Pharmacokinetics of i.v. and rectal pethidine in children undergoing ophthalmic surgery

We have studied the pharmacokinetics of i.v. and rectal pethidine in 20 children age 4-8 yr undergoing ophthalmic surgery. After i.v. administration, the clearance of pethidine was mean 10.4 (SD 1.7) ml kg-1 min-1, volume of distribution at steady state 2.8 (0.6) litre kg-1 and elimination half-life 3.0 (0.5) h. After rectal administration, plasma pethidine concentrations varied greatly and peak concentrations appeared late, at 147 (44) min. The mean systemic bioavailability after rectal administration was approximately 55%. Because the bioavailability of rectal pethidine varies greatly, this route is not encouraged in the management of acute pain.

Pharmacokinetic-pharmacodynamic modelling of meperidine in goats (I): Pharmacokinetics

Plasma and cerebrospinal fluid (CSF) pharmacokinetics of meperidine were investigated after intramuscular (i.m.) or intravenous (i.v.) administration at a dose of 5 mg/kg in adult goats. After i.m. dosing, the plasma profile was best described by a one-compartment open model. In healthy (n = 16) and post-operative (n = 16) goats, the parameters were, respectively: tmax 8.3 +/- 3.9 and 9.2 +/- 5.5 min, Vd 2.763 +/- 1.231 and 3.929 +/- 2.101 l/kg, Clb 0.125 +/- 0.036 and 0.087 +/- 0.025 l/kg/min, Ke 0.0563 +/- 0.0358 and 0.0271 +/- 0.0136 min-1. The plasma profile was best fitted by a two-compartment open model following i.v. injection. In this case, the parameters for healthy (n = 7) and post-operative (n = 13) goats were, respectively: Vd 5.212 +/- 1.992 and 5.085 +/- 2.288 l/kg, Clb 0.096 +/- 0.028 and 0.075 +/- 0.026 l/kg/min, beta 0.0211 +/- 0.0093 and 0.0160 +/- 0.0052 min-1. There were, however, a few individuals with a prolonged elimination phase. Bioavailability of i.m. meperidine was 66.5 +/- 15.8% in healthy (n = 6) goats, but much higher in postoperative (n = 10) ones at 94.6 +/- 30.0%. Meperidine diffused into and out of CSF according to a first-order rate process. The time-course of CSF drug concentration was simulated by a biexponential function. CSF kinetic parameters of i.m. meperidine for healthy (n = 7) and postoperative (n = 13) goats were: elimination rate constant (K(ef)) 0.0269 +/- 0.0131 and 0.0305 +/- 0.0177 min-1, peak CSF concentration time (Tmaxf) 15.9 +/- 5.0 and 17.0 +/- 6.9 min. For the i.v. dosed healthy (n = 6) and postoperative (n = 8) animals, K(ef) was 0.0408 +/- 0.0107, 0.0414 +/- 0.0123 min-1 and Tmaxf was 10.0 +/- 5.0 and 7.7 +/- 2.5 min, respectively. It was demonstrated that an obviously lower peak concentration can be reached significantly later in CSF than in plasma, and the kinetic behaviour of meperidine in plasma is different from that in the CSF, indicating meperidine analgesia might not be predicted by simple extrapolation from the kinetic data.

An estimate of the rate of direct drug diffusion from the surface of heart and kidney--implications for their representation as compartments

In many regional pharmacokinetic experiments and models, the anatomical boundaries of the heart and kidney are intrinsically assumed to be barriers for drug diffusion such that these organs can be represented as one or more compartments. To test this, an experimental preparation was developed in which the heart and kidney of anaesthetized sheep were surrounded with 0.9% saline. The rate of drug diffusion from the surface of the organs into the saline was examined during constant-rate i.v. drug infusions. It was found that the maximum clearances of lidocaine and procainamide into the pericardial saline were 10.3-11.6 and 0.6-2.1 ml min-1 respectively, and the values for the kidney were 0.3-0.6, 0.1-1.0 and 0.4-1.3 ml min-1, for lidocaine, procainamide, and meperidine respectively. These corresponded to calculated times of 4-481 min to reach the steady-state saline concentration depending on the drug and the organ. The steady-state ratio of the saline concentrations over the arterial blood drug concentrations usually ranged from 0.5-1.0. It is concluded that drugs can rapidly enter regions of low or no perfusion surrounding these organs, and that the concept of treating the heart and kidney as compartments may not be valid in certain 'worst-case' situations.

Hepatic enzyme activity is the major factor determining elimination rate of high-clearance drugs in cirrhosis

The relative importance of alterations in hepatic enzyme activity, blood flow and drug binding to drug elimination in patients with liver disease remains controversial. In addition, liver disease appears to selectively impair drug oxidation pathways while leaving glucuronidation preserved. These studies using isolated perfused rat livers were designed to examine the effects of liver disease on the hepatic extraction and clearance and intrinsic clearance of morphine, a glucuronidated drug, and meperidine, an oxidized drug, under controlled blood flow and drug-binding conditions. We chose chronic carbon tetrachloride-induced cirrhosis as the liver disease. At a flow rate of 12 ml/min, the extraction of meperidine was reduced from 0.91 +/- 0.02 ml/min in controls to 0.76 +/- 0.04 ml/min (p < 0.05) in cirrhosis, hepatic clearance was reduced from 10.9 +/- 0.3 ml/min in controls to 9.15 +/- 0.48 ml/min (p < 0.05) in cirrhosis and intrinsic hepatic clearance was reduced from 179 +/- 35 ml/min in controls to 69 +/- 14 ml/min (p < 0.05) in cirrhosis. In contrast, for morphine we saw no significant changes: extraction ratio, 0.59 +/- 0.02 in controls and 0.49 +/- 0.04 in cirrhosis; hepatic clearance, 7.02 +/- 0.26 ml/min in controls and 6.04 +/- 0.42 ml/min in cirrhosis; and hepatic intrinsic clearance, 15.4 +/- 1.2 ml/min in controls and 13.9 +/- 2.3 ml/min in cirrhosis. Regression analysis of hepatic clearance vs. hepatic blood flow and hepatic clearance vs. hepatic intrinsic clearance demonstrate that in normal livers the elimination of both morphine and meperidine is mainly dependent on blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Biotransformation of pethidine: a comparative study of 24 h urine in three ethnic groups

