- Email: [email protected]
Reduction of isoflurane MAC by fentanyl or remifentanil in rats
Veterinary Anaesthesia and Analgesia, 2003, 30, 250^256
Reduction of isoflurane MAC by fentanyl or remifentanil in rats Ana B Criado
MRCVS
& Ignacio A Go¤mez de Seguray
LAS Department, GlaxoSmithKline, UK
yDepartment of Experimental Surgery, University Hospital La Paz, Spain
Correspondence: Ana Criado Gonza¤lez, GlaxoSmithKline, Medicines Research Centre, Gunnels Wood Road, Stevenage, HERTS SG1 2NY UK. E-mail: [email protected]
Abstract Objective The main objective of the study was to determine the e¡ects of three di¡erent infusion rates of fentanyl and remifentanil on the minimum alveolar concentration (MAC) of iso£urane in the rat. A secondary objective was to assess the cardiovascular and respiratory e¡ects of the two opioid drugs. Animal population Thirty-seven male Wistar rats were randomly allocated to one of six treatment groups. Material and methods For all treatment groups anaesthesia was induced with 5% iso£urane in oxygen using an induction chamber. A 14-gauge catheter was used for endotracheal intubation, and anaesthesia was maintained with iso£urane delivered in oxygen via a T-piece breathing system. A baseline determination of the minimum alveolar concentration of iso£urane (MACISO) was made for each animal. Fentanyl (15, 30, 60 mg kg1 hour1) or remifentanil (60, 120, 240 mg kg1 hour1) were infused intravenously into a previously cannulated tail vein. Thirty minutes after the infusion started, a second MACISO (MACISOþdrug) was determined. The carotid artery was cannulated to monitor the arterial pressure and to take samples for arterial gas measurements. Cardiovascular (heart rate and arterial pressure) and respiratory (respiratory rate and presence/absence of apnoea) e¡ects after opioid infusion were also recorded. Results Fentanyl (15, 30, 60 mg kg1 hour1) and remifentanil (60, 120, 240 mg kg1 hour1) similarly 250
reduced iso£urane MAC in a dose-dependent fashion: by 10% at lower doses, 25% at medium doses and by 60% at higher doses of both the drugs. Both opioids reduced the respiratory rate in a similar way for all doses tested. No episodes of apnoea were recorded in the remifentanil groups, while administration of fentanyl resulted in apnoea in three animals (one at each dose level). The e¡ects on the cardiovascular system were similar with both drugs. Conclusions We conclude that the intraoperative use of remifentanil in the rat reduces the MAC of iso£urane, and that this anaesthetic sparing e¡ect is dose-dependent and similar to that produced by fentanyl at the doses tested. Clinical relevance The use of remifentanil during inhalant anaesthesia in the rat can be considered an intravenous alternative to fentanyl, providing similar reduction in iso£urane requirements. Due to its rapid o¡set, it is recommended that alternative pain relief be instituted before it is discontinued. Keywords fentanyl, iso£urane, MAC reduction, rats, remifentanil.
Introduction The minimum alveolar concentration (MAC) required to prevent movement in 50% of patients was originally de¢ned by Eger and coworkers in 1965 (Eger et al.1965), and has become the standard measure of volatile anaesthetic potency. Opioids markedly reduce the concentrations of volatile anaesthetic agents required to maintain anaesthesia. Evaluation
Iso£urane MAC in rats AB Criado & IA Go¤mez de Segura
of this reduction in the MAC of volatile anaesthetic agents byopioids may be used as a comparison of relative opioid potency. Remifentanil is a novel m-(OP3) receptor agonist with an analgesic e¡ect similar to that of fentanyl. It is metabolized by circulating and tissue esterases, giving a pharmacokinetic pro¢le of rapid onset, short duration and rapid o¡set of action. This should, in clinical practice, prove to be a drug whose e¡ects are easy to titrate. The use of this metabolic pathway results in less interpatient pharmacokinetic variability as a result of factors such as age, obesity, hepatic or renal failure. These clinical advantages may be of interest in laboratory animals, particularly when potent analgesia is required as well as precise control of drugs administered, i.e. rapid elimination and reduced interference with other drugs or measurements. Therefore, the aim of the present study was to compare the analgesic potency of remifentanil and fentanyl in the rat, in terms of reduction of iso£urane MAC (MACISO).
Materials and methods After obtaining institutional Animal Care Committee approval, the reduction of MACISO in response to fentanyl or remifentanil given by continuous infusion was evaluated in rats. Iso£urane was obtained from Abbott (Forane; Abbott Laboratories, Madrid, Spain), fentanyl from Roche (Fentanest; Laboratorio Roche S.A., Madrid, Spain) and remifentanil from Glaxo-Wellcome (Ultiva; Glaxo-Wellcome Laboratories, Madrid, Spain). Thirty-seven male Wistar rats (CRIFFA, Barcelona, Spain), with an average weight of 357 32 g, were housed in groups of 8^10, with 12 hours light12 hours dark cycle, relative humidity of 50^70%, 20 2 8C ambient temperature. Food (B&K Universal) and water were provided ad libitum. Animals were allowed to acclimatize for at least 1 week. All the studies were performed during the morning (starting at 9:00 am). Rats were placed in an induction chamber into which 5% iso£urane in a continuous oxygen £ow of 3 L minute1 was directed (Iso£urane Vaporizer Ohmeda Isotec 3; BOC Heath Care, Steeton, UK). Two to three minutes later, the iso£urane vaporizer setting was reduced to 2.5^3%. Endotracheal intubation was performed using a 14-gauge polyethylene catheter with the animal positioned in dorsal recumbency. A cold light was
