UPTAKE OF SUFENTANIL, ALFENTANIL AND MORPHINE IN THE LUNGS OF PATIENTS ABOUT TO UNDERGO CORONARY ARTERY SURGERY

British Journal of Anaesthesia 1992; 68: 370-375

UPTAKE OF SUFENTANIL, ALFENTANIL AND MORPHINE IN THE LUNGS OF PATIENTS ABOUT TO UNDERGO CORONARY ARTERY SURGERY F. BOER, J. G. BOVILL, A. G. L. BURM AND R. A. G. MOOREN

We have studied the pulmonary extraction and retention of sufentanil, alfentanil and morphine using a double indicator technique in 30 patients undergoing elective aortocoronary bypass surgery. Patients were allocated to three groups (10 each) to receive sufentanil 43 ng, alfentanil 672 fig or morphine 1887 ng, mixed with indocyanine green as indicator. After sufentanil, mean peak extraction was 93.7% (95% confidence interval87.1-100.4%) and release occurred after 16.1 (13.8-18.4) s; firstpass retention was 61.1 (51.3-70.9)%. After alfentanil, peak extraction was 67.4 (42.7-92.0) % and re/ease occurred after 9.9 (8.5-11.3) s; firstpass retention was 10.1 (3.5-16.7) %. After morphine, peak extraction was 58.3 (44.4-72.2) % and release occurred after 7.2 (4.4-10.1) s; first-pass retention was 7.1 (-4.7-19.9)%. Both peak extraction and first-pass retention were significantly greater after sufentanil. There was no significant difference in the peak extraction and first-pass retention between alfentanil and morphine. KEY WORDS Analgesics: alfentanil, morphine, sufentanil. Lungs: pulmonary uptake.

The lungs have important non-respiratory functions, including the elimination, production and excretion of vasoactive substances and conversion of endogenous compounds, for example angiotensin I to angiotensin II [1-4]. Many drugs are subjected also to significant uptake and retention during passage through the lungs [5-7]. For most drugs, uptake and retention are temporary and release occurs as incoming blood concentrations decrease. First-pass retention is significant only for drugs that are both lipophilic and basic; for neutral and acidic drugs, pulmonary first-pass retention is negligible [8,9]. Opioids are basic drugs that, with the exception of morphine, are moderately to highly lipophilic. The pulmonary uptake of morphine is negligible [10], but pethidine, fentanyl and alfentanil undergo significant first-pass retention in the lungs [10-13]. In order to obtain information about the importance of lipid solubility and pKa, we compared the first-pass pulmonary uptake of sufentanil, alfentanil and

morphine—drugs that vary considerably in lipid solubility and pATa. PATIENTS AND METHODS

We studied 30 patients undergoing elective aortocoronary bypass surgery. The study was approved by the local medical Ethics Committee and all patients gave informed consent. Patients with unstable angina pectoris, poor left ventricular function as assessed by preoperative angiography, with valvular abnormalities, or with pulmonary disease were excluded. Patients who were taking beta adrenoceptor blocking drugs, calcium entry blocking drugs and nitrates had these medications continued until the morning of surgery. Pulmonary uptake of the drugs was studied in three consecutive groups of 10 patients, in the order alfentanil, sufentanil and morphine. Patients were given lorazepam 4-6 mg orally 90 min before surgery. All studies were conducted before induction of anaesthesia. In the operating theatre an i.v. cannula and an 18-gauge radial artery catheter were inserted. A pulmonary artery catheter was introduced via the right internal jugular vein. Indocyanine green 25 mg (Cardio-green, BBL Microbiology Systems, Becton Dickinson) was dissolved in 2.5 ml of solvent and mixed in a plastic syringe with sufentanil 1 ml (50 |ig ml"1), alfentanil 1 ml (500 ug ml"1) or morphine 0.2 ml (10 mg ml"1). The volume was made up to 5 ml with autologous blood, which was added to prevent precipitation of the indocyanine green. One millilitre of this mixture was stored for later measurement of the concentrations of indocyanine green and the opioid. The syringe was weighed before and after the bolus injection. The exact doses of indocyanine green and opioid administered were calculated from the injected volume and the measured concentrations. The remaining opioid—indocyanine green mixture was injected over 1-2 s through the atrial port of the pulmonary artery catheter, followed by a 10-s saline flush. Immediately after the bolus injection, blood sampling commenced. Freely running blood from F. BOER, M.D.; J. G. Bovnx, M.D., PH.D., F.F.A.R.C.S.I.; A. G. L. BURM, PH.D. ; R. A. G. MOOREN, B.SC. ; Department of Anacs-

