlled analgesia should be renamed 'patient-controlled nausea', there is little evidence in support of the notion that postoperative nausea and vomiting are exacerbated by the method. Indeed, data indicate that opioid-sparing techniques are not associated with less postoperative nausea and vomiting. Although some evidence suggests that certain opioids are less emetogenic than others, this too does not stand scrutiny when compared across patients, although research is still required to find whether individual patients are better treated with a particular opioid. Similarly, the emerging practice of combining anti-emetics with patient-controlled analgesia needs wider study before it can be supported.

Highly sensitive gas chromatographic-mass spectrometric method for morphine determination in plasma that is suitable for pharmacokinetic studies

A sensitive method was devised to determine morphine plasma concentrations by gas chromatography-mass spectrometry (GC-MS) using selected ion monitoring (SIM) with nalorphine as the internal standard. This method was rugged, reliable, selective and sensitive and was used to determine the morphine content of over 2000 samples. Sample preparation involved extraction of basified sample using n-butyl chloride-chloroform (5:1) and evaporation of the extract to dryness. The residue was derivatised with pentafluoropropionic anhydride, evaporated to dryness, reconstituted in 40 microl toluene and injected onto the GC-MS. For a sample size of 1 ml, the limit of quantitation was 0.75 ng/ml (S/N ratio 10:1) and the estimated limit of detection was calculated to be 0.2 ng/ml (S/N ratio 3:1), expressed as morphine base. Precision (n=5) was 4.9% at 0.75 ng/ml, 6.8% at 1.5 ng/ml, 3.0% at 37.5 ng/ml and 2.3% at 150 ng/ml. Standard curves for the range of 0-750 ng/ml morphine in plasma were linear with all r2 values greater than 0.99. No interfering peaks were seen for either morphine or internal standard in the blank samples. The method is suitable for pharmacokinetic studies after subclinical doses of morphine where it has been used to study morphine plasma concentrations for 6 h after a dose of only 2 mg.

Pharmacokinetics of thiopentone enantiomers following intravenous injection or prolonged infusion of rac-thiopentone

AIMS

Thiopentone is administered as a racemate (rac-thiopentone) for induction of anaesthesia as well as for neurological and neurosurgical emergencies. The pharmacokinetics and pharmacodynamics of rac-thiopentone have been extensively studied but the component R-(+)- and S-(-)- enantiomers, until very recently, have been largely ignored.

METHODS

The present study analyses the pharmacokinetics of R-(+)- and S-(-)-thiopentone in 12 patients given rac-thiopentone intravenously for induction of anaesthesia and five patients given a prolonged infusion of rac-thiopentone used for treatment of intracranial hypertension.

RESULTS

The mean total body clearance (CLT) and apparent volume of distribution at steady-state (Vss) showed trends towards higher values for R-(+)- than for S-(-)-thiopentone in both patient groups; CLT and Vss of unbound fractions of R-(+)- and S-(-)-thiopentone, however, did not show these trends. The time courses of R-(+)- and S-(-)- thiopentone serum concentrations were so similar that EEG effect could not be attributed to one or other enantiomer. Serum protein binding for S-(-)-thiopentone was greater than for R-(+)-thiopentone (P = 0.02) and 24 h urinary excretion of R-(+)-thiopentone was greater than for S-(-)-thiopentone (P = 0.03). In one patient, concomitant measurement of CSF and serum thiopentone concentrations found that serum: CSF equilibration of unbound fractions of both enantiomers was essentially complete.

CONCLUSIONS

The study was unable to determine any pharmacokinetic difference of clinical significance between the R-(+)- and S-(-)-thiopentone enantiomers and concludes that minor differences in CLT and Vss could be explained by enantioselective difference found in serum protein binding.

Morphine blood concentrations in elderly postoperative patients following administration via an indwelling subcutaneous cannula

The pharmacokinetics of morphine in venous blood after a 5 mg bolus dose via an indwelling subcutaneous cannula were characterised in 22 elderly patients undergoing elective major surgery. In a subgroup of seven patients, the kinetics were also characterised after a second 5 mg dose of morphine administered 180 min after the first dose. Blood morphine concentrations following the single dose were highly variable--the coefficients of variation of Cmax, Tmax and the AUC up to 180 min (AUC180) were 54, 37 and 39%, respectively, with mean values of 86.6 ng.ml-1, 15.9 min and 3954 ng.ml-1, respectively. These mean values for the second dose were not statistically different to those of the first dose but were more variable. It was concluded that the injection of morphine via an indwelling subcutaneous cannula results in blood concentrations that are comparable to, and as variable as, those arising from intramuscular injection.

Determination of subnanogram concentrations of fentanyl in plasma by gas chromatography-mass spectrometry: comparison with standard radioimmunoassay

A method was devised to determine fentanyl plasma concentrations by GC-MS using selected-ion monitoring (SIM) with sufentanil as internal standard. This was compared with a commonly used commercial radioimmunoassay (RIA). Sample preparation for GC-MS involved basification of plasma then extraction using n-butyl chloride followed by concentration to dryness and reconstitution in toluene for chromatography. Using 1-ml plasma samples, the estimated limit of detection of fentanyl was 20 pg/ml. Blood samples for pharmacokinetic studies were split and assayed by GC-MS and RIA which had a limit of detection of 200 pg/ml. Pearson's r (r - 0.80, p < 0.0001) indicated the methods were highly correlated at all plasma concentrations. Owing to the greater sensitivity of the method, GC-MS is recommended over RIA for subnanogram determination of fentanyl in plasma.

Urineless estimation of glomerular filtration rate and renal plasma flow in the rat

A simple method to perform serial renal clearance studies without urine collection in rats is described. This was applied to nonradiolabelled para-aminohippurate sodium (PAH) and iothalamate sodium (IOT) which were used respectively to estimate renal plasma flow (RPF) and glomerular filtration rate (GFR). Under isoflurane-anesthesia the jugular vein and carotid artery of Fischer 344 rats were cannulated, and a loading dose followed by a continuous infusion of PAH and IOT was administered. Steady state was reached after 30 min, and four arterial blood samples were collected over the next 30 min for analysis by HPLC (day 0). The clearances of PAH and IOT were calculated according to Fick's principle as estimates of RPF and GFR from the ratio of the infusion rates to the median solute concentrations. Three days later the femoral artery and vein were cannulated, and the same study was repeated (day 3). There was no significant difference in renal clearances of PAH and IOT between days 0 and 3. The described method gives values that compare well with others in the literature based on other methods and presents an accurate and simple way of detecting changes in renal function before and after a potentially nephrotoxic treatment regimen.