applied externally over the trachea to aid visualization of the larynx within the oral cavity. Then a £exible, blunt-tip, wire guide was inserted into the trachea with an otoscope and used to direct the endotracheal catheter. After the correct position of the catheter was ascertained, it was connected to a small T-piece breathing system with minimal dead space. Fresh gas £ow to the T-piece was adjusted to 1 L minute1 of oxygen, and iso£urane concentration was adjusted as necessary. Opioids were administered with an infusion pump (Harvard model 55^2226; Harvard Apparatus, Millis, MA, USA) by means of a 24-gauge polyethylene catheter inserted in a tail vein. Monitoring The carotid artery was catheterized with PE50 tubing after surgical cut-down. This access allowed arterial blood sampling and blood pressure measurement via a calibrated pressure transducer (Transpac IV; Abbot Laboratories, Abbot Park, IL, USA). Systolic, diastolic and mean arterial blood pressure, and lead II electrocardiograph (needle electrodes placed on the left and right forelimb and right hindlimb) were recorded continuously (CM-8; Schiller, Switzerland). Arterial blood (0.3 mL) was collected for blood gas analysis (Blood gas analyser, Statnova pro¢le-1; Nova Biomedical, Waltham, MA, USA) at the end of study to ensure values (at that time point) were within normal limits of pH (7.35^7.45), partial pressure of oxygen (PaO2) [>12 kPa (90 mm Hg)], and pressure of carbon dioxide (PaCO2) [4.65^6.3 kPa (35^ 47 mm Hg)]. Rectal temperature was also monitored and maintained between 37 and 38 8C by means of a circulating water warming blanket (Heat Therapy Pump, Model TP-220; Gaymar, Orchand Park, NY, USA) and a heating light. Determination of the minimum alveolar concentration Intratracheal gas sampling was used to measure anaesthetic gas concentration for determination of the MAC value. This method has been described in detail previously (Pajewski et al.1996). In brief, a ¢ne catheter with 0.9-mm external diameter was inserted through the endotracheal catheter, with the tip located at the level of the carina. The proximal end of the catheter was connected to a 10-mL gastight glass syringe (Model 100/383/118; Portex, Hythe, UK). Samples were obtained by withdrawing
# Association of Veterinary Anaesthetists, 2003, 30, 250^256
251
Iso£urane MAC in rats AB Criado & IA Go¤mez de Segura
10 mL gas over 5 minutes using a Harvard infusion pump (Model 55^2226; Harvard Apparatus, Millis, MA, USA). Samples were obtained consecutively in triplicate to ensure constant alveolar concentration, and the ¢nal value was the mean, at every iso£urane concentration step. The catheter was withdrawn between samples. After every step change in anaesthetic concentration delivered by the anaesthetic breathing system, at least 15 minutes were allowed for equilibration before gas samples were obtained. Gas samples were assayed using a side-stream infrared analyser (5330 Agent Monitor; Datex-Ohmeda, Hertfordshire, UK). The MACISO and MACISOþdrug values were established according to the method described by Eger and coworkers. (Eger et al. 1965; Quasha et al. 1980). A painful noxious stimulus was applied with a long haemostat (8-inch Rochester Dean Hemostatic forceps) clamped to the ¢rst ratchet lock on the tail for 60 seconds while the third gas sample was obtained from the lung. The tail was always stimulated proximal to a previous test site. A positive response was considered to be a gross purposeful movement of the head, extremities or body. A negative response was considered to be the lack of movement or grimacing, swallowing, chewing or tail £ick. Where a negative response was seen, the iso£urane concentration was reduced in decrements of 0.10^0.15% until the negative response became positive. The MAC was considered to be the concentration midway between the highest concentration that permitted movement in response to the stimulus and the lowest concentration that prevented such movement. Experimental design Once the animals were anaesthetized and instrumented, a basal MACISO was determined in each animal, each animal acting as its own control. Opioids were given intravenously in the tail vein. Both opioids were given by continuous infusion, and no loading dose was used. Thirty minutes after drug administration, the MACISOþdrug was obtained by decreasing the iso£urane concentration as described previously. Drug groups Thirty-six animals were used. They were randomly assigned to one of six groups (n ¼ 6), and baseline MACISO was determined in all the animals prior to starting the opioid infusion. All animals within a group received the same dose of drug. Doses of15, 30, 252
60 mg kg1 hour1 of fentanyl (F-15, F-30, F-60) and 60, 120, 240 mg kg1 hour1 of remifentanil (R-60, R-120, R-240) were used. Two animals in the 60 mg kg1 hour1 fentanyl group could not be included in the analysis; an extra rat was used giving a ¢nal n-value for that group of ¢ve (see Results section). Animals were euthanased with pentobarbital given intravenously (Eutalender; Laboratorios Norton, Madrid, Spain) while still deeply anaesthetized. Unwanted e¡ects The e¡ects of drug administration on heart and respiratory rates were continuously monitored. Heart and respiratory rates were recorded immediately before each MAC step, and after 30 minutes of drug administration. A continuous printed record of arterial blood pressure was obtained throughout the experiment and used to obtain the peak value in mean arterial blood pressure reduction. Statistical analysis N-query advisor program (N-query advisor 2.0: Statistical solutions; Saugus, MA, USA) was used to determine an adequate n-value from previously published data (Go¤mez de Segura et al.1998). The n-value was calculated assuming a one-way analysis of variance, and found to be a minimum of three animals per group.When the sample size in each of the three groups is six, a one-way analysis of variance will have >90% power to detect at the 0.050 level a di¡erence in means, based on previous data characterized by a variance of means of 0.050, and assuming that the common standard deviation is 0.140, also data from the same work (Go¤mez de Segura et al.1998). Avalue of six animals per group was considered adequate, and also allowed us further comparisons. To examine the e¡ect on MACISOþdrug of three different infusion rates of remifentanil and fentanyl, a two-way analysis of variance (ANOVA) was conducted on the percentage of reduction of MAC (di¡erence between MACISO and MACISOþdrug expressed as a percentage). First, an overall comparison to see if fentanyl and remifentanil were di¡erent was carried out, and then an overall comparison to examine differences between low, medium and high levels. Finally, planned comparisons between low, medium and high levels of both drugs were made using a Fishers LSD (least signi¢cant di¡erence) test. A 95% con¢dence interval was established. A p-value of <0.05 was set to indicate statistical
# Association of Veterinary Anaesthetists, 2003, 30, 250^256
Iso£urane MAC in rats AB Criado & IA Go¤mez de Segura
signi¢cance. Percentage of reduction of mean arterial pressure, respiratory rate and heart rate were analysed in a similar way. All analyses were performed using the SAS statistical package (version 6.12 for Windows; SAS Institute Inc., SAS Campus Drive, Cary NC, USA). Results are presented as mean SD.
MACISOþdrug by10%, the medium doses by 25%, and the highest doses by 60%. Individual group MACISO values and those following opioid administration (MACISOþdrug) are shown in Table 1. Individual MACISOþdrug values are shown in Fig. 1.