219 surgical patients of either Caucasian, Chinese or Nepalese origin were given pethidine 1 mg/kg by intramuscular injection as pre-operative medication. Urine was collected for 24 h and analysed for pethidine, pethidinic acid, pethidinic acid conjugates, norpethidine, norpethidinic acid, and norpethidinic acid conjugates. The mean proportion of the percentage of metabolites attributable to oxidative demethylation, hydrolysis and conjugation was almost identical in each ethic group (P > 0.2). It was concluded that there were no differences in the metabolic variability of the biotransformation of pethidine in Asians and Caucasians in whom the urine pH had not been acidified.

Pethidine pharmacokinetics in a heroin addict: a case report

Tolerance to some of the pharmacological actions of the opiates in drug addicts is well known. This report illustrates the effect on the pharmacokinetics of pethidine of heroin addiction compared with a well-matched control population.

A physiological model for the distribution of injected agents, with special reference to pethidine

The model is based on Mapleson's Model P for inhaled anaesthetics, but has compartments for lungs, peripheral shunt, kidneys, portal bed, liver, other viscera, muscle, other lean, fat, brain, i.m. injection site and for various blood "pools". Intracellular and extracellular fluids are represented separately in each compartment and in blood. In each fluid, four forms of the agent are distinguished: unionized dissolved in water ("standard" form), ionized dissolved in water, unionized dissolved in lipid, and bound to protein. Equations define the equilibrium between these four forms in any one fluid and between blood and tissue. After changing two of the more uncertain numbers in the quantification for pethidine, good agreement was obtained between computed and published venous concentrations after single i.v. and i.m. injections, continuous i.v. infusions and repeated i.m. injections. The model can be used to make a wide variety of "what if" predictions.

Pharmacokinetics and pharmacodynamics of intravenous meperidine in neonates and infants

The pharmacokinetics of meperidine (pethidine) was studied in 21 infants who received a single intravenous dose of 1 mg/kg after surgery (n = 18) or during mechanical ventilation because of respiratory distress (n = 3). Eleven patients were younger than 1 week old, 10 patients were aged from 3 weeks to 5 months, and five of the patients were premature. The pharmacokinetics of meperidine varied greatly between the subjects, with a median elimination half-life of 10.7 hours (range, 3.3 to 59.4 hours), median clearance of 8.0 ml/kg/min (range, 1.8 to 34.9 ml/kg/min), median volume of the central compartment of 2.4 L/kg (range, 0.5 to 4.8 L/kg), and median steady-state volume of distribution 7.2 L/kg (range, 3.3 to 11.0). The great interindividual variability in meperidine pharmacokinetics should be taken into consideration when meperidine is administered to neonates. Although no life-threatening or serious side effects were observed in this study, appropriate care should be exercised when prescribing meperidine for this age group.

Neonatal pattern of breathing during active and quiet sleep after maternal administration of meperidine

The aim of this study was to reappraise the effects of maternal meperidine administration on breathing pattern during the first hours of life taking into account the state of alertness. Because breathing instability is more pronounced during active sleep, we hypothesized that meperidine administration might create a greater risk for respiratory instability during active sleep, the prominent sleep state in newborns. We studied eight full-term, healthy newborns whose mothers had received a continuous i.v. infusion of meperidine (81 +/- 9 mg) that was terminated 5.5 +/- 2.1 h before delivery. These infants were compared with a control group of eight full-term newborns whose mothers did not receive any opioids. In both groups, all babies were delivered vaginally after a normal labor and had Apgar scores of 9 or 10 at 1 and 5 min. Neonatal gastric secretion and maternal venous and umbilical venous blood were sampled at delivery for determination of meperidine concentration. From 60 to 300 min after delivery, behavioral sleep states and thoracic and abdominal movement as well as transcutaneous arterial oxygen saturation (SaO2) were monitored continuously. The number of apneic spells lasting more than 3 s during 100 min of recording and the percentage of time with SaO2 below 90% in each sleep state were recorded. During quiet sleep, all respiratory variables were similar in both groups. During active sleep, there were significantly more apneic episodes (37.1 +/- 25.1 versus 11.2 +/- 13.9) and a higher percentage of time with SaO2 less than 90% (14.3 +/- 16.7% versus 1.3 +/- 1.5%) in the meperidine group than in the control group (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of hepatic drug biotransformation by carrageenan-induced inflammation in the rat: effect of sex hormone alterations

Following in vivo treatment with carrageenan, sex-related differences in alteration of hepatic drug metabolism were found in the rat. In adult male rats, marked decreases were observed in hepatic 9000 x g supernatant cytochrome P-450 content and in the biotransformation of hexobarbital, aminopyrine, ethylmorphine, and meperidine. Hexobarbital hypnosis was significantly prolonged by carrageenan treatment in intact and testectomized animals as compared to their respective controls. Although carrageenan-treated intact animals slept 480% longer, carrageenan-treated testectomized rats slept only 60% longer than the respective control animals. However, testectomy or administration of 17 beta-estradiol to testectomized male rats did not inhibit the monooxygenase activities by carrageenan-treatment. Furthermore, administration of testosterone to ovariectomized female rats did not antagonize the inhibitory effects of the carrageenan-induced inflammation. The inhibitory effects produced by carrageenan-induced inflammation on the microsomal enzyme system were observed only in mature male rats and were not observed in mature female rats or in sexually immature rats of either sex. Thus, these results suggest that the inhibitory effects of carrageenan-induced inflammation on hepatic 9000 x g supernatant monooxygenases in the male rat are partially mediated through the toxic action of carrageenan-induced inflammation on androgen-dependent factors in this enzyme system.