thesiology, University Hospital Leiden, Postbox 9600, 2300 RC Leiden, The Netherlands. Accepted for Publication: October 27, 1991.

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SUMMARY

PULMONARY UPTAKE OF OPIOID S

371 100 -,

TABLE I. Patient data, opioid dose and haemodynamics of the three groups {mean (SD)). * P < 0.05

SufentanU

10

10

174 (9) 77.9(10.0) 1.91(0.16)

169 (7) 77.8(5.4) 1.88(0.09)

Morphine

11 11

50

10

172 (6) 83.3(7.6) 1.96(0.10)

a?

0

a> o

-50

o 8 7 4

7 8 5

5.34(1.22)

4.59(1.07)*

10 7 4

5.99(0.94)

LJJ

1 rwi

-150 -200

the radial artery catheter was directed via short extension tubing to a drum, rotating at 1 r.p.m., containing 60 sampling tubes. Each tube contained heparin 40 ul (5000 u ml"1). Individual tubes were exposed to the blood flow for 1 s. Blood sampling was continued until 60 s after injection. From each sample, 0.2 ml was separated for the measurement of indocyanine green concentration, which was performed within 1 h after the experiment. The remaining portion of the whole blood samples was stored at —20 °C until opioid assay. Indocyanine green concentrations were measured by absorption spectrometry at 805 nm. Sufentanil concentrations were measured by radioimmunoassay, alfentanil concentrations by gas chromatography and morphine concentrations by fluoroimmunoassay. (Details of these assays are described in the Appendix.) Calculations Extraction and retention were calculated as described by Geddes and colleagues [5] and Jorfeldt and colleagues [6,14]. The indocyanine green fraction vs time curve was taken as the reference curve representing zero uptake by the lungs. The curve was corrected for recirculation by log-linear extrapolation of the terminal part of the descending portion. The concentrations of indocyanine green and opioids in each sample were expressed as a fraction of the administered dose per millilitre of blood (hereafter referred to as fraction) by dividing the concentration (ng ml"1) by the dose (ng). This fraction was used for further calculation of extraction and retention. For each sampling time, extraction was calculated by assuming that any difference between the fraction of opioid and the fraction of indocyanine green resulted from retention of the opioid in the lungs. Extraction was calculated as: opioid^\ FICG1

)xl00/o

where Fopioid' and FICG' are the respective fractions of the administered dose per millilitre of blood at sampling time t.

10 Time (s)

15

20

FIG. 1. Mean (SD) pulmonary extraction (Extr.) ( • ) and retention (Reten.) (O) of sufentanil after rapid i.v. bolus injection of 43.4 (4.9) ug. Time 0 = time of first appearance of indocyanine green at the sample site.

Retention was calculated as: /?'=!-

AUCO_,ICG

1x100%

where A U C ^ is the area under the concentration vs time curve up to time t. AUC was calculated by the trapezoidal rule method. First-pass retention was defined as the retention up to the time when 95 % of the total area under the indocyanine green curve was reached. Because possible recirculation of opioid was assumed to have occurred when recirculation of indocyanine green was observed, data from patients in whom recirculation occurred before 95 % of the AUC of indocyanine green was reached were not included in the calculation of extraction and retention. Cardiac output was calculated from the area under the curve and the dose of indocyanine green administered, using the formula: CO =

£)ICG AUCICG

Results are expressed as mean (SD) or mean (95 % confidence interval of the mean). Data were analysed using analysis of variance, followed by NewmanKeul's range test when indicated, or x2 test. P < 0.05 was taken as statistically significant. RESULTS