The systemic and cerebral kinetics of thiopental in sheep: enantiomeric analysis

Thiopental is used as a racemate (rac-thiopental). Enantiomeric pharmacokinetic differences could therefore influence the onset and duration of anesthesia of rac-thiopental. We studied the systemic and cerebral kinetics of R(+)- and S(-)-thiopental in five adult ewes after a 2-min intravenous infusion of 500 mg rac-thiopental sodium. Systemic kinetic values were determined from the time course of concentrations in arterial plasma; cerebral kinetic values were deduced from the time course of the concentration differences between arterial and superior sagittal sinus blood plasma. Enantiomeric differences were found in both sites, with the (R:S) ratio of thiopental enantiomer blood concentrations initially being > or = 1 then decreasing to < 1 after approximately 60 min. This is consistent with the finding of the mean total body clearance of R(+)-thiopental being 17% (SD 12%) greater than that of S(-)-thiopental (P = .04). Sagittal sinus plasma concentrations of both enantiomers followed closely behind those in arterial plasma and this is consistent with facile bidirectional exchange of thiopental between plasma and brain. No significant differences were found between enantiomers in the rates of brain influx or efflux. Onset and regression of anesthesia occurred while the enantiomer blood concentrations were similar. Hence published pharmacokinetic-pharmacodynamic models of the onset of thiopental effects probably are not significantly compromised by neglecting the enantiomeric duality of thiopental, but models based on its elimination kinetics could be compromised if enantiomeric differences are neglected.

Comparison of the respiratory and systemic kinetics of nitrous oxide in the sheep

BACKGROUND

To determine whether discrepancies in views on the kinetics of nitrous oxide (N2O) may have a methodological basis, we compared its kinetics, simultaneously, in the respiratory system and systemic circulation.

METHODS

Six merino ewes (40-50 Kg) were previously prepared with catheters in the pulmonary artery and aorta. The animals were anaesthetised with thiopentone then ventilated on a mixture of 70% N2O, 1% halothane in oxygen for 4 h. Simultaneous serial arterial and pulmonary arterial blood samples were assayed for N2O by gas chromatography and respiratory gases were monitored continuously by mass spectrometry.

RESULTS

Marked differences were observed between the respiratory and systemic kinetics of N2O uptake. While the expired/inspired N2O concentration ratio rose within 30 min to a value close to unity, the pulmonary arterial/arterial blood N2O concentration ratio did not reach unity during the 4 h of each study, but approached a constant rate of uptake shown by the mean ratio of 0.94 (SD 0.01) from about 2 h onward.

CONCLUSIONS

Discrepancies in fluid flow between respiratory gas and the cardiovascular systems, a concentration effect of N2O in the lungs, the relative solubility of N2O in blood and tissues, and ventilation/perfusion inequalities all may contribute to the observed differences. The ongoing uptake is consistent with persistent extrapulmonary losses. There remains a need for experimental data on the pharmacokinetics of N2O. Unequivocal studies on the disposition of N2O can be undertaken only by using direct measurement of fluxes of N2O across relevant organs or tissues.

High-performance liquid chromatographic determination of thiopentone enantiomers in sheep plasma

An HPLC method was developed to determine the plasma concentrations of R(+)- and S(-)-thiopentone for pharmacokinetic studies in sheep. The method required separation of the thiopentone enantiomers from the corresponding pentobarbitone enantiomers which are usually present as metabolites of thiopentone. Phenylbutazone was used as an internal standard. After acidification, the plasma sample were extracted with a mixture of ether and hexane (2:8). The solvent was evaporated to dryness and the residues were reconstituted with sodium hydroxide solution (pH 10). The samples were chromatographed on a 100 mm x 4 mm I.D. Chiral AGP-CSP column. The mobile phase was 4.5% 2-propanol in 0.1 M phosphate buffer (pH 6.2) with a flow-rate of 0.9 ml/min. This gave k' values of 1.92, 2.92, 5.71, 9.30 and 11.98 for R(+)-pentobarbitone, S(-)-pentobarbitone, R(+)-thiopentone, S(-)-thiopentone, and phenylbutazone, respectively. At detection wavelength of 287 nm, the limit of quantitation was 5 ng/ml for R(+)-thiopentone and 6 ng/ml for S(-)-thiopentone. The inter-day coefficients of variation at concentrations of 0.02, 0.1 and 8 micrograms/ml were, respectively, 4.8, 4.4 and 3.5% for R(+)-thiopentone and, respectively, 5.0, 4.3 and 3.9% for S(-)-thiopentone (n = 6 each enantiomer). At the same concentrations, the intra-day coefficients of variation from six sets of replicates (measured over six days) were, respectively, 8.0, 8.0 and 8.8% for R(+)-thiopentone and 8.8, 7.4 and 9.6% for S(-)-thiopentone. Linearity over the standard range, 0.01-40 micrograms/ml, was shown by correlation coefficients > 0.998. This method has proven suitable for pharmacokinetic studies of thiopentone enantiomers after administration of rac-thiopentone in human plasma also and would be suitable for pharmacokinetic studies of the pentobarbitone enantiomers.

Opioids: a pharmacologist's delight!

1. Opioids, in one form or another, have been used for their pain-relieving properties from prehistoric times: they are still the first line medication for the treatment of severe nociceptive pain and are likely to remain so for the foreseeable future. 2. The therapeutic index of opioids used for pain management is low: opioid side effects are essentially extensions of therapeutic effects and no available agent has a marked advantage over the others. When used for opioid 'anaesthesia', differences in therapeutic index are more obvious due to differences in non-opioid effects. 3. Opioid receptors in brain and spinal cord periphery are the main 'therapeutic targets' and clinical dosage strategies have been derived using a variety of systemic (indirect or blood-borne) methods as well as intraspinal and intracerebroventricular (direct) methods: no method, however, is without potential side effects. Peripheral opioid effects are now being exploited with intra-articular injection. 4. Opioid pharmacokinetics and pharmacodynamics are characterized by high inter-subject variability: accordingly, patient-controlled dosage strategies are found to be more successful for pain control than deterministic recipes.

1994 John J. Bonica Lecture. The clinical effects of morphine pharmacology

BACKGROUND AND OBJECTIVES

The literature is replete with studies of morphine. It is not surprising that some viewpoints expressed tend to be often repeated without critical analysis. The goal of this lecture is to probe some commonly held viewpoints about the pharmacology of morphine. This involves retracing some of its history, re-examining its physicochemical nature, re-evaluating its physiologic disposition and the pharmacologic activity of its metabolites.

METHODS

The background material selected for inclusion consists of historic pharmacologic secondary sources, selections from the recent primary literature on morphine pharmacokinetics and metabolism, and vignettes from the author's personal research in progress with colleagues in Australia.

RESULTS

The author's perspective is that of a chemical pharmacologist with more than a passing interest in pharmacokinetics. The synthesis evaluates the therapeutic consequences of the pharmacologic aspects chosen.