Elimination of bupivacaine and pethidine from the rabbit feto-placental unit

Bupivacaine 12.5 mg and pethidine 12.5 mg were administered as an i.v. infusion over 80 min in 15 near-term pregnant New Zealand white rabbits. After the infusion, pups were removed via individual hysterotomies every 30 min. Bupivacaine and pethidine were measured by gas chromatography in amniotic fluid, placenta, fetal plasma, fetal brain and maternal plasma, sampled as each fetal sac was opened. Pethidine was cleared from all compartments more rapidly than bupivacaine and for both drugs elimination rates were maternal plasma > placenta > amniotic fluid > fetal brain > fetal plasma. Concentrations of both drugs in fetal plasma correlated significantly only with those in fetal brain and not with those in maternal plasma, placenta or amniotic fluid, while there were positive correlations between the last three compartments. Nested analysis of covariance showed that only maternal plasma and placental concentrations of the two drugs decreased significantly with time. Maternal plasma half-lives for pethidine and bupivacaine were 1.0 and 2.0 h, and placental half-lives 1.9 and 2.5 h, respectively. The apparent fetal plasma half-life of pethidine was 9.9 h while there was apparently no net elimination of bupivacaine from fetal plasma.

Pethidine pharmacokinetics after intramuscular dose: a comparison in Caucasian, Chinese and Nepalese patients

The pharmacokinetics of pethidine after a single intramuscular injection were studied in 30 male patients of Caucasian, Chinese and Nepalese extraction. There were no significant differences between the three ethnic groups in the mean time for maximum absorption (tmax) and peak plasma concentration (Cmax) of pethidine. The mean (+/- S.D.) elimination half life (t1/2) of pethidine was shorter in Caucasians (4.5 +/- 1.3 h) compared with Nepalese (6.3 +/- 1.6 h) and Chinese (8.1 +/- 3.1 h) (p < 0.01). The plasma clearance of pethidine was greater in Caucasians (14.2 +/- 4.8 ml.min-1.kg-1) than in Nepalese (12.6 +/- 2.9 ml.min-1.kg-1) and Chinese (10.0 +/- 2.9 ml.min-1.kg-1) (p < 0.05); yet the apparent renal clearance was similar (64.1 +/- 22.9, 86.7 +/- 44.5 and 61.4 +/- 30.1 ml.min-1.kg-1, respectively, for the Chinese (n = 6), Caucasian (n = 6) and Nepalese (n = 9) patients). No apparent ethnic differences were found in the tmax and Cmax of norpethidine which emerged as the major metabolite in the plasma in the three races. An apparently higher area under plasma concentration-time curve (AUC infinity [symbol: see text]) and longer elimination t1/2 of the metabolite were observed in the two Asian patient groups. It appears that both the Chinese and Indian groups did not eliminate pethidine as effectively as the Caucasians after a single intramuscular injection, which may be the result of interethnic variability in the metabolism of pethidine. Caution may be required on multiple dosing of pethidine in Asian patients due to the possible accumulation of the parent drug and its toxic metabolite, norpethidine.

Lethal effects of normeperidine

Normeperidine, having a longer half-life than meperidine, can accumulate faster. Its excitatory neurotoxicity is manifest with administration of multiple meperidine doses or in cases of renal failure. This is a report of the lethal effects of normeperidine in a patient with renal failure who had abused meperidine.

The influence of surgery and anaesthesia on the pharmacokinetics of pethidine in the horse

The plasma concentration of pethidine was measured in 16 horses, after its administration intravenously (i.v.) at a dose rate of 1 mg/kg bodyweight (bwt). In eight animals studied before surgery, the plasma levels of the drug decreased in a bi-exponential manner with a distributive half life of 3 mins and an elimination half life of 57.7 mins. Total body clearance was 17.7 ml/kg bwt/min. The remaining horses were investigated immediately after a period of anaesthesia and surgery and in these animals the drug exhibited smaller volumes of distribution (V1 cand Vdarea) and a significantly lower clearance rate of 12.8 ml/kg bwt/min in these animals. Correlation between the plasma concentration of pethidine and information published previously on the duration of action of the agent in the horses suggests that plasma levels of approximately 0.4 microgram/ml are required to produce analgesia in this species.

Bioavailability and analgesic effect of sustained release cetobemidone capsules in cancer patients with chronic pain of malignant origin

Ever since its introduction in 1952, Ketogan has consisted of a combination of the opioid cetobemidone and a spasmolytic drug, A29, in the ratio of 1:5. Its main limitations are the relatively short duration of action and an apparent ceiling effect due to A29. We therefore developed a cetobemidone formulation with sustained release properties (cetobem. s.r.). Forty-seven patients with advanced cancer with moderate/severe pain, treated with regular doses of Ketogan tablets took part in an open phase 1 type pharmacologic/toxicologic study comparing Ketogan tablets and cetobem. s.r. The patients spent at least 4 days in the department. After evaluation during 12 h with Ketogan tablets (3-4 dosing intervals), a loading phase lasting for 36 h with cetobem. s.r. given twice/24 h (total dose of Ketogan tablets per 24 h divided by 2), and synchronous reduction of the dose of Ketogan tablets, a similar evaluation period of 12 h was performed when on treatment with cetobem. s.r. alone. Comparison of data from the two observation periods showed the following: Average relative bioavailability 0.77 SD 0.28. Only 2 patients were remedicated before 12 h when on cetobem. s.r. Duration of action mean 11.9 h, SD 0.3 h. There was no significant difference between Ketogan tablets and cetobem. s.r. periods concerning 'time with no pain'. Side-effects were few. The only significant difference was dry mouth being more severe during the tablet period. Vital signs were unaffected by treatment. It is concluded that cetobem. s.r. can be given every 12 h to cancer patients with chronic pain with satisfactory analgesic effect, a very modest change in side-effect profile and no significant effects on vital signs when substituting cetobem. s.r. for Ketogan tablets in equal daily doses.