Groups were comparable in height, weight, body surface area and preoperative medication (table I). Cardiac output was significantly less in patients in the alfentanil group, compared with those in the other groups. The administered doses of sufentanil, alfentanil and morphine were 43.4 (4.9) ug, 672 (113) ug and 1887 (769) ug, respectively. In all patients, more than 95 % of the AUC of indocyanine green was reached before recirculation was observed. For all

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No. patients Height (cm) Weight (kg) Body surface area (m1) Medications (No. patients) Beta blockers Nitrates Ca1+ entry blockers Cardiac output (litre min"1)

Alfentanil

BRITISH JOURNAL OF ANAESTHESIA

372 100 n

x

50 •

I I I JI5

II;

c CD


-50 \

I[

-100

Q

I 1

C1 cJ

Ijl

(pivot point) are shown in table II. In one patient in the morphine group, calculation of the first-pass retention was not possible. In another patient in this group, the pivot point was 32 s after injection, whereas in the remaining nine patients the time to pivot point ranged from 4 to 11 s; the value given for the time to pivot point for morphine in table II is the mean value calculated from these nine patients. The peak extraction and the first-pass retention were significantly greater and the time to the pivot point was significantly longer after sufentanil than after alfentanil or morphine.

n

-150 •

DISCUSSION

10 Time (s)

20

15

FIG. 2. Mean (SD) pulmonary extraction (Extr.) ( • ) and retention (Reten.) (O) of alfentanil after rapid i.v. bolus injection of 672 (113) ug. Time 0 = time of first appearance of indocyanine green at the sample site. 100 n

50 •

1 J

4-?c ec

1 1 i J 11 j 1 1 r I

I

ill

-50 •

5 -100 -150

-200 10 Time (s)

15

20

FIG. 3. Mean (SD) pulmonary peak extraction (Extr.) ( • ) and firstpass retention (Reten.) (O) of morphine after rapid i.v. bolus injection of 1887 (769) ug. Time 0 = time of first appearance of indocyanine green at the sample site.

three drugs, peak extraction occurred within 3 s after the appearance of indocyanine green. Extraction then gradually decreased and after 10-20 s there was net release of opioid from the lungs. The changes in extraction and retention with time for sufentanil, alfentanil and morphine are shown in figures 1-3. The average peak extraction, first-pass retention and the time at which extraction changed to release

We have shown that there are marked differences in first-pass pulmonary retention of the opioids sufentanil, alfentanil and morphine. Both the initial extraction and the first-pass retention of sufentanil were significantly greater than those of alfentanil and morphine. The last two drugs were indistinguishable in both peak extraction and first-pass retention. The pulmonary uptake of morphine in our study was small and comparable to that reported by Roerig and co-workers [10]. The first-pass retention of alfentanil (0-31 %) in our study was less than that reported by Taeger and co-workers [13]. In their study, first-pass retention of alfentanil varied between 35.9 and 79.8%. Taeger and co-workers studied patients anaesthetized with nitrous oxide and enflurane, receiving intermittent positive pressure ventilation. Our patients were studied during spontaneous ventilation before induction of anaesthesia. It is possible that anaesthesia, positive pressure ventilation, or both, could have contributed to the greater pulmonary retention of alfentanil in the study of Taeger and co-workers. The first-pass retention of sufentanil found is comparable to that of pethidine (64.5 %), less than that of fentanyl (75.2 %) reported by Roerig and coworkers [10], and comparable to the first-pass retention of fentanyl as reported by Taeger and coworkers, who found values of 43.0-86.9% [13]. The difference in pulmonary uptake between various opioids may be explained by differences in their physicochemical properties. All opioids are basic drugs, but they differ in lipophilicity, p/Cm and protein binding—factors known to influence pulmonary uptake [15—17]. Morphine and sufentanil have similar p/Ca values (7.9 and 8.0, respectively), but morphine is highly hydrophilic, whereas sufentanil is highly lipophilic [18]. The difference in lipid solubility may explain the marked difference in