CONCLUSIONS

Morphine in various forms has been used successfully in the alleviation of pain for at least several millenia but it is only relatively recently that a rationale for its action has been found. Morphine is a relatively poorly lipophilic opioid but equilibrates with tissue of the brain relatively easily. While the metabolism of morphine occurs in the liver, it is metabolized in the kidneys and brain as well. The clearance of morphine is not notably decreased by renal dysfunction, but clearance of its glucuronide metabolites is decreased markedly. While the metabolites produced may be shown to be pharmacologically active in particular experimental preparations, it is not yet certain that they contribute pharmacologically after systemic administration of morphine in experimental animals or humans with generally normal physiology.

Comparative disposition of morphine-3-glucuronide during separate intravenous infusions of morphine and morphine-3-glucuronide in sheep. Importance of the kidney

The disposition of morphine-3-glucuronide (M3G) in sheep was compared during separate constant infusions of morphine and M3G. Five ewes received a 15-min loading dose, followed by a constant infusion of morphine sulfate (10 mg/hr) or M3G (4 mg/hr for 4 sheep, 7.5 mg/hr for 1 sheep) for a further 5.75 hr. During the 5th-6th hr of infusion, blood was collected simultaneously from the aorta, pulmonary artery, hepatic vein, hepatic portal vein, renal vein, and posterior vena cava. Additional samples were collected from the aorta from 0 to 5 hr and from 6 to 48 hr. Urine was collected via an indwelling catheter from 0 to 6 hr, with further free-flowing urine up to 48 hr. An HPLC assay was used to determine simultaneously morphine, M3G, and morphine-6-glucuronide (M6G) in plasma and urine. Constant concentrations of morphine, M3G, and M6G in plasma were achieved during the 5- to 6-hr period of infusion with morphine, as were the concentrations of M3G while M3G was infused. Regional net extraction ratios and total and regional clearances were calculated during the 5- to 6-hr period. After the infusions were ceased, there was prolonged elimination of M3G formed in situ from morphine compared to when infused as M3G. No morphine or M6G was detected in the plasma during and after infusion with M3G, nor were they found in urine collected up to 6 hr.(ABSTRACT TRUNCATED AT 250 WORDS)

Bupivacaine enantiomer pharmacokinetics after intercostal neural blockade in liver transplantation patients

Bupivacaine, being a racemic local anesthetic, exists as an equal mixture of its component enantiomers R(+)- and S(-)-bupivacaine, which behave pharmacokinetically as independent drugs after injection into the body. Intercostal neural blockade using bupivacaine was performed for postoperative analgesia in 12 patients after orthotopic liver transplantation. Arterial blood, sampled serially, was assayed by enantioselective high-performance liquid chromatography for R(+)- and S(-)-bupivacaine. The average of the simultaneous R(+):S(-) ratios of blood bupivacaine concentrations in the 12 patients was 0.74 (SD 0.11); however, the use of a population mean value or a mean value for any patient denies the time-dependence of this entity. The blood enantiomer concentration difference was reflected in the maximum measured concentrations which, after the first dose, were, respectively, 0.38 (SD 0.19) and 0.52 (SD 0.28) mg.L-1.100 mg-1 RS-bupivacaine administered (P = 0.0003). The difference in blood concentrations between the enantiomers, reflected by the R(+):S(-) ratio being less than unity, could be explained by a greater mean total body clearance and a larger apparent volume distribution of R(+)-bupivacaine. Elimination of both enantiomers was prolonged in these patients after liver transplantation compared to data from the literature, but there was no tendency for either enantiomer to accumulate selectively, even upon repeated dosing. We conclude that this demonstration of differences in pharmacokinetics (and, in laboratory studies, also in pharmacodynamics) between the bupivacaine enantiomers points to the need for future studies to recognize the enantiomeric duality of this local anesthetic.

Pharmacokinetics of bupivacaine enantiomers in sheep: influence of dosage regimen and study design

Bupivacaine is used as a racemate. In previous studies the mean total body clearance of R(+)-bupivacaine was found to be greater than S(-)-bupivacaine by 65% after iv bolus dose of separate enantiomers and by 20% after iv infusion to steady state of racemate. The present studies were performed to determine whether different study designs using different iv dosage regimens could influence the pharmacokinetic parameters determined for either bupivacaine enantiomer. rac-Bupivacaine.HCl was administered iv to 6 adult Merino ewes by bolus, brief infusion, and prolonged infusion. Arterial blood concentrations of R(+)- and S(-)-bupivacaine were measured by enantioselective HPLC. These regimens consistently produced lower arterial blood concentrations of R(+)-bupivacaine than S(-)-bupivacaine due to R(+)-bupivacaine having a greater initial dilution volume by 16 (95% CI = 3-29)%, volume of distribution at steady state equilibrium by 32 (95% CI = 17-32)% and mean total body clearance by 28 (95% CI = 21-35)%. The slow half-life of R(+)-bupivacaine, however, was found to be 15 (95% CI = 0-31)% longer than that of S(-)-bupivacaine. The difference between enantiomers in mean total body clearance thus was similar to the previous study based upon infusion to steady state of rac-bupivacaine. Differences in pharmacokinetics attributable to the dosage regimen consisted of a greater mean total body clearance for R(+)-bupivacaine along with a smaller terminal half life with the bolus regimen and a longer half-life of S(-)-bupivacaine after prolonged infusion. Differences in pharmacokinetics between the bupivacaine enantiomers occurred consistently in both distribution and clearance but the magnitude of the effect was less than 50% in each case. Systematic differences in pharmacokinetics associated with the dosage regimen were found mainly in terminal half-life. Dosage regimen, thus, was found to influence the pharmacokinetic results found experimentally and is therefore a significant variable in its own right.

The pharmacokinetics of meperidine in the myocardium of conscious sheep

The pharmacokinetics of meperidine in blood and myocardium were studied in five conscious sheep. After an intravenous (i.v.) bolus of 100, 200, or 300 mg meperidine HCl, the maximum arterial blood concentrations (mean +/- SD) were 27.8 +/- 4.6, 66.8 +/- 13.3, and 114.5 +/- 23.1 micrograms/mL, respectively, and coronary sinus blood concentrations were 2.7 +/- 1.0, 7 +/- 1.5, and 13.7 +/- 2.5 micrograms/mL, respectively. These were linearly related to dose. Net uptake of meperidine into the myocardium occurred during the first minute and the maximum rates of uptake (influxes) were 5.1 +/- 2.6, 15.1 +/- 4.6, and 27.5 +/- 8.7 mg/min. Myocardial meperidine concentrations, calculated using mass balance principles for the 100-, 200-, and 300-mg doses, respectively, were 9.0 +/- 1.8, 20.5 +/- 8.6, and 34.1 +/- 7.4 micrograms/g at peak and had decreased to 57% +/- 11% of peak 5 min after injection. No pseudoequilibrium between blood and myocardial meperidine concentrations had been reached within the 15-min study period. Myocardial perfusion and blood-myocardial concentration gradients were both important determinants of meperidine myocardial pharmacokinetics. The fast uptake and brief sojourn of meperidine in the myocardium agreed with its rapid but transient negative inotropic effect reported previously.