Pethidine blood concentrations at time of birth

Pethidine levels in maternal and fetal blood were estimated at birth after variable periods of intramuscular administration--delivery intervals in forty pregnant women. The results suggest that maternal--fetal equilibrium occurs at a fast rate with a regression coefficient initially of 0.54. There appeared to be a tendency towards trapping of the drug in the fetal circulation resulting in a rise in the regression coefficient after 3 h reaching 0.85. The pharmacokinetics and the clinical significance of these observations are discussed.

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.

Exogenous opioids in human breast milk and acute neonatal neurobehavior: a preliminary study

Opioid analgesia requirements, distribution into breast milk, and influence on neonatal neurobehavior were evaluated in ten parturient-neonate pairs nursing after elective cesarean section during epidural anesthesia. Five patients received first a loading dose of intravenous meperidine after umbilical cord clamping, then patient-controlled analgesia (PCA) with intravenous meperidine, and finally meperidine tablets as needed. Five patients received morphine in the same manner. Treatment groups showed no differences with respect to neonatal Apgar scores or visual analog scale (VAS) pain or satisfaction scores at 24 and 48 h postpartum. Breast milk specimens, obtained at 12, 24, 36, 48, 72, and 96 h postpartum and analyzed for opioids and metabolites, showed persistently elevated normeperidine concentrations in the meperidine group. A blinded psychologist evaluated each infant once on the 3rd day of life with the Brazelton Neonatal Behavioral Assessment Scale (NBAS). A priori, the "alertness" and three "human orientation" outcomes of the NBAS were chosen for analysis as best measures of opioid-induced effects. On all four outcomes, neonates in the morphine group scored significantly higher (P less than 0.05) than neonates in the meperidine group. We conclude that post-cesarean delivery PCA with morphine provides equivalent maternal analgesia and overall satisfaction as that provided by PCA with meperidine, but with significantly less neurobehavioral depression among breast-fed neonates on the 3rd day of life.

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.

Effect of umbilical perfusate pH and controlled maternal hypotension on placental drug transfer in the rabbit

The purpose of this study was to explore the effect that fetal acidosis and poor maternal placental blood flow may have on the rate of passage across the placenta of drugs commonly used in labor: bupivacaine and meperidine. The rabbit placenta perfused in situ on the fetal side was used, and antipyrine (phenazone) was included as an index of placental exchange and of maternal placental flow. Bupivacaine and meperidine (both 1.25 mg/mL) and antipyrine (4 mg/mL) were infused into a maternal external jugular vein, at 3 mL/h, after loading, in seven anesthetized does. A single placenta in each doe was perfused via the umbilical arteries with Krebs' bicarbonate buffer at pH 7.5 (phase 1), 7.0 (phase 2), and 7.5 (phases 3-5). During phase 4 maternal blood was withdrawn so as to reduce maternal arterial pressure by 35%, and in phase 5 it was reinfused. Concentrations of drugs were measured during each of the five phases in maternal arterial plasma (Cma) and in the effluent perfusate collected from the umbilical vein (Cuv). From these data, placental clearance rates (Cuv x umbilical flow/Cma) were calculated for each drug at each phase. Clearance of bupivacaine and meperidine increased (P less than 0.03) during phase 2, whereas that of antipyrine did not. Clearance of meperidine and antipyrine, but not of bupivacaine, decreased during phase 4. If these results can be extrapolated to the clinical situation, they suggest that the adverse effects of drugs may well be exacerbated in the presence of fetal compromise.

The effect of clinical hepatic disease on the distribution and elimination of pethidine administered post-operatively to dogs

The pharmacokinetics of pethidine were investigated in dogs suffering from portosystemic vascular shunts and hepatic parenchymal disease. The drug was administered post-operatively at a dose rate of 2.0 mg/kg either intravenously or intramuscularly. When the drug was given intravenously to dogs suffering from intrahepatic shunts, the elimination half life was 75.5 +/- 4.2 min and the clearance rate was 29.0 +/- 2.3 ml/kg min. When the drug was given intramuscularly its rate of absorption was very slow (57.9 +/- 6.1 min) and the time taken to reach the maximum plasma concentration was long at 26.7 +/- 6.4 min. The elimination half-life for the intramuscularly administered drug was also very slow at 108.0 +/- 13 min, reflecting the reduced ability of these dogs to metabolize and eliminate pethidine. These findings suggest that caution should be exercised in the administration of pethidine to dogs suffering from hepatic dysfunction. In particular, the slower rate of absorption should be remembered in judging whether or not maximum effects have been achieved and the longer elimination half-life indicates the need to extend the intervals between repeat doses of this analgesic.