Results
Cardiovascular e¡ects
Side-e¡ects
A decrease in mean arterial blood pressure was observed during the intravenous infusion of both fentanyl and remifentanil. No signi¢cant di¡erences were found in the peak reduction in mean arterial blood pressure between drugs, at all doses used. The overall percentage of mean arterial pressure reduction was 22.9% (16.6). Heart rate (measured 30 minutes after onset of infusion) decreased during both
MACISO, MACISOþdrug and MACISO reduction (%) The mean baseline MACISO value determined in all the rats was 1.29% (0.08). Fentanyl and remifentanil infusion reduced MACISOþdrug in a dose-dependent fashion. For the three infusion rates studied, fentanyl and remifentanil were found to be equipotent, i.e. the lowest doses of either drug similarly reduced
Table 1 Baseline (MACISO) minimum alveolar concentration of iso£urane, postdrug, fentanyl or remifentanil,(MACISOþdrug) minimum alveolar concentration of iso£urane and postdrug, fentanyl or remifentanil, percentage of minimum alveolar concentration of iso£urane reduction
Fentanyl
lg kg n
1
hour
1
MACISO MACISOþdrug %MACISO (red.)
Remifentanil
15 6
30 6
60 5
1.27 (0.10) 1.15 (0.11) 10.2 (1.9)
1.31 (0.06) 0.97 (0.07) 26.1 (4.6)
1.35 (0.08) 0.51 (0.12) 61.7 (10.2)
60 6
1.29 (0.08) 1.16 (0.09) 10.6 (4.2)
120 6
1.28 (0.10) 0.97 (0.09) 24.1 (8.7)
240 6
1.27 (0.09) 0.45 (0.06) 64.6 (5.6)
Data are expressed as mean (SD).
Figure 1 Individual MACISO values of rats given either fentanyl (15, 30 and 60 mg kg1 hour1) or remifentanil (60,120 and 240 mg kg1 hour1). # Association of Veterinary Anaesthetists, 2003, 30, 250^256
253
Iso£urane MAC in rats AB Criado & IA Go¤mez de Segura
Figure 2 Mean percentage of heart rate (HR) reduction 30 minutes after administration of either fentanyl (15, 30 and 60 mg kg1 hour1) or remifentanil (60, 120 and 240 mg kg1 hour1). Low, medium and high dose are signi¢cantly di¡erent in both drugs, and the di¡erence is similar in both treatments. Data are expressed as mean, boxes representing values within mean standard error, and whiskers values within mean SD.
fentanyl and remifentanil infusions: e¡ects of low, medium and high doses are signi¢cantly di¡erent in both drugs, and the di¡erence was similar in both treatments (Fig. 2). Respiratory e¡ects A reduction in respiratory rate was found in all groups 30 minutes after opioid infusion. No signi¢cant di¡erences could be found between drugs or infusion rates. The overall reduction in respiratory rate was 28.1% (23). No periods of apnoea were recorded in the remifentanil groups, even when the highest dose was used. Fentanyl resulted in apnoea at all doses used: one rat receiving15 mg kg1 hour1, one rat 30 mg kg1 hour1, and one 60 mg kg1 hour1. Two animals with the highest fentanyl dose could not be included in the study due the high levels [11.25 and 9.34 kPa (84.6 and 70.3 mm Hg)] of PaCO2 at the end of the experiment. In all the remifentanil animals, PaCO2 was below 6.0 kPa (45 mm Hg) at the end of the study.
Discussion Di¡erent methods can be used to compare the relative potencies of opioids, but the primary measure of opioid e⁄cacy is analgesia. The ability of a drug to reduce the MAC of an inhalant anaesthetic provides an objective measurement of analgesic e⁄cacy during anaesthesia. In this study, remifentanil and fentanyl showed a dose-related reduction of iso£urane MAC, and no differences could be found when low, medium and high doses of both drugs were compared. Nevertheless, plasma concentrations of both opioids were not measured at the time of MAC determination and we cannot con¢rm that the maximum concentration 254
in plasma of fentanyl had been reached at that time. The sparing e¡ect of opioids on the requirements of inhalant anaesthetics is well known. Up to 82% reduction in iso£urane MAC was reported with fentanyl in human beings (McEwan et al. 1993), while up to 91% was found with remifentanil (Lang et al. 1996). In Rhesus monkeys, a decrease in iso£urane MAC between 20 and 60% has been reported, with no ceiling e¡ect at the doses tested (Ste¡ey et al. 1994). Similar studies showed a ceiling e¡ect close to 70% en£urane MAC reduction with both, fentanyl (Murphy & Hug 1982) and remifentanil (Michelsen et al. 1996) in dogs. To our knowledge, no previous studies on iso£urane MAC reduction with either fentanyl or remifentanil have been conducted in rats. Similar studies have been carried out in the rat using di¡erent opioids (Hecker et al. 1983; Lake et al. 1985; Criado et al. 2000). The maximum degree of reduction of inhalant anaesthetics varied between approximately 90% in the case of sufentanil to only 50% when alfentanyl was administered. All the data from these and similar studies suggest that opioid potency is variable and species dependent. Remifentanil is a relatively new opioid with some speci¢c pharmacokinetic characteristics that make it a potentially useful option for the management of many situations, i.e. short and painful procedures, intraoperative analgesia, and in general all clinical situations where a strong analgesic is needed together with a prompt recovery. Esterase hydrolysis is a metabolic pathway that can be exploited to increase the rate of metabolism and elimination and so reduce the duration of the pharmacodynamic e¡ects of drugs. Previously applied to b-adrenergic blocking agents (esmolol) and muscle relaxants (mivacurium), the same concept was used to develop the ¢rst esterase-metabo-
# Association of Veterinary Anaesthetists, 2003, 30, 250^256
Iso£urane MAC in rats AB Criado & IA Go¤mez de Segura