Determination of ketorolac enantiomers in plasma using enantioselective liquid chromatography on an alpha 1-acid glycoprotein chiral stationary phase and ultraviolet detection

A chirally selective high-performance liquid chromatographic assay was developed to measure the R(+) and S(-) enantiomers of ketorolac in plasma for pharmacokinetic studies. Naproxen sodium [S(+) enantiomer] (10 micrograms) was used as an internal standard. Plasma samples (0.5 ml) were acidified (50 microliters of 4 M H3PO4 to pH 1.5), extracted into 0.4 ml of 10% pentan-2-ol in hexane and back-extracted into 0.15 ml of base (20 mM NaOH pH to 7-8), of which samples (5 microliters) were chromatographed on a 100 x 4 mm I.D. column packed with an HPLC chiral stationary phase based upon immobilized alpha 1-acid glycoprotein (Chiral AGP-CSP) with 4% propan-2-ol in 0.1 M NaH2PO4 pH 5.5, at 0.9 ml/min. Detection was at 325 nm and run time was 10 min. Retention times of R- and S-ketorolac and of S(+)-naproxen were 3.3, 4.8 and 6.4 min, respectively. The metabolite p-hydroxyketorolac was not resolved enantiomerically and had a retention time of 2.2 min. The assay was linear over the range 0.5-10 mg/l, with precisions < 5% C.V. Good separations (alpha > 1.35) and resolutions (Rs > 3.23) between peaks were achieved. The sensitivity could be extended to 35 micrograms/l with less precision by increasing the injection volume to 100 microliters.

The hemodynamic effects of intravenous bolus doses of meperidine in conscious sheep

The hemodynamic effects of 100, 200, and 300 mg of meperidine injected intravenously were studied in five chronically instrumented adult ewes. The maximum rate of increase of left ventricular pressure was decreased, respectively, by 27.4% +/- 3.9%, 37.5% +/- 5.6%, and 31.9% +/- 13.0%, and recovery occurred by 5, 8, and 0.5 min, respectively. Mild central nervous system stimulatory effects (agitation) were observed in three of five sheep at 200 mg and moderate effects (rigor and jumping movements) were observed in four of five sheep at 300 mg. These doses also produced increases in heart rate (43%-64%) and mean arterial blood pressure (17%-27%). At these doses, cardiac output was increased for 0.5 min by approximately 25% without changes in stroke volume and left ventricular stroke work. Coronary blood flow was increased by 44%-81% for 0.5 min. We conclude that, in unpremedicated sheep, meperidine has a brief direct negative inotropic effect on the myocardium, but that at larger doses this is overridden by stimulatory central nervous system (CNS) and indirect hemodynamic effects.

Disposition of morphine and its 3- and 6-glucuronide metabolites during morphine infusion in the sheep

A sheep preparation was used to examine the regional formation and extraction of morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), relative to the regional extraction of morphine, at four morphine dose rates. On separate occasions, four ewes received a 15-min loading infusion of morphine sulfate, followed by a constant infusion at 2.5, 5, 10, or 20 mg/hr for an additional 5.75 hr. During the 5th to 6th hr of infusion, blood samples were collected simultaneously from the aorta, pulmonary artery, hepatic vein, hepatic portal and renal veins, posterior vena cava, and coronary and sagittal sinuses. Urine was collected for 48 hr. Morphine, M3G, and M6G in plasma and urine were determined simultaneously by HPLC. The blood/plasma concentration ratio (lambda) for morphine, M3G, and M6G was determined in spiked "blank" blood. Steady-state plasma concentrations were achieved during the sampling period, and dose-normalized concentrations were independent of the infusion rate. There was significant (p < 0.05) extraction (mean +/- SD) of morphine by the liver (0.676 +/- 0.014) and kidney (0.602 +/- 0.039), net extraction of M3G (0.106 +/- 0.046) and M6G (0.104 +/- 0.030) by the kidney, and net formation of M3G (-0.057 +/- 0.017) by the gut. The mean lambda for morphine, M3G, and M6G was 1.25 +/- 0.17, 0.80 +/- 0.03, and 0.82 +/- 0.09, respectively. The mean total body clearance of morphine with respect to blood was 1.58 +/- 0.27 liters/min. Mean (+/-SD) percentage urinary recoveries as morphine, M3G, and M6G were 14.7 +/- 8.5, 75.4 +/- 11.1, and 0.49 +/- 0.39, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Patient-controlled analgesia. Pharmacokinetic and therapeutic considerations

The opioid analgesic agents exhibit relatively large pharmacokinetic differences between drugs, and there is substantial pharmacokinetic and pharmacodynamic variability across subjects or patients with each agent. The advent of patient-controlled analgesic administration techniques and their widespread use in contemporary pain management, especially in postsurgical and cancer patients, has decreased the unfortunate impact of interpatient variability on achieving the optimal balance between pain relief and opioid adverse effect intensity. The improvements in pain management provided by patient-controlled analgesia do not, however, decrease the importance of knowledge of opioid pharmacokinetics towards enlightened use of these drugs and attainment of maximal benefits from them in any patient. Future improvements in patient-controlled analgesia technology will probably be based on the pharmacokinetic behaviour of different opioid analgesic agents in specific receptor-containing regions. Finally, physicochemical and pharmacokinetic characteristics of these agents are important determinants of the speed of onset of effects, duration of action and spinal selectivity of epidurally and intrathecally administered analgesics. Thus, effective patient-controlled analgesia depends on an understanding of the differential pharmacokinetics of opioids self-administered by a variety of possible modes.

Tissue distribution of bupivacaine enantiomers in sheep

rac-Bupivacaine HCl was infused intravenously to constant arterial blood drug concentrations in sheep using a regimen of 4 mg/min for 15 min followed by 1 mg/min to 24 h. At 24 h, arterial blood was sampled, the animal was killed with a bolus of KCl solution, then rapidly dissected and samples were obtained from heart, brain, lung, kidney, liver, muscle, fat, gut, and rumen. Tissue:blood distribution coefficients for (+)-(R)-bupivacaine exceeded those of (-)-(S)-bupivacaine (P < 0.05) for heart, brain, lung, fat, gut, and rumen by an overall mean of 43%. Blood:plasma distribution coefficients of (-)-(S)-bupivacaine exceeded those of (+)-(R)-bupivacaine by a mean of 29% and this offset the tissue:blood distribution coefficients so that the previously significant enantioselective differences disappeared. It is concluded that although enantioselectivity of bupivacaine distribution is shown by the measured tissue:blood distribution coefficients, it is not shown when tissue:plasma water distribution coefficients are calculated, suggesting that there is no intrinsic difference between the bupivacaine enantiomers in tissue affinity. Sheep given fatal intravenous bolus doses of rac-bupivacaine had significantly greater concentrations of (+)-(R)-bupivacaine than (-)-(S)-bupivacaine in brain (P = 0.028) and ventricle (P = 0.036); these could augment the greater myocardial toxicity of this enantiomer found in vitro.