Pharmacokinetics of pethidine administered intramuscularly and intravenously to dogs over 10 years old

The pharmacokinetics of pethidine administered postoperatively both intravenously and intramuscularly was investigated in dogs aged over 10 years. When the drug was given intravenously (2 mg kg-1), plasma levels declined in a biexponential manner, with an elimination half-life of 62.7 minutes. Following its intramuscular administration at the same dose rate, absorption was very slow and two distinct rates of absorption were observed. Maximum plasma concentrations were not achieved until 33 minutes after drug administration. The elimination of the intramuscularly administered drug was also slow with a t 1/2 beta' of 145.9 minutes. Thus, it seems that in elderly animals pethidine has a long elimination half-life and a slowed rate of absorption. However, the total body clearance of the drug does not seem to be affected by age.

Ion-trapping of meperidine: influence of antacid treatments on serum and gastric fluid concentrations

The ascorbic acid pretreated beagle dog was used to demonstrate the phenomenon of ion-trapping and the influence of three different antacid treatments on the serum and gastric fluid concentration of meperidine: pretreatment with magnesium-aluminum hydroxide (MA), pretreatment with cimetidine (C), and periodic total aspiration of gastric fluid (ASP). A significant correlation between pH and the meperidine concentration in gastric fluid was found; the lower the pH the higher is the secretion of meperidine into gastric fluid. The gastric fluid meperidine concentrations were 8-30 times as high as the respective serum concentrations. No significant differences between the meperidine concentration in the gastric fluid were found among the three treatments, MA, C, or ASP, versus control. Meperidine serum concentrations differed significantly versus control on two times for MA and on one time for ASP. The elimination half-life [h] and the mean residence time [h] differed significantly in the MA treatment (1.15 +/- 0.02 h and 1.23 +/- 0.24 h) versus control (1.90 +/- 0.19 h and 2.00 +/- 0.15 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Disposition of pethidine in man under acidic urinary pH. 4. Kinetics after oral dose in caucasian, Chinese and Indian subjects

The pharmacokinetics of low dose pethidine (450 micrograms kg-1) after oral administration were determined in 9 Caucasian, 9 Chinese and 9 Indian healthy volunteers under conditions of acidic urinary pH. Plasma and urine concentrations of pethidine and norpethidine were determined simultaneously by gas liquid chromatography. In all three ethnic groups the oral absorption of pethidine was rapid. The Tmax was faster in the Caucasian group (0.75 h) compared with the Chinese group (1.0 h) and the Indian group (1.5 h). No significant difference was observed in their respective lag time while the absorption t1/2 was significantly shortest in the Caucasian group who also had the highest Cmax (95.5 +/- 7.8 ng ml-1) compared with the Chinese (85.9 +/- 11.0 ng ml-1) and the Indian (58.2 +/- 3.0 ng ml-1) groups. Moderate exercise and upright posture of the Asian students might interfere with absorption and distribution of pethidine, due possibly to change in blood flow during the early stage of the study while the Caucasian subjects were in supine position. No significant difference was observed in the elimination t1/2 of pethidine between the Caucasian (8.3 +/- 0.2 h) and the Chinese (8.2 +/- 0.2 h) groups, although the Indian subjects significantly had the longest elimination t1/2 (0.1 +/- 0.3 h); this could possibly be due to their significantly higher apparent volume distribution. Under acidic urinary conditions both Chinese and Indian subjects excreted significantly more norpethidine in the urine while no difference was observed in the recovery of unchanged pethidine; this may suggest an interethnic difference in the oxidative demethylation of pethidine.

The pharmacologic management of pain in children

We have attempted to dispel many of the myths and misconceptions surrounding the use of narcotic analgesics in the treatment of childhood pain. Our hope is that an improved understanding and application of effective and safe therapies will minimize the suffering of the child with acute or chronic pain.

[Distribution of 14C-pethidine in the blood of certain species]

Binding of 14C-pethidine to erythrocytes of some species, particularly human, rat and rabbit, was examined in two different concentrations. This binding was investigated by direct measurement of the relative representation in erythrocytes, plasma proteins and the free fraction in blood. In the case of human erythrocytes the relative representation was 33%, in the rat ones 44%, and in the rabbit ones 54%. The fraction bound to plasma proteins was 45% in man, 29% in rats, and 27% in rabbits. The portion of the free fraction was 22% in man, 27% in rats, and 19% in rabbits. The ratio of the concentrations of the fraction bound to erythrocytes and the fraction free in the blood was 1.72 in man, 2.09 in rats, and 3.92 in rabbits. On the basis of these results it can be concluded that binding of pethidine to erythrocytes does not play an insignificant part in total pharmacokinetics of pethidine.

Pre-stimulus/post-stimulus relations in EEG spectra and their modulations by an opioid and an antidepressant

Parametric spectral analysis of late cerebral potential components (later than 80 msec) evoked by painful somatosensory stimuli reveals a stimulus-induced increase of power in the low frequencies, delta and theta. This paper investigates the effect kinetics of the opioid meperidine (150 mg, p.o.) and the antidepressant imipramine (100 mg, p.o.) on spontaneous and evoked EEG activity in a placebo-controlled double-blind cross-over study with 20 healthy male subjects. Brief electrical stimuli (5 msec) were intracutaneously applied on the finger tip with randomized intensities above pain threshold and intervals between 10 and 20 sec. Spectra of short (500 msec) pre- and post-stimulus EEG epochs were evaluated using the maximum entropy method (model order 20). The main findings were: (1) The maximum effects of the 2 drugs upon spontaneous and evoked EEG activity were comparable: in the pre-stimulus (spontaneous) EEG both drugs increased the power in the low frequencies and decreased the power in the alpha range. In the post-stimulus (evoked) EEG the drugs decreased the power in all frequency bands. (2) Since in the low frequency range the drugs exhibited contrary effects upon spontaneous and evoked EEG activity, the pre-/post-stimulus relationship of the delta power was found to be the most sensitive measure for monitoring the cerebral bioavailability of the tested drugs. (3) The time courses of development of the effects of the two drugs were different: maximal effects of meperidine were obtained 85-105 min, and of imipramine 190-210 min after medication. The differences in the effect kinetics agreed with the different pharmacodynamics, with time constants for absorption and elimination of 40 min and 240 min for meperidine, and 240 min and 840 min for imipramine. (4) The most important difference between the 2 drugs was the different effect kinetic. Furthermore, in contrast to meperidine the effects of imipramine upon beta activity could not be separated from placebo, either in the spontaneous or in the evoked EEG activity.