lized opioid, remifentanil. The metabolism by nonspeci¢c esterases in the blood and tissues also prevents accumulation, even when given at high dosages over prolonged periods. Clinical recovery from anaesthesia is very rapid, irrespective of the age or physical status of the patient or the type or duration of surgery (Hughes et al.1992). The haemodynamic responses to remifentanil are similar to those found with another m-agonist (Joshi et al. 1993). In human volunteers, doses up to 2 mg kg1 were found to produce minimal haemodynamic alterations (Pitts et al.1992), whereas doses of up to 10 mg kg1 resulted in arterial blood pressure decreases of 10^20%, and mild bradycardia (Glass 1995). The larger decreases in arterial blood pressure are mainly due to bradycardia, which can be prevented by pre-treating patients with glycopyrrolate or atropine (Pitts et al. 1992). The incidence of muscle rigidity was also found to be similar to that with alfentanil (Glass et al. 1993). The lack of such undesired e¡ects encountered in the present study may have been related to the withholding of loading doses (Bu«rkle et al. 1996). With a zero order infusion, the time to reach a steady-state concentration is very rapid (approximately 10 minutes) (Glass 1995). Respiratory depression is a common side-e¡ect of OP3-agonist opioids (Amin et al. 1995; Munday et al. 1995) like remifentanil, but di¡erences may be observed when two drugs are compared (Lauwers et al. 1997). No periods of apnoea were recorded in the remifentanil group, even at the higher dose and in all the animals PaCO2 values at the end of the experience were below 5.33 kPa (40 mm Hg). All rats in the remifentanil group regained the basal MAC value. Apnoea was recorded in fentanyl groups at all doses tested and two animals did not regain the basal MAC value due to high PaCO2 values and could not be included in the study. A further advantage of remifentanil is derived from its short halflife so the undesirable respiratory and cardiovascular e¡ects are not expected to last for more than 10^15 minutes after discontinuation of infusion. However, the present study was not designed to accurately study the cardiovascular and respiratory e¡ects and further studies are needed to better address this point. There is no report on the use of remifentanil as a sole anaesthetic agent in rats. The maximum MAC reduction found with remifentanil in the dog is 70%, and this reduction was unchanged by two or three times higher doses (Michelsen et al. 1996). Although
there are major di¡erences in opioid potency amongst di¡erent species, it is unlikely that this drug alone could be used as a complete anaesthetic because the limiting factor will be the maximum intrinsic activity inherent in all OP3-type opioids. In our study, we could not ¢nd a ceiling e¡ect with fentanyl or with remifentanil at the doses tested. In conclusion, remifentanil produces a dosedependent reduction in the MAC of iso£urane in the rat similar to that induced by fentanyl when equipotent doses are considered.
Acknowledgements The authors wish to thank Belinda Farn¢eld for the review of the manuscript.
References Amin HM, Sopchak AM, Esposito BF (1995) Naloxoneinduced and spontaneous reversal of depressed ventilatory responses to hypoxia during and after continuous infusion of remifentanil or alfentanil. J Pharmacol Exp Ther 274,34^33. Bu«rkle H, Dunbar S, Van Aken H (1996) Remifentanil: a novel, short-acting, m-opioid. Anesth Analg 83,646^651. Criado AB, Go¤mez de Segura IA, Tendillo FJ et al. (2000) Reduction of iso£urane MAC with buprenophine and morphine in rats. Lab Animals 34, 252^259. Eger EIII, Saidman LJ, Brandstater B (1965) Minimum alveolar anesthetic concentration: a standard of anesthetic potency. Anesthesiology 26,756^763. Glass PSA (1995) Pharmacology of remifentanil. Eur J Anaesthesiol12 (Suppl.10),73^74 (Abstract). Glass PSA, Hardman D, KamiyamaYet al. (1993) Preliminary pharmacokinetics and pharmacodynamics of an ultrashort-acting opioid: remifentanil (GI87084B). Anesth Analg 77,1031^1040. Go¤mez de Segura IA, Criado AB, Santos M et al. (1998) Aspirin synergistically potentiates iso£urane minimum alveolar concentration reduction produced by morphine in the rat. Anesthesiology 89,1489^1494. Hecker BR, Lake CL, DiFazio CA et al. (1983) The decrease of the minimum alveolar anaesthetic concentration produced by sufentanil in rats. Anesth Analg 62,987^990. Hughes MA, Glass PSA, Jacobs JR (1992) Context-sensitive half-time in multicompartment pharmacokinetic models for intravenous anesthetic drugs. Anesthesiology 76, 334^341. Joshi P, Jhaveri R, BaumanVet al. (1993) Comparative trial of remifentanil and alfentanil for anesthesia induction. Anesthesiology 79 (Suppl.3A),379 (Abstract). Lake CL, DiFazio CA, Moscicki JC et al. (1985) Reduction in halothane MAC: comparison of morphine and alfentanil. Anesth Analg 64,807^810.
# Association of Veterinary Anaesthetists, 2003, 30, 250^256
255
Iso£urane MAC in rats AB Criado & IA Go¤mez de Segura
Lang E, Kapila A, Shlugman D et al. (1996) Reduction of iso£urane minimal alveolar concentration by remifentanil. Anesthesiology 85,721^728. Lauwers M, Camu F,Vanlersberghe C (1997) Remifentanil, an esterase-metabolised opioid.What advantages does it o¡er in analgesia and anaesthesia? CNS Drugs 8 (3),189^198. McEwan AI, Smith C, Dyar O (1993) Iso£urane minimum alveolar concentration reduction by fentanyl. Anesthesiology 78,864^869. Michelsen LG, Salmenpera« M, Hug CC Jr. et al. (1996) Anesthetic potency of remifentanil in dogs. Anesthesiology 84,865^872. Munday I, Ward PM, Sorooshian S (1995) Interaction between remifentanil and iso£urane in spontaneously breathing patients during ambulatory surgery. Anesthesiology 83 (Suppl.3A), A23 (Abstract).
256
Murphy MR, Hug CC Jr (1982) The anesthetic potency of fentanyl in terms of its reduction of en£urane MAC.Anesthesiology 57, 485^488. Pajewski T, DiFazio CA, Moscicki JC et al. (1996) Nitric oxide synthase inhibitors, 7-nitro indazole and nitro G-L -arginine methyl ester, dose dependently reduce the threshold for iso£urane anesthesia. Anesthesiology 85,1111^1119. Pitts MC, Palmore MM, Salmenpera« M et al. (1992) Hemodynamic e¡ects of intravenous GI87084B (GI) in patients undergoing elective surgery. Anesthesiology 77 (Suppl.3A), 101 (Abstract). Quasha AL, Eger EIII, Tinker JH (1980) Determination and applications of MAC. Anesthesiology 53,315^334. Ste¡ey EP, Baggot JD, Eisele JH et al. (1994) Morphine^iso£urane interaction in dogs, swine and rhesus monkeys. J Vet Pharmacol Ther17, 202^210.