The pharmacological relief of pain — contemporary issues

OBJECTIVE

To provide a general description of the pharmacological principles of pain management particularly the management of severe pain with opioid analgetic agents.

DATA SOURCES

Literature updating previous reviews by the authors in this Journal and the Proceedings of the VIth World Congress on Pain, Adelaide, 1-6 April 1990.

STUDY SELECTION

Studies were selected under the subheadings: opioid receptors; endogenous opioids; development of new agents; recognition of the need to improve analgesia; pharmacokinetics and pharmacodynamics of opioids; and novel drug delivery methods.

DATA EXTRACTION

The extracted information was more descriptive than quantitative.

DATA SYNTHESIS

The pharmacological relief of pain involves treatment of patients with drugs characterised by extremely large interpatient variability in pharmacokinetics and pharmacodynamics by way of an extremely complex interaction with endogenous control mechanisms. It is not surprising that management of pain in many patients is less than adequate.

CONCLUSIONS

Although much vigorous interdisciplinary research is being undertaken to develop a scientific basis for understanding pain and analgesia, improvements in the clinical management of pain can only occur if practitioners recognise the need for individualised methods of pain relief and treat their patients accordingly.

Failure of the nitrous oxide tissue equilibration method for the determination of brain and myocardial blood flow under controlled conditions

1. Two adult merino ewes were prepared with intravascular cannule for sampling aortic root blood, sagittal sinus blood and coronary sinus blood. 2. One week after preparation the animals were anaesthetized then ventilated with a gas mixture containing 10% nitrous oxide (N2O) for 60 min. Serial measurements of brain and myocardial blood flow were made using the N2O tissue equilibration method of Kety and Schmidt. 3. N2O failed to achieve matching arteriovenous blood concentration equality and saturation of the relevant tissues. Valid use of the Kety-Schmidt method, therefore, could not be confirmed. 4. Because of the failure of the arteriovenous equilibration, serially determined brain and myocardial blood flows were found to decrease with time. 5. The use of this method in circumstances where tissue saturation with the indicator gas cannot be ascertained is arbitrary.

Do the pharmacodynamics of the nonsteroidal anti-inflammatory drugs suggest a role in the management of postoperative pain?

Until recently, nonsteroidal anti-inflammatory drugs (NSAIDs) were regarded as weak analgesic agents with a potent antiplatelet effect that severely limited their perioperative usefulness. However, the recent development of injectable NSAIDs has stimulated a re-evaluation of the potential role of this class of drugs in postoperative pain management. In general surgery, NSAIDs have been shown to be effective analgesics when administered after surgery, as judged by either a reduction in pain scores and/or by an opioid sparing effect. Parenteral NSAIDs alone, notably ketorolac and diclofenac, may be adequate or even preferred analgesic agents after minor surgery. In dental surgery, NSAIDs produce greater initial analgesia than steroids, although the latter produce greater suppression of swelling and less functional loss. NSAID pretreatment results in only modest suppression of swelling compared with placebo. These data suggest that the acute analgesic effects of NSAIDs in oral surgery and probably other models result from suppression of a nociceptive process, rather than a generalised anti-inflammatory effect. This view challenges the traditional association between inhibition of prostaglandin synthesis and the therapeutic effects of these drugs. The variety of NSAIDs leads to a range in half-lives from short, e.g. diclofenac (1 h), intermediate, e.g. ketorolac (5h), to long, e.g. tenoxicam (60h), which has implications for both convenience of the dosage regimen and drug accumulation. For some racemic NSAIDs (e.g. ibuprofen), metabolic 'activation' of the inactive R-enantiomer to the active S-enantiomer occurs. Renal dysfunction may increase both the plasma concentration and body residence time of NSAIDs, thereby increasing the risk of adverse effects. The concomitant effects of anaesthesia have not yet been studied. The principal concern regarding the use of perioperative NSAIDs is the risk of decreased haemostasis and wound healing. Although it has been found that NSAIDs prolong bleeding times in patients, values generally remain below the upper limits of those in generally healthy patients. Healing of gastrointestinal anastomoses may be compromised by NSAID administration but corneal healing and bone remodelling are not. There is a need for further research into the potential for renal side effects with NSAIDs in the perioperative setting, where the effects of anaesthesia and surgery may increase the risk of side effects, particularly in elderly patients. The main benefits of NSAIDs derive from opioid sparing (e.g. reduction in perioperative nausea and vomiting and improvement in ventilation), although some studies allude to an enhanced quality of analgesia from the combination compared with either NSAID or opioid alone. The question of pre- vs postinjury treatment with NSAIDs remains unresolved.

Pharmacokinetics and patient-controlled analgesia

With patient control over dosage regimens as in PCA, the pharmacodynamic properties of the opioid analgesic agents emerge as being of greater importance than the pharmacokinetic properties in obtaining a salutary result. However, the prescription of opioid analgesic agents for PCA is more complex than for the same drugs under conventional control. There are major differences in the physicochemical, receptor selectivity and pharmacokinetic properties among opioid analgesic agents. Improved understanding of these differences, it is contended, can lead to more optimized strategies for the clinical management of pain whether under conventional or patient control. The impact of these properties on the components of the PCA prescription is discussed.

Physiological disposition of i.v. morphine in sheep

In a crossover design study we have measured the total body and regional clearances of morphine. Thirteen experiments were performed in four conscious sheep that had been prepared previously with appropriate intravascular cannulae. Morphine (as sulphate pentahydrate) was infused i.v. at 2.5, 5, 10 and 20 mg h-1 to produce constant blood concentrations. Morphine (base) concentrations were measured in blood, urine and tissues with a specific HPLC method. The mean (SEM) total body clearance of morphine was 1.63 (0.21) litre min-1; this comprised 1.01 (0.10) litre min-1 clearance by the liver and 0.55 (0.06) litre min-1 by the kidneys. There was no evidence of dose-dependent clearance or significant extraction of morphine by the lungs, brain, heart, gut or hindquarters at any dose. The kidney clearance of morphine was greater than the 0.21 (0.06) litre min-1 renal clearance determined from the product of the mean total body clearance and the 12.3 (2.4%) of the administered dose recovered as unmetabolized morphine from 48 h urine collection (P less than 0.05). It was concluded that the liver and kidneys account for the majority of morphine clearance, and that the kidneys both excrete and metabolize morphine.