Pharmacokinetic disposition of pethidine under tolerance

Under tolerance, evoked by multiple doses of pethidine (PD), the serum and brain tissue content of PD was related to diminished analgesic activity. Even though in tolerant rats no enhancement of PD biotransformation in the liver could be recognized (as followed by the measurement of hepatic esterase and N-demethylase activity), the amounts of both PD and nor-PD excreted in urine were increased under tolerance. The authors conclude that the faster disposition of PD may contribute to the development of tolerance.

Pharmacokinetics of meperidine in pregnancy

The disposition of meperidine was studied in 11 term pregnant humans after the administration of a single 50 mg intravenous dose of meperidine through 48 h post-injection. Half-lives of the rapid and terminal elimination phases were calculated as 2.3 and 13.3 h, respectively. These values are much greater than previously reported half-lives which were based on data collected over less than 8 h after injection. An accurate value for t1/2 beta may be particularly important in sequential dosing of analgesic medication. These pharmacokinetic constants calculated on data collected through 48 h in this study may have important clinical correlates.

Arterial and ventricular CSF pharmacokinetics after intrathecal meperidine in humans

In order to investigate the mechanisms leading to respiratory depression after lumbar administration of opioids, plasma and ventricular CSF pharmacokinetics of intrathecal meperidine (1 mg.kg-1) were studied in five head-injured patients undergoing surgery for lower limb fracture. Meperidine was detected both in the plasma (arterial catheter) and in the ventricular CSF (intracranial catheter) soon after intrathecal administration: 45 +/- 17 min and 100 +/- 14 min, respectively. The maximal plasma concentration was 341 +/- 133 ng.ml-1, whereas, in ventricular CSF, it was 64.5 +/- 14.9 ng.ml-1. The ventricular CSF-plasma ratio increased with time (r = 0.82) from 0.18 +/- 0.04 at the first hour to 0.38 +/- 0.1 at 16th hour. It is concluded that the putative risk of respiratory depression appears to be mainly related to the absorption into the systemic circulation and to redistribution back into CSF.

Intravenous, deltoid, or gluteus administration of meperidine during labor?

It has been suggested, but not well verified, that drug absorption from traditional intramuscular injection sites is altered during labor. This study tested the hypothesis that absorption of meperidine from the gluteus muscle would be impeded when compared with deltoid or intravenous administration. Five patients in labor were given 50 mg intravenously, 10 were given 50 mg in the gluteus muscle, and five were given 50 mg in the deltoid muscle. Five nonpregnant subjects served as their own controls for all three routes of administration. Blood samples were obtained at intervals after injection, meperidine was determined by gas chromatography/mass spectrometry, and mean plasma levels of meperidine versus time after administration were plotted. Repeated measures analysis of variance was used to determine whether the curves were significantly different from 30 to 150 minutes. Maternal plasma levels after gluteus injection were significantly lower than those after intravenous injection (F[1,8] = 10.53; p less than 0.01). In nonpregnant subjects, drug levels were not significantly different from 30 to 150 minutes after either gluteus or intravenous injection. Plasma levels after deltoid injection were always higher than those after gluteus injection in both pregnant and nonpregnant subjects (F[1,8] = 9.7; p less than 0.02; F[1,8] = 14.5; p less than 0.004). These findings support impaired absorption of drugs from the gluteus muscle and suggest the deltoid muscle as the favored intramuscular site during labor.

Pharmacokinetics of intramuscularly administered pethidine in dogs and the influence of anaesthesia and surgery

The plasma concentration of pethidine was measured after it had been administered intramuscularly to fully conscious dogs, and to dogs in the postoperative period and during general anaesthesia. The absorption of the drug was erratic except in the anaesthetised animals and the plasma concentrations of the drug were also higher in this group. Correlation of plasma concentrations of the drug with its analgesic activity revealed a 'critical' concentration of pethidine of 0.4 micrograms/ml for complete analgesia; useful though not complete analgesia was achieved with concentrations above 0.2 micrograms/ml. These concentrations were maintained for 90 minutes after the administration of the drug at a dose of 2.0 mg/kg and for 120 minutes after a dose of 3.5 mg/kg.

Distribution of 11C-labelled morphine and pethidine after spinal administration to Rhesus monkey

The kinetics of 11C-labelled morphine and pethidine were studied by positron emission tomography (PET) at different levels of the spinal canal (C4, T4, T5, T6, L1 and L6). Studies were performed in the Rhesus monkey after intrathecal and extradural administration of the drugs at the lumbar level (L3-L4 or L4-L5, seven experiments). Radioactivity 100-300 times higher than with even distribution in the body was measured initially near the site of injection for both morphine and pethidine, irrespective of the route of administration. After injection of pethidine, high activity was observed at the L6 and L1 levels, whilst the radioactive uptake was lower at T6 (10-20% of those at lumbar level). Morphine-derived 11C-radioactivity showed more constant levels along the spinal canal, except at C4 where radioactivity was low. In CSF taken from the cervical level the peaks of radioactivity of the two drugs appeared 80-170 min after injection. The importance of different distribution routes was quantified in a pharmacokinetic compartment model, using the above results. The systemic distribution was extensive, irrespective of drug or route of administration. From the site of injection the systemic distribution was at least 60 times larger than the rostral distribution within the spinal canal.