TABLE II. Peak pulmonary extraction and first-pass retention after bolus injection of three opioids (mean (95% confidence interval of the mean)). Pivot point = time after first appearance of drug in the arterial blood samples when extraction became zero. * P < 0.05. t n = 9 (see text)

Sufentanil

Alfentanil

Morphine 58.3(44.4-72.2)

Peak extraction

93.7(100.4-87.1)*

67.4(42.7-92.0)

First-pass retention (%) Pivot point

61.1(51.3-70.9)*

10.1(3.5-16.7)

16.1(13.8-18.4)*

9.9(8.5-11.3)

(s)

7.1(-4.7-19.9)t 7.2(4.4-10.1)t

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-200

PULMONARY UPTAKE OF OPIOIDS

tissue is complete, or until the pulmonary arterial blood concentration decreases to less than the equilibrium concentration in lung tissue. Pulmonary uptake of basic lipophilic amines is extensive and saturable. The mean first-pass lung uptake of propranolol was 75% in patients who had not previously taken the drug and 33% in patients receiving regular oral treatment with propranolol [5]. Interactions have been shown also between propranolol and fentanyl [21]. Studies in a rat isolated, perfused lung preparation co-perfused with propranolol and fentanyl demonstrated inhibition of fentanyl uptake by propranolol. In patients not taking propranolol mean (SE) first-pass pulmonary uptake of fentanyl was 86 (1.4) % whereas in a group that had received propranolol 30-120 mg day"1 for at least 1 month, first-pass pulmonary uptake of fentanyl was 53 (6.3)%, and in one patient it was only 20.3% [21]. The authors calculated that, in patients receiving chronic propranolol therapy, the decrease in first-pass pulmonary uptake of fentanyl would result in two to four times more of an injected dose entering the systemic circulation in the first 30-60 s after injection. Compartmental pharmacokinetic models are based on the assumption that, after i.v. bolus injection, a drug is mixed instantaneously and completely within its central compartment. In practice this is never the case. It has been suggested diat a pharmacokinetic model that took account of pulmonary uptake would better describe the complexities of early drug distribution and of the dynamics of pharmacological effect [22]. Therefore, early mixing in this central compartment should be distinguished from early drug distribution to the peripheral compartments [23,24]. In conclusion, we have shown that sufentanil undergoes significant first-pass retention in the lungs. The pulmonary retention of alfentanil and morphine was insignificant. Sufentanil is a basic amine, is highly lipophilic and has a pK% that is greater than 7.5. These physicochemical properties are important for pulmonary retention. APPENDIX Indocyanine green measurement Whole blood 0.2 ml was diluted with distilled water 2 ml and vortexed vigorously to lyse the red cells. The solution was centrifuged for 15 min at 4000 r.p.m. Absorption at 805 nm was measured in the supernatant with an absorption spectrophotometer (Milton Roy type Spectrotronic 301). The mean absorption of the first five to seven samples, in which no indocyanine green was present, was taken as the background absorption and was subtracted from the absorption measured in the remaining samples. The concentration in each sample was calculated from a reference curve, constructed by adding a known amount of indocyanine green to whole blood samples from eight patients. The coefficient of variation was 2.6-3.9 % for the relevant range of concentrations.

Sufentanil assay Whole blood sufentanil concentrations were determined by radioimmunoassay. To 0.3 ml of whole blood, 1 ml of sodium hydroxide 0.1 mol litre"1 and n-heptane-isoamyl alcohol 4 ml