Disposition of mepivacaine and bupivacaine enantiomers in sheep

Mepivacaine and bupivacaine are used clinically as racemic mixtures of enantiomers. In these studies the enantiomers of each agent were administered separately to sheep by i.v. bolus injection on separate occasions. Enantioselective disposition was deemed if the R:S ratio of the relevant pharmacokinetic parameter differed significantly from unity. Both enantiomers of both agents were cleared principally by the liver; urinary excretion of unmetabolized agents accounted for less than 2% of the doses. For R(-)-and S(+)-mepivacaine, respective mean (SEM) values of parameters were: total body clearance 1.20 (0.29) litre min-1 and 0.97 (0.20) litre min-1 (ns); total volume of distribution 144 (39) litre and 80 (21) litre (P less than 0.05); slow half-life 120 (40) min and 84 (22) min (ns); mean hepatic extraction ratio 0.50 (0.14) and 0.52 (0.09) (ns); mean hepatic clearance 0.75 (0.23) litre min-1 and 0.75 (0.18) litre min-1 (ns). For R(+)- and S(-)-bupivacaine, respective values were: total body clearance 0.77 (0.33) litre min-1 and 0.53 (0.26) litre min-1 (P less than 0.05); total volume of distribution 40 (10) litre and 43 (10) litre (ns); slow half-life 57 (10) min and 104 (21) min (P less than 0.05); mean hepatic extraction ratio 0.46 (0.15) and 0.29 (0.13) (P less than 0.05); mean hepatic clearance 0.85 (0.31) litre min-1 and 0.54 (0.26) litre min-1 (P less than 0.05). Thus there was enantioselective distribution of mepivacaine and enantioselective clearance of bupivacaine, but the magnitude of the effect was relatively small.

Cardiovascular effects and regional clearances of i.v. bupivacaine in sheep: enantiomeric analysis

All currently available aminoacylaniline local anaesthetics, except lignocaine, contain a chiral centre but are used as racemates, a fact usually ignored in pharmacokinetic studies. This study reports the cardiovascular effects, and the regional and total body clearances of the enantiomers of bupivacaine determined at two steady state periods (3-4 h and 23-24 h) during continuous i.v. infusion to subtoxic concentrations in conscious sheep. Racemic (RS)-bupivacaine hydrochloride 1 mg min-1, was infused in five sheep that had been prepared at least 1 week previously with appropriate intravascular cannulae. The infusion of RS-bupivacaine produced constant arterial R(+)- and s(-)-bupivacaine concentrations of 0.20-0.68 mg litre-1 and 0.22-0.94 mg litre-1, respectively. This caused no appreciable cardiovascular effects. The hepatic clearance of R(+)- was greater than that of S(-)-bupivacaine (P less than 0.05) with mean (SD) clearance at the two respective time periods being 1.37 (0.78) and 1.47 (0.57) litre min-1 and 1.01 (0.72) and 1.29 (0.47) litre min-1. There was no significant clearance of either enantiomer by the lungs, brain, heart, gut, kidneys or hindquarters. It was concluded that, although the clearances of the enantiomers differed, the total body clearance of both enantiomers was accounted for by hepatic clearance exclusively. There was no evidence of time dependent kinetics.

An assessment of methods for sampling blood to characterize rapidly changing blood drug concentrations

The accuracy of different blood sampling methods used to characterize rapidly changing blood drug concentrations was examined both in vitro and in vivo. It was shown in vitro that blood sampling methods based on the fraction collection principle failed to characterize a "square wave" change in drug concentration, and there was a 9-16-s delay before achieving 95% of the expected drug concentration. Varying the catheter size and length did not improve the response. This observation is consistent with laminar and/or turbulent flow producing dispersion and mixing of blood of different drug concentrations in the catheter. A sampling method (flush and withdrawal) was developed to minimize these effects. In vivo studies showed that peak blood drug concentrations obtained using this method after an iv bolus of a drug were approximately 25-28% higher than those simultaneously obtained by methods based on fraction collection principles. It is concluded that blood sampling methods based on fraction collection principles can produce significant errors in measured blood drug concentrations. The error is greater the greater the rate of change of the blood drug concentrations.

Failure of the Kety-Schmidt nitrous oxide method for determination of myocardial blood flow

1. The reliability of the Kety-Schmidt nitrous oxide (N2O) blood-tissue equilibration method was examined in 50 studies of myocardial blood flow in seven conscious, unrestrained sheep using a newly developed carefully validated gas chromatographic assay for N2O. 2. In 10 studies the arterial and coronary sinus N2O blood concentration-time curves converged as expected at the end of the 10 min sampling period. In 14 studies they crossed over, and in 26 studies, the curves failed to converge. 3. A survey of the literature revealed that such results have been encountered previously but have not been accorded particular significance. An ultimate matching equilibrium between arterial and venous blood N2O concentration-time curves is, however, fundamental to the validity of the method. 4. The results indicate that the use of the Kety-Schmidt method with N2O as the indicator gas is invalid as applied to the measurement of myocardial blood flow in this preparation.

Uptake and elution of chlormethiazole, meperidine, and minaxolone in the hindquarters of sheep: implications for clearance calculations

Mass balance principles were used to describe the uptake and elution of chlormethiazole, meperidine, and minaxolone in the hindquarters of sheep. Sheep received a right atrial infusion of either chlormethiazole (3.71 mg/min) or meperidine (2.70 mg/min) for 180 min, or minaxolone (0.37 mg/min) for 120 min. Paired arterial and inferior vena cava (draining the hindquarters) blood samples were taken during and after the infusion. The mean and SD (n = 4) of the time-averaged extraction ratios across the hindquarters (determined from the relevant arterio-venous area under blood concentration--time curves) were 0.12 (0.10), 0.36 (0.13), and 0.27 (0.05) for chlormethiazole, meperidine, and minaxolone, respectively. The rank order of the rate of uptake of the drugs into the hindquarters was the same as the rank order of their lipophilicity, and uptake still continued when both the arterial and inferior vena cava drug concentrations were essentially constant. For chlormethiazole, meperidine, and minaxolone, 48% (44), 4% (6), and 35% (17), respectively, of the drug taken into the hindquarters eluted from the hindquarters after the infusion. Drug uptake and retention in extravisceral tissues, represented here by the hindquarters, can result in the mean total body drug clearance being overestimated when determined by traditional systemic pharmacokinetic methods.

The in vitro uptake and metabolism of lignocaine, procainamide and pethidine by tissues of the hindquarters of sheep

1. In vitro studies using tissue slices or tissue homogenates of liver, skeletal muscle, fat skin and blood were conducted to determine whether the uptake of procainamide, lignocaine and pethidine into the hindquarters of sheep was due to distribution or metabolism. Both homogenates and slice preparations of liver showed significant metabolism or uptake, confirming the viability of the preparations. 2. None of the drugs was metabolized in blood and there was minimal uptake of the drugs into the skin. 3. There was metabolism of pethidine in skeletal muscle and substantial uptake of pethidine into fat, indicating that the rapid rate of uptake and prolonged elution of pethidine in the hindquarters was due to both distribution and metabolism. 4. No metabolism of lignocaine in muscle was found, but there was substantial uptake into fat, indicating that the rapid rate of uptake and prolonged elution of lignocaine in the hindquarters was due to its distribution into fat. 5. There was negligible uptake of procainamide into either muscle or fat, presumably due to its relatively low lipophilicity.