# Association of Veterinary Anaesthetists, 2003, 30, 250^256
Reduction of isoflurane MAC by fentanyl or remifentanil in rats Ana B Criado
MRCVS
& Ignacio A Go¤mez de Seguray
LAS Department, GlaxoSmithKline, UK
yDepartment of Experimental Surgery, University Hospital La Paz, Spain
Correspondence: Ana Criado Gonza¤lez, GlaxoSmithKline, Medicines Research Centre, Gunnels Wood Road, Stevenage, HERTS SG1 2NY UK. E-mail: [email protected]
Abstract Objective The main objective of the study was to determine the e¡ects of three di¡erent infusion rates of fentanyl and remifentanil on the minimum alveolar concentration (MAC) of iso£urane in the rat. A secondary objective was to assess the cardiovascular and respiratory e¡ects of the two opioid drugs. Animal population Thirty-seven male Wistar rats were randomly allocated to one of six treatment groups. Material and methods For all treatment groups anaesthesia was induced with 5% iso£urane in oxygen using an induction chamber. A 14-gauge catheter was used for endotracheal intubation, and anaesthesia was maintained with iso£urane delivered in oxygen via a T-piece breathing system. A baseline determination of the minimum alveolar concentration of iso£urane (MACISO) was made for each animal. Fentanyl (15, 30, 60 mg kg1 hour1) or remifentanil (60, 120, 240 mg kg1 hour1) were infused intravenously into a previously cannulated tail vein. Thirty minutes after the infusion started, a second MACISO (MACISOþdrug) was determined. The carotid artery was cannulated to monitor the arterial pressure and to take samples for arterial gas measurements. Cardiovascular (heart rate and arterial pressure) and respiratory (respiratory rate and presence/absence of apnoea) e¡ects after opioid infusion were also recorded. Results Fentanyl (15, 30, 60 mg kg1 hour1) and remifentanil (60, 120, 240 mg kg1 hour1) similarly 250
reduced iso£urane MAC in a dose-dependent fashion: by 10% at lower doses, 25% at medium doses and by 60% at higher doses of both the drugs. Both opioids reduced the respiratory rate in a similar way for all doses tested. No episodes of apnoea were recorded in the remifentanil groups, while administration of fentanyl resulted in apnoea in three animals (one at each dose level). The e¡ects on the cardiovascular system were similar with both drugs. Conclusions We conclude that the intraoperative use of remifentanil in the rat reduces the MAC of iso£urane, and that this anaesthetic sparing e¡ect is dose-dependent and similar to that produced by fentanyl at the doses tested. Clinical relevance The use of remifentanil during inhalant anaesthesia in the rat can be considered an intravenous alternative to fentanyl, providing similar reduction in iso£urane requirements. Due to its rapid o¡set, it is recommended that alternative pain relief be instituted before it is discontinued. Keywords fentanyl, iso£urane, MAC reduction, rats, remifentanil.
Introduction The minimum alveolar concentration (MAC) required to prevent movement in 50% of patients was originally de¢ned by Eger and coworkers in 1965 (Eger et al.1965), and has become the standard measure of volatile anaesthetic potency. Opioids markedly reduce the concentrations of volatile anaesthetic agents required to maintain anaesthesia. Evaluation
Iso£urane MAC in rats AB Criado & IA Go¤mez de Segura
of this reduction in the MAC of volatile anaesthetic agents byopioids may be used as a comparison of relative opioid potency. Remifentanil is a novel m-(OP3) receptor agonist with an analgesic e¡ect similar to that of fentanyl. It is metabolized by circulating and tissue esterases, giving a pharmacokinetic pro¢le of rapid onset, short duration and rapid o¡set of action. This should, in clinical practice, prove to be a drug whose e¡ects are easy to titrate. The use of this metabolic pathway results in less interpatient pharmacokinetic variability as a result of factors such as age, obesity, hepatic or renal failure. These clinical advantages may be of interest in laboratory animals, particularly when potent analgesia is required as well as precise control of drugs administered, i.e. rapid elimination and reduced interference with other drugs or measurements. Therefore, the aim of the present study was to compare the analgesic potency of remifentanil and fentanyl in the rat, in terms of reduction of iso£urane MAC (MACISO).
Materials and methods After obtaining institutional Animal Care Committee approval, the reduction of MACISO in response to fentanyl or remifentanil given by continuous infusion was evaluated in rats. Iso£urane was obtained from Abbott (Forane; Abbott Laboratories, Madrid, Spain), fentanyl from Roche (Fentanest; Laboratorio Roche S.A., Madrid, Spain) and remifentanil from Glaxo-Wellcome (Ultiva; Glaxo-Wellcome Laboratories, Madrid, Spain). Thirty-seven male Wistar rats (CRIFFA, Barcelona, Spain), with an average weight of 357 32 g, were housed in groups of 8^10, with 12 hours light12 hours dark cycle, relative humidity of 50^70%, 20 2 8C ambient temperature. Food (B&K Universal) and water were provided ad libitum. Animals were allowed to acclimatize for at least 1 week. All the studies were performed during the morning (starting at 9:00 am). Rats were placed in an induction chamber into which 5% iso£urane in a continuous oxygen £ow of 3 L minute1 was directed (Iso£urane Vaporizer Ohmeda Isotec 3; BOC Heath Care, Steeton, UK). Two to three minutes later, the iso£urane vaporizer setting was reduced to 2.5^3%. Endotracheal intubation was performed using a 14-gauge polyethylene catheter with the animal positioned in dorsal recumbency. A cold light was
applied externally over the trachea to aid visualization of the larynx within the oral cavity. Then a £exible, blunt-tip, wire guide was inserted into the trachea with an otoscope and used to direct the endotracheal catheter. After the correct position of the catheter was ascertained, it was connected to a small T-piece breathing system with minimal dead space. Fresh gas £ow to the T-piece was adjusted to 1 L minute1 of oxygen, and iso£urane concentration was adjusted as necessary. Opioids were administered with an infusion pump (Harvard model 55^2226; Harvard Apparatus, Millis, MA, USA) by means of a 24-gauge polyethylene catheter inserted in a tail vein. Monitoring The carotid artery was catheterized with PE50 tubing after surgical cut-down. This access allowed arterial blood sampling and blood pressure measurement via a calibrated pressure transducer (Transpac IV; Abbot Laboratories, Abbot Park, IL, USA). Systolic, diastolic and mean arterial blood pressure, and lead II electrocardiograph (needle