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pulmonary uptake between the two drugs. We found that the pulmonary uptake of alfentanil and morphine are similar, although they differ in lipid solubility. Alfentanil is moderately lipid soluble [18], whereas morphine has poor lipid solubility. Pethidine is also moderately lipid soluble [19], but its first pass retention is large and comparable to that of fentanyl [10]. Alfentanil differs from all other opioids in that it has a pKt (6.5) less than 7.5. It is likely that this is the explanation for the lesser pulmonary uptake of alfentanil. Pulmonary uptake is significant only for basic substances with a pKA greater than 7.5 [16]. Although protein binding influences the pulmonary uptake of non-basic drugs, it is not a major factor in determining the pulmonary uptake of basic drugs, such as the opioids. In an isolated, perfused rat lung model, the pulmonary uptake of methadone increased only from 40 % to 45 % when 4.5 % bovine serum albumin was omitted from the perfusate [17]. The protein binding of morphine (63 %) is less than that of sufentanil (93%), and the lung uptake of morphine is insignificant compared with the latter drug. In contrast, the protein binding of alfentanil and sufentanil are similar [20], but the drugs differ markedly in pulmonary uptake. All our patients were receiving drugs before surgery, and 25 of 30 patients were taking beta adrenoceptor blocking drugs. Propranolol decreases the pulmonary accumulation of fentanyl both in humans and in an isolated, perfused rat lung preparation [21]. None of our patients was taking propranolol, but it is possible that other beta adrenoceptor blocking drugs similarly affect pulmonary first-pass retention, as these drugs also are basic amines. However, the pulmonary uptake of opioids in the five patients who were not taking beta adrenoceptor blocking drugs was not different from that in other patients in the same group. There is no available information about the influence of other drugs used by our patients, such as calcium entry blockers, on pulmonary uptake of opioids. The double indicator dilution technique, although easy to use, has the disadvantage that only information on pulmonary uptake over a very short time interval after injection is possible. An alternative approach is to use a mass balance technique. Using this method, Taeger and co-workers [13] showed that the release of fentanyl was described best by a bi-exponential equation with half-lives of 0.22 min and 5.78 min. An important disadvantage of die mass balance technique is that it relies on the assumption that pulmonary blood flow remains constant during the measurement period. This is often difficult to achieve, as many drugs alter the haemodynamic state. Pulmonary uptake of drugs has clinical implications. For drugs with extensive uptake, the lungs act as a temporary capacitor, reducing peak arterial concentration and thus the risk of side effects whenever a change in pulmonary arterial blood concentration occurs, for example after central venous or peripheral venous bolus injection, or a change in an infusion rate [1]. Pulmonary uptake continues until partitioning between blood and lung

373

374

Alfentanil assay Whole blood alfentanil concentrations were determined by capillary gas chromatography. To 0.2 ml of whole blood, the internal standard, R 38527 (Janssen Pharmaceutica, Beerse, Belgium), was added and, after vortexing, the sample was extracted for 30 s with 5 ml of redistilled reagent grade n-pentane (Merck, Darmstadt, GFR) on a whirl mixer. After centrifugation, the organic phase was transferred to a conical centrifuge tube and evaporated to dryness in a stream of dry nitrogen on a water bath at 40 °C. The residue was dissolved in analytical grade ethanol 50-100 ul (Merck, Darmstadt, GRF) and 1 ul was introduced into the gas chromatograph via a falling-needle solid injection system (Chrompack, Middelburg, The Netherlands). A Hewlett-Packard 5890A gas chromatograph was equipped with a nitrogen detector and a capillary fused silica column (length 10 m, internal diameter 0.32 mm) with CP-Sil-5-CB (Chrompack, Middelburg, The Netherlands) as the stationary phase. The operating temperatures of the injection port, column oven and detector were 300 °C, 230 °C and 300 °C, respectively. Helium was used as the carrier gas (flow rate 4 ml min"1) and an auxiliary flow of helium 26 ml min"1 was fed into the detector. Retention times for alfentanil and the internal standard were 1.8 min and 2.6 min, respectively. The detection limit was 1 ng ml"1. The coefficient of variation did not exceed 5 % in the concentration range encountered in this study.