Regional pharmacokinetics. III. Modelling methods

Regional pharmacokinetics is the study of drug concentrations in specific regions of the body due to drug uptake and elution. Mathematical methods of interpreting regional pharmacokinetic data can vary greatly in their complexity depending on their intended use (i.e. to describe or predict), but must reinforce rather than replace experimental pharmacokinetics. 'Black box' analysis provides and empirical method for the study of complex pharmacokinetic systems using either statistical moment or linear systems analysis. However, these methods are only applicable to linear and time-invariant systems, and ignore the large body of information concerning the physiological and physiochemical basis of regional pharmacokinetics. Clearance concepts are suitable for describing linear drug uptake processes, but mass balance principles have wider applications in describing the rate and extent of both drug uptake and elution. Compartmental models of a region can vary from single compartment descriptions based on the concept of venous equilibrium to complex multi-compartmental models of the intravascular, interstitial, and intracellular spaces, in which drug transport between compartments is a function of drug binding and ionization. Ultimately, as more regional pharmacokinetic information is obtained, more complex three dimensional models may be necessary such as those used to describe the uptake of oxygen from capillaries.

Insight into interstitial drug disposition: lymph concentrations of lidocaine, procainamide and meperidine in the hindquarters of unanesthetized and anesthetized sheep

Drug concentrations in the lymph of the hindquarters of sheep were used to gain insight into drug disposition within the interstitial space. Lidocaine, procainamide and meperidine were each infused into the right atrium of adult merino ewes for 220 min. The time courses of drug concentrations in arterial and inferior vena caval (IVC, draining the hindquarters) blood and hindquarter lymph were determined. It was found that in unanesthetized sheep the mean (+/- S.D., n = 3) lidocaine, procainamide and meperidine concentrations in the hindquarter lymph, were 74 +/- 11, 107 +/- 25 and 94 +/- 17%, respectively, of the arterial and 92 +/- 15, 113 +/- 27 and 134 +/- 28%, respectively, of the IVC blood drug concentrations. Drug binding in lymph, determined by equilibrium dialysis, was not significantly higher than that in blood, except for lidocaine at an initial buffer concentration of 5 micrograms/ml. When the studies were repeated with different animals under halothane anesthesia, the lymph/arterial and lymph/IVC blood drug concentration ratios of the three drugs decreased to 66 +/- 9, 73 +/- 13 and 71 +/- 13% and 77 +/- 8, 88 +/- 19 and 85 +/- 10%, respectively. These values were all significantly lower than those in unanesthetized animals. The results suggest that drug protein binding in interstitial fluid and the status of the microcirculation are important determinants of drug distribution in the interstitial and intracellular spaces. The data also confirm the assumption made in pharmacokinetic studies based upon mass balance principles that the rate of drug removal from organs by lymph is negligible.

The in vivo blood, fat and muscle concentrations of lignocaine and bupivacaine in the hindquarters of sheep

1. A method was developed for sampling muscle and fat from the hindquarters of sheep undergoing spinal anaesthesia. The method was used to measure the concentrations of lignocaine and bupivacaine in the blood, muscle and fat of the hindquarters of sheep during and after 180 min constant-rate infusions of the drugs. 2. For both drugs the muscle drug concentrations were a relatively constant ratio of the simultaneous arterial blood drug concentrations during and after the infusion. 3. There was uptake of both lignocaine and bupivacaine into subcutaneous fat during the infusions. At the end of the infusion the ratio of the fat: arterial blood drug concentrations were 1.54 (SD = 0.57, n = 4) and 3.1 (SD = 1.4, n = 4) for lignocaine and bupivacaine, respectively. 4. The drug concentrations in fat declined relatively slowly after the infusion. The ratio of the fat: arterial blood drug concentrations 180 min after the end of the infusion was 21.5 (SD 4.0, n = 3) and for lignocaine, and 120 min after the end of the infusion was 9.54 (SD 5.2, n = 3) for bupivacaine. 5. It was concluded that the concentrations of lignocaine and bupivacaine in muscle were essentially in equilibrium with the arterial concentrations during and after the infusion. However, the concentrations of lignocaine and bupivacaine in fat were not in equilibrium with the arterial concentrations in the post-infusion period.

Regional pharmacokinetics. II. Experimental methods

Regional pharmacokinetics is the study of the drug concentrations in specific regions of the body. It allows greater insight into the mechanisms of drug disposition than the study of systemic blood concentrations. Experimental methods in regional pharmacokinetics and their applications and limitations are reviewed. Post-mortem tissue biopsies give the drug concentrations in highly specific regions of the body, but require a large number of animals. Serial tissue biopsies yield the time-course of drug concentrations in individual animals, but have limited applications. Regional blood sampling in vivo requires catheterization of blood vessels and a measure of regional blood flow, but allows repeated measurements of the time-course of regional drug concentrations in an individual. In contrast, artificially perfused regions allow greater control of perfusate flow and composition, but are less representative of the in vivo situation. These factors can be retained in some animals by surgically transplanting organs to another location to increase access. Tissues slices and cell cultures can examine drug uptake in the absence of perfusion, and tissue homogenates can be used to study the in vitro rates of drug metabolism and tissue drug binding.

Cardiovascular effects of propofol and of thiopentone anaesthesia in the sheep

We have examined the effects on the cardiovascular system and on regional blood flow of propofol and thiopentone when administered with IPPV (FIO2 0.4). A longitudinal study design was used in which 16 studies were performed in eight sheep for 30 min before, during the last 30 min of 70 min anaesthesia, and for 6 h after anaesthesia. During anaesthesia with propofol and thiopentone, mean total body oxygen consumption decreased, respectively, by 47% (P less than 0.001) and 24% (P less than 0.01) of pre-anaesthesia baseline values, mean heart rate increased by approximately 50% (P less than 0.05) with both agents, mean arterial pressures increased by approximately 50% (P less than 0.05) with both agents and the mean cardiac output was unaltered with propofol anaesthesia but was decreased by 20% (P less than 0.05) with thiopentone anaesthesia. The changes in arterial pressure and heart rate were unexpected and may have been a result of a species-specific effect. Mean hepatic blood flow decreased consistently by a mean of 17% (P less than 0.01) during propofol anaesthesia, and inconsistently during thiopentone anaesthesia so that it was not significantly different from baseline values. Mean renal blood flow decreased during propofol anaesthesia by 7% (P less than 0.05) and by 27% (P less than 0.001) during thiopentone anaesthesia. Whereas most variables returned to baseline values within 2 h after propofol anaesthesia, this took 5 h after thiopentone anaesthesia.