electrodes placed on the left and right forelimb and right hindlimb) were recorded continuously (CM-8; Schiller, Switzerland). Arterial blood (0.3 mL) was collected for blood gas analysis (Blood gas analyser, Statnova pro¢le-1; Nova Biomedical, Waltham, MA, USA) at the end of study to ensure values (at that time point) were within normal limits of pH (7.35^7.45), partial pressure of oxygen (PaO2) [>12 kPa (90 mm Hg)], and pressure of carbon dioxide (PaCO2) [4.65^6.3 kPa (35^ 47 mm Hg)]. Rectal temperature was also monitored and maintained between 37 and 38 8C by means of a circulating water warming blanket (Heat Therapy Pump, Model TP-220; Gaymar, Orchand Park, NY, USA) and a heating light. Determination of the minimum alveolar concentration Intratracheal gas sampling was used to measure anaesthetic gas concentration for determination of the MAC value. This method has been described in detail previously (Pajewski et al.1996). In brief, a ¢ne catheter with 0.9-mm external diameter was inserted through the endotracheal catheter, with the tip located at the level of the carina. The proximal end of the catheter was connected to a 10-mL gastight glass syringe (Model 100/383/118; Portex, Hythe, UK). Samples were obtained by withdrawing
# Association of Veterinary Anaesthetists, 2003, 30, 250^256
251
Iso£urane MAC in rats AB Criado & IA Go¤mez de Segura
10 mL gas over 5 minutes using a Harvard infusion pump (Model 55^2226; Harvard Apparatus, Millis, MA, USA). Samples were obtained consecutively in triplicate to ensure constant alveolar concentration, and the ¢nal value was the mean, at every iso£urane concentration step. The catheter was withdrawn between samples. After every step change in anaesthetic concentration delivered by the anaesthetic breathing system, at least 15 minutes were allowed for equilibration before gas samples were obtained. Gas samples were assayed using a side-stream infrared analyser (5330 Agent Monitor; Datex-Ohmeda, Hertfordshire, UK). The MACISO and MACISOþdrug values were established according to the method described by Eger and coworkers. (Eger et al. 1965; Quasha et al. 1980). A painful noxious stimulus was applied with a long haemostat (8-inch Rochester Dean Hemostatic forceps) clamped to the ¢rst ratchet lock on the tail for 60 seconds while the third gas sample was obtained from the lung. The tail was always stimulated proximal to a previous test site. A positive response was considered to be a gross purposeful movement of the head, extremities or body. A negative response was considered to be the lack of movement or grimacing, swallowing, chewing or tail £ick. Where a negative response was seen, the iso£urane concentration was reduced in decrements of 0.10^0.15% until the negative response became positive. The MAC was considered to be the concentration midway between the highest concentration that permitted movement in response to the stimulus and the lowest concentration that prevented such movement. Experimental design Once the animals were anaesthetized and instrumented, a basal MACISO was determined in each animal, each animal acting as its own control. Opioids were given intravenously in the tail vein. Both opioids were given by continuous infusion, and no loading dose was used. Thirty minutes after drug administration, the MACISOþdrug was obtained by decreasing the iso£urane concentration as described previously. Drug groups Thirty-six animals were used. They were randomly assigned to one of six groups (n ¼ 6), and baseline MACISO was determined in all the animals prior to starting the opioid infusion. All animals within a group received the same dose of drug. Doses of15, 30, 252
60 mg kg1 hour1 of fentanyl (F-15, F-30, F-60) and 60, 120, 240 mg kg1 hour1 of remifentanil (R-60, R-120, R-240) were used. Two animals in the 60 mg kg1 hour1 fentanyl group could not be included in the analysis; an extra rat was used giving a ¢nal n-value for that group of ¢ve (see Results section). Animals were euthanased with pentobarbital given intravenously (Eutalender; Laboratorios Norton, Madrid, Spain) while still deeply anaesthetized. Unwanted e¡ects The e¡ects of drug administration on heart and respiratory rates were continuously monitored. Heart and respiratory rates were recorded immediately before each MAC step, and after 30 minutes of drug administration. A continuous printed record of arterial blood pressure was obtained throughout the experiment and used to obtain the peak value in mean arterial blood pressure reduction. Statistical analysis N-query advisor program (N-query advisor 2.0: Statistical solutions; Saugus, MA, USA) was used to determine an adequate n-value from previously published data (Go¤mez de Segura et al.1998). The n-value was calculated assuming a one-way analysis of variance, and found to be a minimum of three animals per group.When the sample size in each of the three groups is six, a one-way analysis of variance will have >90% power to detect at the 0.050 level a di¡erence in means, based on previous data characterized by a variance of means of 0.050, and assuming that the common standard deviation is 0.140, also data from the same work (Go¤mez de Segura et al.1998). Avalue of six animals per group was considered adequate, and also allowed us further comparisons. To examine the e¡ect on MACISOþdrug of three different infusion rates of remifentanil and fentanyl, a two-way analysis of variance (ANOVA) was conducted on the percentage of reduction of MAC (di¡erence between MACISO and MACISOþdrug expressed as a percentage). First, an overall comparison to see if fentanyl and remifentanil were di¡erent was carried out, and then an overall comparison to examine differences between low, medium and high levels. Finally, planned comparisons between low, medium and high levels of both drugs were made using a Fishers LSD (least signi¢cant di¡erence) test. A 95% con¢dence interval was established. A p-value of <0.05 was set to indicate statistical
# Association of Veterinary Anaesthetists, 2003, 30, 250^256
Iso£urane MAC in rats AB Criado & IA Go¤mez de Segura
signi¢cance. Percentage of reduction of mean arterial pressure, respiratory rate and heart rate were analysed in a similar way. All analyses were performed using the SAS statistical package (version 6.12 for Windows; SAS Institute Inc., SAS Campus Drive, Cary NC, USA). Results are presented as mean SD.
MACISOþdrug by10%, the medium doses by 25%, and the highest doses by 60%. Individual group MACISO values and those following opioid administration (MACISOþdrug) are shown in Table 1. Individual MACISOþdrug values are shown in Fig. 1.