Morphine assay Whole blood morphine concentrations were measured by polarization fluoroimmunoassay—a technique which has become available for the measurement of opioids in whole blood [25,26]. In these studies, correlation of measured morphine concentrations with spiked whole blood calibration curves was 0.999 and the coefficient of variation in these studies was in the range 4.7-12%. We used this fluoroimmunoassay under the assumption that no significant metabolism of morphine occurred during the study period, before recirculation of morphine had occurred. During this period, the only likely source for metabolism of morphine is the lungs. Metabolism of morphine does not occur in isolated, perfused rabbit lung [27] or in the lungs of patients [28]. Measurements were performed on an Abbott TDx Analyzer. Whole blood 0.3 ml was mixed with absolute alcohol 0.05 ml and saline 0.3 ml and vortexed. The solution was centrifuged (TDx centrifuge, 13000 r.p.m. for 10 min) and 75 ul of the supernatant added to the TDx cups. The TDx opioids reagent pack was used. The TDx Analyzer was calibrated with standard whole blood samples at concentrations of 0, 125, 500, 1000 and 2000 ng ml"1. Spiked controls of 374 and 2888 ng ml"1 were analysed with each batch. The deviations of these known values were 0.5 and 4% of the true value and the between-run coefficients of variation were 7.1 and 6.8%, respectively.

ACKNOWLEDGEMENT The authors thank Janssen Pharmaceutica, Beerse, Belgium for donation of the 3H-sufentanil and antiserum. REFERENCES 1. Bahkle YS. Pharmacokinetic and metabolic properties of the lung. British Journal of Anaesthesia 1990; 65: 79-93. 2. Ryan US, Grantham CJ. Metabolism of endogenous and xenobiotic substances by pulmonary vascular endothelial cells. Pharmacology and Therapeutics 1989; 42: 235-250. 3. Nunn JF. Non-respiratory functions of the lung. In: Applied Respiratory Physiology. London: Butterworth, 1987; 284— 293. 4. Orton TC, Anderson MW, Pickett RD, Eling TE, Fouts JR. Xenobiotic accumulation and metabolism by isolated perfused rabbit lungs. Journal of Pharmacology and Experimental Therapy 1973; 186: 482-197. 5. Geddes DM, Nesbitt K, Traill T, Blackburn JP. First pass uptake of 14C propranolol by the lung. Thorax 1979; 34: 810-813. 6. Jorfeldt L, Lewis DH, Loefstroem JB, Post C. Lung uptake of lidocaine in healthy volunteers. Acta Anaesthesiologica Scandinavica 1979; 23: 567-574. 7. Post C. Studies on the pharmacokinetic function of the lung with special reference to lidocaine. Acta Pharmacologica et Toxicologica 1979; 44 (Suppl. I): 1-53. 8. Ludden TM, Schanker LS, Lanman RC. Binding of organic compounds to rat liver and lung. Drug Metabolism and Disposition 1976; 4: 8-16. 9. Roerig DL, Kotrly KJ, Dawson CA, Ahlf SB, Gualtieri JF, Kampine JP. First-pass uptake of verapamil, diazepam and thiopental in the human lung. Anesthesia and Analgesia 1989; 69: 461^66. 10. Roerig DL, Kotrly KJ, Vucins E, Ahlf SB, Dawson CA, Kampine JP. First pass uptake of fentanyl, meperidine and morphine in the human lung. Anesthesiology 1987; 67: 466-472. 11. Persson MP, Wiklund L, Hartvig P, Paalzow L. Potential pulmonary uptake and clearance of morphine in postoperative patients. European Journal of Clinical Pharmacology 1986; 30: 567-574. 12. Persson MP, Hartvig P, Paalzow L. Pulmonary disposition of pethidine in postoperative patients. British Journal of Clinical Pharmacology 1988; 25: 235-241. 13. Taeger K, Weninger E, Schmelzer F, Adt M, Franke N, Peter K. Pulmonary kinetics of fentanyl and alfentanil in surgical patients. British Journal of Anaesthesia 1988; 61: 425-434. 14. Jorfeldt L, Lewis DH, Loefstroem JB, Post C. Lung uptake of lidocaine in man as influenced by anaesthesia, mepivacaine infusion or lung insufficiency. Acta Anaesthesiologica Scandinavica 1983; 27: 5-9. 15. Henderson RF, Bechtold WE, Medinsky MA, P-Fischer J, Lee TT. The effect of molecular weight/lipophilicity on clearance of organic compounds from lungs. Toxicology and Applied Pharmacology 1988; 95: 515-521. 16. Bend JR, Serabjit-Singh CT, Philpot RM. The pulmonary uptake, accumulation and metabolism of xenobiotics. Annual Review of Pharmacology and Toxicology 1985; 25: 97-125. 17. Roerig DL, Dahl RR, Dawson CA, Wang RIH. Effect of plasma protein binding on the uptake of methadone and diazepam in the isolated perfused rat lung. Drug Metabolism and Disposition 1984; 12: 536-542. 18. Mather LE. Clinical pharmacokinetics of fentanyl and its newer derivatives. Clinical Pharmacokinetics 1983; 8: 422446. 19. Hug CC. Pharmacokinetics and dynamics of narcotic analgesia. In: Prys-Robert C, Hug CC, eds. The Pharmacokinetics of Anaesthesia. Oxford: Blackwell Scientific Publications, 1984; 187-234. 20. Meuldermans WEG, Hurkmans RMA, Heykants JJP. Plasma protein binding and distribution of fentanyl, sufentanil, alfentanil and lofentanil in blood. Archives Internationales de PharmacodynamU et de Therapie 1982; 257: 4-19. 21. Roerig DL, Kotrly KJ, Ahlf SB, Dawson CA, Kampine JP. Effect of propranolol on the first-pass uptake of fentanyl in the human and rat lung. Anesthesiology 1989; 71: 62-68.