Improved method for morphine determination in biological fluids and tissues: rapid, sensitive and selective

Morphine was assayed using a simple two step solvent extraction--acid back extraction sample preparation method, coupled with normal phase high-performance liquid chromatography (HPLC) and dual electrode coulometric detection. HPLC is performed with a 1.0 M Tris-methanol (5:95) mobile phase with subtle pH adjustments to separate morphine and internal standard from any interfering compounds. The use of normal phase HPLC (silica column) substantially reduces problems from interfering lipophilic substances sometimes encountered with reverse phase HPLC following solvent extraction and which would otherwise require more time-consuming sample preparation. Dual electrode detection further improves the selectivity for morphine and gives excellent sensitivity (0.5 ng mL-1), reproducibility and stability for automated sample injection. This method has proven suitable for pharmacokinetic studies of morphine.

Effects of propofol and of thiopentone anaesthesia on the renal clearance of cefoxitin in the sheep

We have examined the renal extraction ratios and clearances of cefoxitin in three groups of adult merino ewes. One group (n = 3) was studied for 12 h without perturbation; these were designated control studies. The other two groups (n = 4 each) were studied before (baseline values), during and after the induction and 70-min maintenance of anaesthesia with propofol or thiopentone. In the control studies, mean renal extraction ratio and clearance for cefoxitin were, respectively, 0.67-0.92 and 0.66-0.91 litre min-1 and were consistent throughout the entire study period in individual animals. Comparable values were obtained as baseline values in the anaesthesia groups. Compared with individual baseline values, blood concentrations of cefoxitin doubled during anaesthesia with each agent. At the same time, renal extraction ratio and clearance for cefoxitin each decreased significantly to about 50-60% of their control values. Recovery to control values of arterial blood concentrations and renal extraction ratio of cefoxitin took at least 5 h, but recovery of renal clearance was more rapid. The results indicate that renal elimination of an organic anion such as cefoxitin may be affected by changes in renal blood flow and in renal function produced by propofol and thiopentone; these effects may last for several hours after recovery of renal blood flow.

Effects of propofol and of thiopentone anaesthesia on the regional kinetics of pethidine in the sheep

We have examined the extraction ratios, net fluxes and clearances of pethidine by the liver, kidneys and hindquarters in sheep before, during and after continuous anaesthesia (70 min) with propofol or thiopentone. Before anaesthesia, the overall mean respective regional pethidine extraction ratios were 0.98 (SD 0.01), 0.20 (0.06) and 0.44 (0.13), the corresponding net fluxes were 47 (7), 5 (2) and 20 (10)% dose min-1 and the clearances 1.44 (0.22), 0.17 (0.07) and 0.80 (0.39) litre min-1. During propofol anaesthesia, arterial blood concentrations of pethidine approximately doubled (P less than 0.05), mean pethidine hepatic extraction ratio was unchanged, flux was increased to 145 (20)% and clearance decreased to 79 (10)% (P less than 0.05) of baseline values; mean pethidine renal extraction ratio, flux and clearance were 73 (34), 112 (43) and 69 (31)% of baseline values; mean hindquarter pethidine extraction ratio decreased to 65 (25)% (P less than 0.05) of baseline values. During thiopentone anaesthesia, arterial blood concentrations of pethidine approximately doubled (P less than 0.01), mean pethidine hepatic extraction ratio was 97 (2)% of baseline values and flux and clearance were unchanged, mean pethidine renal extraction ratios, flux and clearance decreased to 37 (21), 54 (18) and 27 (19)% (all P less than 0.05) of baseline values and mean pethidine hindquarter extraction ratio was 81 (20)% of baseline values. In spite of only modest changes in hepatic and renal blood flow during anaesthesia, blood concentrations of pethidine doubled and pethidine kinetics were disturbed for several hours after anaesthesia. Overall, however, the changes were of smaller magnitude and shorter duration than those that have been described for anaesthesia with the volatile anaesthetic agents.

Plasma concentrations of the stereoisomers of prilocaine after administration of the racemate: implications for toxicity?

A chiral high pressure liquid chromatography method was developed to measure the separate isomers of prilocaine in plasma after administration of the racemate. The concentrations of the isomers in six patients were similar (S(+)/R(-) = 1.06 (SD 0.06)) after brachial plexus block with 1.5% (RS)-prilocaine hydrochloride 35 ml, suggesting that a higher systemic safety margin may not be achieved by substituting racemic prilocaine by one of its isomers. Much higher plasma concentrations of the S(+)- than the R(-)-form after oral administration of 300 mg of the racemate (n = 4) indicated a large difference in intrinsic metabolic clearance of the isomers on first pass through gut, liver or both organs.

Variables of patient-controlled analgesia. 3: Test of an infusion-demand system using alfentanil

Patient-controlled infusion-demand analgesia was studied using alfentanil. We were unable to identify an optimal dose and administration rate; doses required range from 100 to 900 micrograms alfentanil. The mean concentration of alfentanil in blood associated with return of pain (i.e. immediately before demand) was 58 ng/ml on day 1 and 37 ng/ml on day 2. This difference was despite similar drug consumption on both days.

Cardiovascular effects and regional clearances of intravenous ropivacaine in sheep

The purpose of this study was to determine the cardiovascular effects and the total body and regional clearances of ropivacaine during its continuous intravenous infusion to subtoxic levels in five conscious unrestrained sheep that had been previously prepared with appropriate intravascular cannulas. Ropivacaine HCl.H2O, 1 mg/min, produced constant arterial blood concentrations which ranged from 0.70 to 1.84 mg/L. This caused no appreciable cardiovascular effects. The mean total body clearance (+/- SD) of ropivacaine was 1.00 +/- 0.27 L/min. There was significant clearance of ropivacaine by the liver (0.85 +/- 0.32 L/min), gut (0.09 +/- 0.07 L/min), and kidneys (0.04 +/- 0.03 L/min). There was no significant clearance of ropivacaine by the lungs, brain, heart, or hindquarters. It was concluded that the liver accounts for the majority of ropivacaine clearance.

Physiological and biochemical consequences of electroimobilisation in conscious sheep

An electroimmobilisation device has been developed to facilitate the automated shearing of sheep, but there is little information on its effects on the body. We have studied its effects on the cardiovascular system and on intermediary metabolism in sheep. Electroimmobilisation caused statistically significant increases in mean arterial pressure, heart rate, cardiac output, renal and hepatic and hindquarter glucose and lactate flux, organ and whole body oxygen flux, hindquarter blood flow and core temperature and decreases in arterial and posterior vena cava blood pH, renal and hepatic blood flow and PaCO2. Notably, no change occurred in PaO2. The metabolic changes demonstrated the capacity of sheep to respond to the increased muscular and cardiovascular work induced by electroimmobilisation. Pulmonary function was not compromised during electroimmobilisation as judged from blood gas changes, and the acid/base changes were rapidly reversed after electroimmobilisation. The recovery to control conditions for all perturbations generally took no longer than 30 min, consistent with a rapid and physiologically adequate reversal by the animal's homeostatic mechanisms.