Results
Cardiovascular e¡ects
Side-e¡ects
A decrease in mean arterial blood pressure was observed during the intravenous infusion of both fentanyl and remifentanil. No signi¢cant di¡erences were found in the peak reduction in mean arterial blood pressure between drugs, at all doses used. The overall percentage of mean arterial pressure reduction was 22.9% (16.6). Heart rate (measured 30 minutes after onset of infusion) decreased during both
MACISO, MACISOþdrug and MACISO reduction (%) The mean baseline MACISO value determined in all the rats was 1.29% (0.08). Fentanyl and remifentanil infusion reduced MACISOþdrug in a dose-dependent fashion. For the three infusion rates studied, fentanyl and remifentanil were found to be equipotent, i.e. the lowest doses of either drug similarly reduced
Table 1 Baseline (MACISO) minimum alveolar concentration of iso£urane, postdrug, fentanyl or remifentanil,(MACISOþdrug) minimum alveolar concentration of iso£urane and postdrug, fentanyl or remifentanil, percentage of minimum alveolar concentration of iso£urane reduction
Fentanyl
lg kg n
1
hour
1
MACISO MACISOþdrug %MACISO (red.)
Remifentanil
15 6
30 6
60 5
1.27 (0.10) 1.15 (0.11) 10.2 (1.9)
1.31 (0.06) 0.97 (0.07) 26.1 (4.6)
1.35 (0.08) 0.51 (0.12) 61.7 (10.2)
60 6
1.29 (0.08) 1.16 (0.09) 10.6 (4.2)
120 6
1.28 (0.10) 0.97 (0.09) 24.1 (8.7)
240 6
1.27 (0.09) 0.45 (0.06) 64.6 (5.6)
Data are expressed as mean (SD).
Figure 1 Individual MACISO values of rats given either fentanyl (15, 30 and 60 mg kg1 hour1) or remifentanil (60,120 and 240 mg kg1 hour1). # Association of Veterinary Anaesthetists, 2003, 30, 250^256
253
Iso£urane MAC in rats AB Criado & IA Go¤mez de Segura
Figure 2 Mean percentage of heart rate (HR) reduction 30 minutes after administration of either fentanyl (15, 30 and 60 mg kg1 hour1) or remifentanil (60, 120 and 240 mg kg1 hour1). Low, medium and high dose are signi¢cantly di¡erent in both drugs, and the di¡erence is similar in both treatments. Data are expressed as mean, boxes representing values within mean standard error, and whiskers values within mean SD.
fentanyl and remifentanil infusions: e¡ects of low, medium and high doses are signi¢cantly di¡erent in both drugs, and the di¡erence was similar in both treatments (Fig. 2). Respiratory e¡ects A reduction in respiratory rate was found in all groups 30 minutes after opioid infusion. No signi¢cant di¡erences could be found between drugs or infusion rates. The overall reduction in respiratory rate was 28.1% (23). No periods of apnoea were recorded in the remifentanil groups, even when the highest dose was used. Fentanyl resulted in apnoea at all doses used: one rat receiving15 mg kg1 hour1, one rat 30 mg kg1 hour1, and one 60 mg kg1 hour1. Two animals with the highest fentanyl dose could not be included in the study due the high levels [11.25 and 9.34 kPa (84.6 and 70.3 mm Hg)] of PaCO2 at the end of the experiment. In all the remifentanil animals, PaCO2 was below 6.0 kPa (45 mm Hg) at the end of the study.
Discussion Di¡erent methods can be used to compare the relative potencies of opioids, but the primary measure of opioid e⁄cacy is analgesia. The ability of a drug to reduce the MAC of an inhalant anaesthetic provides an objective measurement of analgesic e⁄cacy during anaesthesia. In this study, remifentanil and fentanyl showed a dose-related reduction of iso£urane MAC, and no differences could be found when low, medium and high doses of both drugs were compared. Nevertheless, plasma concentrations of both opioids were not measured at the time of MAC determination and we cannot con¢rm that the maximum concentration 254
in plasma of fentanyl had been reached at that time. The sparing e¡ect of opioids on the requirements of inhalant anaesthetics is well known. Up to 82% reduction in iso£urane MAC was reported with fentanyl in human beings (McEwan et al. 1993), while up to 91% was found with remifentanil (Lang et al. 1996). In Rhesus monkeys, a decrease in iso£urane MAC between 20 and 60% has been reported, with no ceiling e¡ect at the doses tested (Ste¡ey et al. 1994). Similar studies showed a ceiling e¡ect close to 70% en£urane MAC reduction with both, fentanyl (Murphy & Hug 1982) and remifentanil (Michelsen et al. 1996) in dogs. To our knowledge, no previous studies on iso£urane MAC reduction with either fentanyl or remifentanil have been conducted in rats. Similar studies have been carried out in the rat using di¡erent opioids (Hecker et al. 1983; Lake et al. 1985; Criado et al. 2000). The maximum degree of reduction of inhalant anaesthetics varied between approximately 90% in the case of sufentanil to only 50% when alfentanyl was administered. All the data from these and similar studies suggest that opioid potency is variable and species dependent. Remifentanil is a relatively new opioid with some speci¢c pharmacokinetic characteristics that make it a potentially useful option for the management of many situations, i.e. short and painful procedures, intraoperative analgesia, and in general all clinical situations where a strong analgesic is needed together with a prompt recovery. Esterase hydrolysis is a metabolic pathway that can be exploited to increase the rate of metabolism and elimination and so reduce the duration of the pharmacodynamic e¡ects of drugs. Previously applied to b-adrenergic blocking agents (esmolol) and muscle relaxants (mivacurium), the same concept was used to develop the ¢rst esterase-metabo-
# Association of Veterinary Anaesthetists, 2003, 30, 250^256
Iso£urane MAC in rats AB Criado & IA Go¤mez de Segura