Intrathecal opioids, potency and lipophilicity

To determine the relationship between potency and lipophilicity for intrathecal opioids, morphine, normorphine, pethidine and methadone were studied in an electrophysiological model in the rat. Dose-response curves were constructed for the opioid effects on C fibre evoked activity of dorsal horn nociceptive neurones following intrathecal application of each opioid, and the ED50 values were correlated with lipid solubility. A significant inverse correlation was found (P = 0.002; r = 0.998) so that the most lipid soluble drugs were the least potent. The possible mechanism of this relationship, the implications for spinal opioid use and the effect of different administration routes on the relationship between lipid solubility and potency are considered.

Effects of perfusate flow rate on measured blood volume, disse space, intracellular water space, and drug extraction in the perfused rat liver preparation: characterization by the multiple indicator dilution technique

The effect of hepatic blood flow on the elimination of several highly cleared substrates was studied in the once-through perfused rat liver preparation. A constant and low input concentration of ethanol (2.0 mM), [14C]-phenacetin and [3H]-acetaminophen (0.36 and 0.14 microM, respectively), or meperidine (8.1 microM) was delivered once-through the rat liver preparation in five flow periods (greater than 35 min each); control flow periods at 12 ml/min were interrupted by flow changes to 8 or 16 ml/min. The steady-state hepatic availabilities (F or outflow survivals) at 12 ml/min were ethanol, 0.075 +/- 0.038; [14C]-phenacetin, 0.15 +/- 0.059; [3H]-acetaminophen, 0.34 +/- 0.051; meperidine, 0.047 +/- 0.017. Flow-induced changes were different among the compounds: with reduced flow (8 ml/min), F was decreased for ethanol (0.061 +/- 0.032) and [3H]-acetaminophen (0.28 +/- 0.051), as expected, but was increased for [14C]-phenacetin (0.20 +/- 0.068) and meperidine (0.05 +/- 0.03); with an elevation of flow (to 16 ml/min), F was increased for all compounds, as expected of shorter sojourn times: ethanol, 0.13 +/- 0.065; [14C]-phenacetin, 0.22 +/- 0.062; [3H]-acetaminophen, 0.43 +/- 0.063; meperidine, 0.055 +/- 0.022. A marked increase in F for ethanol had occurred when flow changed from 12 to 16 ml/min due to nonlinear metabolism; the latter was confirmed by a reduction in the extraction ratios at increasing concentrations (1.8 to 11.4 mM); this condition was not present for the other compounds. In order to explain the observations, we used the multiple indicator dilution technique to investigate the flow-induced behaviors of tissue distribution spaces of vascular and intracellular references in the perfused rat liver preparation.

The development and clinical use of patient-controlled analgesia

Patient-controlled analgesia has successfully made the transition from research tool to clinical acceptability. Reliable and sophisticated patient-controlled analgesia systems are commercially available. The technique has been most used for control of postoperative pain but has been successfully used during labour, after burns and other trauma and in terminal care. Virtually every opioid has been administered by patient-controlled analgesia using almost every route of administration. It is more effective than the traditional techniques of pain control after surgery but is not automatically so. Choice of opioid and the settings chosen for demand dose and lockout interval greatly influence effectiveness. Patient-controlled analgesia requires active participation by the patient but the psychology of patient-controlled analgesia has generally been under-estimated. Patient-controlled analgesia has developed empirically and many assumptions have been made; there is a need for fundamental research.

The scientific basis of patient-controlled analgesia

The current practice of patient-controlled analgesia has grown from empirical observations. Although several variants of patient-controlled analgesia, bolus doses, infusions, or combinations of both, have been suggested, a scientific basis for advocating one variant over the others has been lacking. Most systems have been based on the simplest system, bolus demand, although the use of a combined bolus and background infusion method has theoretical merit. Similarly, a scientific basis for setting the variables of patient-controlled analgesia, drug choice, incremental dose, maximum dose and lockout interval, also has been lacking. Settings for these variables may be rationalised post hoc on the basis of the physicochemical properties and global pharmacokinetic properties of the opioids used but knowledge of these properties has not helped in setting the variables a priori. Foremost, the drug choice should be based on therapeutic index. Knowledge of the regional kinetics of drug (influx and efflux) from brain may provide a more logical basis for setting the patient-controlled analgesia variables but such information can only come from animal experiments. More research is required if patient-controlled analgesia is to become anything but an empirical tool in the quest for improved analgesia in patients.

The effect of buprenorphine on the analgesic and respiratory depressant effects of pethidine: a preliminary study

The effects on respiration and pain perception of giving 0.6 mg buprenorphine alone and of giving the same dose after the administration of pethidine intravenously to achieve a steady-state blood pethidine level (mean blood level 0.29-0.47 microgram/ml) were studied in 3 healthy male volunteers. Depression of ventilation occurred with both pethidine and buprenorphine, and the combination produced greater depression than did either drug alone. Times to onset of, and tolerance to, experimental pain increased with pethidine and buprenorphine, a greater increase occurring when both drugs were combined. There was no evidence that buprenorphine reversed the respiratory depression produced by pethidine, while maintaining analgesia.