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were added and the solution was mixed for 10 min on a rotary mixer (Protherm type Cenco FT 1510). The solution was centrifuged for 10 min at 4000 r.p.m. (Heraeus Minifuge GL) and the supernatant evaporated to dryness in a stream of dry nitrogen on a water bath at 60 °C. Absolute alcohol 50 ul was added and the solution again vortexed. Bovine serum albumin (Sigma no. A7888 RIA grade) 450 ul in phosphate buffer (pH 7.5) was added and the tube was vortexed again. Fifty microlitre of the solution was added to 2 % bovine serum albumin 450 ul, the mixture vortexed and *H-sufentanil (Janssen, Beerse, Belgium) added. After further vortexing, antiserum 100 ul (Janssen, Beerse, Belgium) was added. The solution was left at room temperature for 2 h, 500 ul of a Norit-Dextran suspension was added to each test tube and the solution was incubated for 1 h. The solution was centrifuged for 10 min at 4000 r.p.m., 20 °C, decanted into a new test tube and 4 ml of liquid scintillation cocktail (Ultima Gold, Packard) added to the supernatant. Standard solutions, initial binding and antiserum-free samples (to determine non-specific binding) were treated similarly. Scintillation was measured (as d.p.m.) in a liquid scintillation analyser (Packard, Tricarb 2000 CA). The detection limit of this assay was 2 ng ml"1 and the average coefficient of variation was 7%. The initial binding was 35 % and the coefficient of non-specific binding was 1.4 % of total binding.

BRITISH JOURNAL OF ANAESTHESIA

PULMONARY UPTAKE OF OPIOIDS 22. Hull CJ. How far can we go with compartmental models? Anesthestology 1990; 72: 399-^102. 23. Henthorn TK, Avram MJ, Krejcic TC. Intravascular mixing and drug distribution: The concurrent disposition of thiopental and indocyanine green. Clinical Pharmacology and Therapeutics 1989; 45: 56-65. 24. Avram MJ, Krejcie TC, Henthom TK. The relationship of age to the pharmacokinetics of early drug distribution: the concurrent disposition of thiopental and indocyanine green. Anesthesiology 1990; 72: 403-^tll. 25. McCord CE, McCutcheon JR. Preliminary evaluation of the

375 Abbott TDx for benzoylecgonine and opiate screening in whole blood. Journal of Analytical Toxicology 1988; 12: 295-297. 26. Lee C-W, Lee H-M. Evaluation of the Abbott TDx Analyzer. Journal of Analytical Toxicology 1989; 13: 50-54. 27. Davis ME, Mehendale HM. Absence of metabolism of morphine during accumulation by isolated perfused rabbit lung. Drug Metabolism and Disposition 1979; 7: 425-428. 28. Ratcliffe FM. Absence of morphine glucuronidation in the human lung. European Journal of Clinical Pharmacology 1989; 37: 537-538.

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