lized opioid, remifentanil. The metabolism by nonspeci¢c esterases in the blood and tissues also prevents accumulation, even when given at high dosages over prolonged periods. Clinical recovery from anaesthesia is very rapid, irrespective of the age or physical status of the patient or the type or duration of surgery (Hughes et al.1992). The haemodynamic responses to remifentanil are similar to those found with another m-agonist (Joshi et al. 1993). In human volunteers, doses up to 2 mg kg1 were found to produce minimal haemodynamic alterations (Pitts et al.1992), whereas doses of up to 10 mg kg1 resulted in arterial blood pressure decreases of 10^20%, and mild bradycardia (Glass 1995). The larger decreases in arterial blood pressure are mainly due to bradycardia, which can be prevented by pre-treating patients with glycopyrrolate or atropine (Pitts et al. 1992). The incidence of muscle rigidity was also found to be similar to that with alfentanil (Glass et al. 1993). The lack of such undesired e¡ects encountered in the present study may have been related to the withholding of loading doses (Bu«rkle et al. 1996). With a zero order infusion, the time to reach a steady-state concentration is very rapid (approximately 10 minutes) (Glass 1995). Respiratory depression is a common side-e¡ect of OP3-agonist opioids (Amin et al. 1995; Munday et al. 1995) like remifentanil, but di¡erences may be observed when two drugs are compared (Lauwers et al. 1997). No periods of apnoea were recorded in the remifentanil group, even at the higher dose and in all the animals PaCO2 values at the end of the experience were below 5.33 kPa (40 mm Hg). All rats in the remifentanil group regained the basal MAC value. Apnoea was recorded in fentanyl groups at all doses tested and two animals did not regain the basal MAC value due to high PaCO2 values and could not be included in the study. A further advantage of remifentanil is derived from its short halflife so the undesirable respiratory and cardiovascular e¡ects are not expected to last for more than 10^15 minutes after discontinuation of infusion. However, the present study was not designed to accurately study the cardiovascular and respiratory e¡ects and further studies are needed to better address this point. There is no report on the use of remifentanil as a sole anaesthetic agent in rats. The maximum MAC reduction found with remifentanil in the dog is 70%, and this reduction was unchanged by two or three times higher doses (Michelsen et al. 1996). Although
there are major di¡erences in opioid potency amongst di¡erent species, it is unlikely that this drug alone could be used as a complete anaesthetic because the limiting factor will be the maximum intrinsic activity inherent in all OP3-type opioids. In our study, we could not ¢nd a ceiling e¡ect with fentanyl or with remifentanil at the doses tested. In conclusion, remifentanil produces a dosedependent reduction in the MAC of iso£urane in the rat similar to that induced by fentanyl when equipotent doses are considered.
Acknowledgements The authors wish to thank Belinda Farn¢eld for the review of the manuscript.
References Amin HM, Sopchak AM, Esposito BF (1995) Naloxoneinduced and spontaneous reversal of depressed ventilatory responses to hypoxia during and after continuous infusion of remifentanil or alfentanil. J Pharmacol Exp Ther 274,34^33. Bu«rkle H, Dunbar S, Van Aken H (1996) Remifentanil: a novel, short-acting, m-opioid. Anesth Analg 83,646^651. Criado AB, Go¤mez de Segura IA, Tendillo FJ et al. (2000) Reduction of iso£urane MAC with buprenophine and morphine in rats. Lab Animals 34, 252^259. Eger EIII, Saidman LJ, Brandstater B (1965) Minimum alveolar anesthetic concentration: a standard of anesthetic potency. Anesthesiology 26,756^763. Glass PSA (1995) Pharmacology of remifentanil. Eur J Anaesthesiol12 (Suppl.10),73^74 (Abstract). Glass PSA, Hardman D, KamiyamaYet al. (1993) Preliminary pharmacokinetics and pharmacodynamics of an ultrashort-acting opioid: remifentanil (GI87084B). Anesth Analg 77,1031^1040. Go¤mez de Segura IA, Criado AB, Santos M et al. (1998) Aspirin synergistically potentiates iso£urane minimum alveolar concentration reduction produced by morphine in the rat. Anesthesiology 89,1489^1494. Hecker BR, Lake CL, DiFazio CA et al. (1983) The decrease of the minimum alveolar anaesthetic concentration produced by sufentanil in rats. Anesth Analg 62,987^990. Hughes MA, Glass PSA, Jacobs JR (1992) Context-sensitive half-time in multicompartment pharmacokinetic models for intravenous anesthetic drugs. Anesthesiology 76, 334^341. Joshi P, Jhaveri R, BaumanVet al. (1993) Comparative trial of remifentanil and alfentanil for anesthesia induction. Anesthesiology 79 (Suppl.3A),379 (Abstract). Lake CL, DiFazio CA, Moscicki JC et al. (1985) Reduction in halothane MAC: comparison of morphine and alfentanil. Anesth Analg 64,807^810.
# Association of Veterinary Anaesthetists, 2003, 30, 250^256
255
Iso£urane MAC in rats AB Criado & IA Go¤mez de Segura
Lang E, Kapila A, Shlugman D et al. (1996) Reduction of iso£urane minimal alveolar concentration by remifentanil. Anesthesiology 85,721^728. Lauwers M, Camu F,Vanlersberghe C (1997) Remifentanil, an esterase-metabolised opioid.What advantages does it o¡er in analgesia and anaesthesia? CNS Drugs 8 (3),189^198. McEwan AI, Smith C, Dyar O (1993) Iso£urane minimum alveolar concentration reduction by fentanyl. Anesthesiology 78,864^869. Michelsen LG, Salmenpera« M, Hug CC Jr. et al. (1996) Anesthetic potency of remifentanil in dogs. Anesthesiology 84,865^872. Munday I, Ward PM, Sorooshian S (1995) Interaction between remifentanil and iso£urane in spontaneously breathing patients during ambulatory surgery. Anesthesiology 83 (Suppl.3A), A23 (Abstract).
256
Murphy MR, Hug CC Jr (1982) The anesthetic potency of fentanyl in terms of its reduction of en£urane MAC.Anesthesiology 57, 485^488. Pajewski T, DiFazio CA, Moscicki JC et al. (1996) Nitric oxide synthase inhibitors, 7-nitro indazole and nitro G-L -arginine methyl ester, dose dependently reduce the threshold for iso£urane anesthesia. Anesthesiology 85,1111^1119. Pitts MC, Palmore MM, Salmenpera« M et al. (1992) Hemodynamic e¡ects of intravenous GI87084B (GI) in patients undergoing elective surgery. Anesthesiology 77 (Suppl.3A), 101 (Abstract). Quasha AL, Eger EIII, Tinker JH (1980) Determination and applications of MAC. Anesthesiology 53,315^334. Ste¡ey EP, Baggot JD, Eisele JH et al. (1994) Morphine^iso£urane interaction in dogs, swine and rhesus monkeys. J Vet Pharmacol Ther17, 202^210.
# Association of Veterinary Anaesthetists, 2003, 30, 250^256