Species differences in the hydrolysis of meperidine and its inhibition by organophosphate compounds

The hydrolysis of meperidine was assayed in washed, unfortified liver microsomal fractions of guinea pig, rat, mouse, dog, and human, by following substrate disappearance as quantitated by high-performance liquid chromatography. Using the method of Lineweaver-Burk plots, the velocity of the meperidine hydrolysis reaction was not detectable in guinea pig, very low in human, and extremely high in dog. Hydrolysis of p-nitrophenyl acetate was also monitored in liver microsomal preparations from the same animal species, with guinea pig showing greatest hydrolytic activity and rat showing least hydrolytic activity for this substrate. The data in the above two assays suggested that meperidine hydrolysis is mediated by a unique esterase not present in guinea pig and very low in human, but present with high activity in dog liver microsomes. From these comparative studies we concluded that liver microsomes from different species may contain different carboxylesterases having different affinities for meperidine. To further characterize meperidine carboxylesterase of dog and rat liver microsomes, inhibitory studies in vitro with two organophosphate compounds--paraoxon (diethyl-p-nitrophenyl phosphate) and soman (pinacolyl methylphosphonofluoridate)--indicated a varied pattern of enzyme inhibition. These results suggested that liver microsomal carboxylesterases are involved in the metabolism of meperidine and that interference with these enzymes by organophosphate compounds may alter pharmacologic and toxicologic effects of meperidine.

Feasibility study for transdermal delivery of meperidine

The present study examines the feasibility of meperidine for transdermal delivery. Meperidine possesses characteristics favorable for percutaneous absorption, i.e., a high lipid/water partition coefficient of 25 at pH 7.4, a low molecular weight of 247, and water solubility. Following solubility and apparent partition coefficient experiments, meperidine base rather than salt, was chosen for permeation experiments. The release of meperidine from a drug delivery device (disc) was studied in vitro. Ex vivo permeation of meperidine through excised rat skin was studied using the Thomas diffusion cell, and the fluxes from different solvents were compared. From the in vitro and ex vivo studies, a disc was chosen for final in vivo evaluation in rats, and compared to I.V. and P.O. administration of meperidine.

Disposition of intrapartum narcotic analgesics in monkeys

Maternal-fetal disposition and neonatal respiratory depressant effect of narcotic analgesics were studied by administration of meperidine (2 mg/kg, IV) or alfentanil (IV infusion, 0.1 mg/kg total dose) during labor in rhesus monkeys. Fetal/maternal plasma ratios were lower for alfentanil, the more highly protein-bound drug (fetal/maternal ratio 0.20 at birth versus 0.46 for meperidine). However, elimination of alfentanil was delayed in the neonate. Indeed, plasma concentrations of alfentanil increased during the first 2 postnatal hours, indicating a compartmental shift from tissues to circulation in the neonate. As regards respiratory depression, six of ten narcotic-treated monkeys had suboptimal (less than 60 breath/min) respiratory rates at birth. Respiratory rate was negatively correlated with cord vein normeperidine and meperidine levels; the strongest correlation was with normeperidine (r = -0.84, P less than 0.01). Neonatal normeperidine elimination in the postnatal period was prolonged, as has also been observed in humans. These studies serve as a basis for comparing the neonatal neurobehavioral effects of the two analgesics and support the use of the rhesus monkey as an animal model to further understanding of the effects of narcotic analgesics on neonatal respiration.

Pharmacokinetic profile of epidural meperidine with and without dextran 70

To evaluate the effect of dextran 70 on the kinetics of epidural meperidine, 10 female patients anesthetized with nitrous oxide and halothane were studied. Meperidine, 1 mg.kg-1, was administered epidurally in either 10 ml dextran 70 in saline solution (group I) or 10 ml saline solution (group II, control subjects). Plasma concentration of meperidine was determined for 10 hours after its administration with GC. Meperidine plasma concentration-time curves could be best resolved into three exponential terms with a lag time in both groups of patients. The disposition kinetics were described adequately by a three-compartment model. This study demonstrated that apart from a significantly longer lag time and smaller k10 (apparent first-order rate constant for elimination of meperidine from the central compartment), the addition of dextran did not alter significantly the kinetic parameters of epidural meperidine.

The correlation of the thermal and mechanical antinociceptive activity of pethidine hydrochloride with plasma concentrations of the drug in sheep

The analgesic activity of pethidine was measured in eight sheep using both thermal and mechanical test systems. Pethidine, at a dose rate of 5 mg/kg body weight given intravenously, produced a significant degree of antinociception to thermal pain for 30 min (on average) but gave only a few minutes of significant analgesia when tested in the mechanical pressure system. Analgesia in both systems was abolished by pretreatment with naloxone. Pharmacokinetic analyses of plasma levels of pethidine after intravenous (i.v.) injection were carried out. Plasma concentrations of the drug exhibited a rapid biexponential pattern of decline with an average distribution half-life of 0.99 min and an elimination half-life of 12.8 min. Correlation of plasma levels of the drug with the presence of a significant degree of antinociception in the thermal test system enabled 'critical' analgesic levels of pethidine to be defined for sheep (0.93 microgram/ml).

Pulmonary disposition of pethidine in postoperative patients

1. Two methods of pethidine administration, namely constant-rate infusion and single i.v. injection, were used to assess the pulmonary disposition of the drug in 10 postoperative patients. Using two sites of blood sampling, the pulmonary extraction ratio was determined. 2. Pronounced pulmonary uptake of pethidine was found in all patients (n = 10). On the other hand, there was no significant evidence of pulmonary clearance. 3. The mean total plasma clearance was 810 ml min-1 and the volume of distribution was 3.11 kg-1. 4. A flow model was used to describe the disposition of pethidine in man. The concentration-time profiles calculated by the model were in accordance with observed data. The data showed that both pulmonary uptake and pulmonary release of pethidine were rapid. 5. Constant-rate infusion was found advantageous in the determination of pulmonary extraction, with respect to the accuracy and precision of the results. The extraction obtained after a single injection may be overestimated on account of uptake of the drug by the lungs.