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Hyperalgesia and increased sevoflurane minimum alveolar concentration induced by opioids in the rat

A randomised experimental study

Abreu, Mariana; Aguado, Delia; Benito, Javier; García-Fernández, Javier; Segura, Ignacio A. Gómez de

Author Information
European Journal of Anaesthesiology (EJA): April 2015 - Volume 32 - Issue 4 - p 232-241
doi: 10.1097/EJA.0000000000000188
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This article is accompanied by the following Invited Commentary:

Weinbroum AA. Role of anaesthetics and opioids in perioperative hyperalgesia: One step towards familiarisation. Eur J Anaesthesiol 2015; 32:230–231.


Opioids are widely used analgesics to treat moderate to severe pain. There is evidence, however, suggesting that opioids activate not only antinociceptive but also pronociceptive mechanisms leading to an increase in pain sensitivity, also known as opioid-induced hyperalgesia (OIH). Opioid-induced hyperalgesia has been defined as a state of nociceptive sensitisation caused by exposure to opioids, whereby a patient receiving opioids for the treatment of pain becomes more sensitive to nociceptive stimulation.1

The μ opioid receptor system has been suggested as playing a pivotal role in the development of OIH.2 Morphine3 and remifentanil4,5 are the drugs most commonly related to OIH, but other opioids such as methadone or buprenorphine have also been implicated.6–9 Furthermore, OIH might be long lasting; a single remifentanil dose has been associated with hyperalgesia in mice for up to 10 days.10 The central glutaminergic system, spinal dynorphins, descending facilitation, genetic mechanisms with decreased reuptake and enhanced nociceptive responses have all been hypothetised to have a role in the development of OIH.11 N-methyl-D-aspartate (NMDA) receptor activation has been shown to be involved in OIH12 and the NMDA antagonist ketamine has been used for its antihyperalgesic properties.13

During inhalational anaesthesia, opioids are usually coadministered and produce a reduction in anaesthetic requirements. The anaesthetic-sparing action of remifentanil, as determined by a reduction of the minimum alveolar concentration (MAC), has been recently shown to rapidly decrease in rats due to an acute tolerance effect.14 This suggests that increasing doses of the opioid are required to maintain a similar anaesthetic level. A previous study suggested a potential relationship between the hyperalgesic action of remifentanil given as an infusion, and its effect on the MAC.15

We hypothesised that other opioids would also increase inhalational anaesthetic requirements secondary to OIH in a manner similar to remifentanil. The aim of this study was, therefore, to determine whether remifentanil, buprenorphine, methadone and morphine administration produce an increase in the MAC associated with OIH, the duration of these effects and whether ketamine administration could prevent them.

Materials and methods

The study was approved by the institutional animal care and use committee [Ethics Animal Experimentation Committee (CEA) of the University Complutense, Madrid, Spain, Chairperson Prof D. José Francisco Tirado Fernández. CEA approval reference date: 29 May 2008] and the care of animal and licensing guidelines under which the study was performed were in accordance with the ARRIVE (Animals in Research: Reporting In Vivo Experiments) statement.


Seventy-two adult male Wistar rats (Charles River Laboratories, Barcelona, Spain), of mean (SD) weight 347 (55) g were used. The animals were housed in groups of six per cage (Macrolon Type IV; Souralit 3000, Souralit S.L., Girona, Spain) and kept on a 12 : 12 h light-dark cycle (lights on between 8 : 00 a.m. and 8 : 00 p.m.) at mean (SD) 22 (2)°C and 40 to 70% relative humidity. Food (A03 maintenance diet for rodents; Scientific Animal Food & Engineering, SAFE, Panlab, Barcelona, Spain) and water were available ad libitum. All experiments were performed during the light period starting at 9 : 00 a.m. (Microsurgery Laboratory, Experimental Surgery Department).

Anaesthesia and minimum alveolar concentration determination

Rats were placed in a methacrylate anaesthetic induction chamber into which 8% sevoflurane (Sevorane; Abbott Laboratories, Madrid, Spain) was delivered in oxygen (3 l min−1) via a sevoflurane vapouriser (Sevorane Dragër Vapor 2000; Lubeck, Germany). Endotracheal intubation was performed using a 14-gauge polyethylene catheter (Terumo Surflo i.v. catheter; Terumo Europe NV, Leuven, Belgium) with the animal positioned in sternal recumbence. A flexible blunt-tip wire guide was inserted into the trachea with the aid of an otoscope and this was used to guide the endotracheal tube into the trachea. After the correct position of the catheter was ascertained by determining gas movement through the catheter with gauze threads and exhaled carbon dioxide with a capnograph, a small T-piece breathing system with minimum dead space was connected.

Heart and respiratory rates, arterial haemoglobin oxygen saturation (via pulse oximetry) and end-tidal carbon dioxide and sevoflurane concentrations were continuously measured (Datex-Ohmeda SA5 monitor; General Electric, Helsinki, Finland). Heart and respiratory rates and end-tidal carbon dioxide concentrations were recorded 60 s prior to tail clamping at each step used to determine the MAC value. Rectal temperature was monitored and maintained between 37°C and 38°C by means of a water-circulating warming blanket (Heat Therapy Pump Model TP-220; Gaymar Industries, Orchard Park, New York, USA) and a standard heating light.

To determinate the MAC, intratracheal gas sampling was used to measure anaesthetic gas concentrations. Before the first MAC determination, the end-tidal anaesthetic concentration was adjusted to 2.4%, a value close to the MAC previously reported for the rat.16 After every step change in the anaesthetic concentration delivered by the vapouriser to each rat, at least 10 min were allowed for equilibration (steady-state end-tidal concentration of sevoflurane) before obtaining gas samples and applying the noxious stimulus.

The MAC and any reduction produced by the opioids were established according to the method described previously.16–18 A noxious stimulus was applied with a long haemostat (230 mm Doyen forceps; Martin, Tutlingen, Germany) clamped to the first ratchet lock on the rat's tail for 60 s, or until a positive response was observed, immediately after the gas sample had been obtained from the trachea. The initial stimulus point was 6 cm distal to the tail base. The tail was always stimulated 1 cm proximal to a previous test site, when a previous response was negative, or 1 cm distal to that point if the response was positive.19 A positive response was considered to be a gross purposeful movement of the head, extremities and/or body or a clear movement of the spine. A negative response was considered to be either the lack of movement or discreet reactions (grimacing, swallowing, chewing or tail flick). When a negative response occurred, the sevoflurane concentration was reduced by 0.2% decrements until a positive response was obtained. When a positive response occurred, the sevoflurane concentration was increased until the positive response became negative. The MAC was considered to be the concentration mid-way between the highest concentration that permitted movement in response to the stimulus and the lowest concentration that prevented such movement. Determination of the MAC was performed in a laboratory at 650 m above sea level and MAC values were corrected to the barometric pressure at sea level using the following formula: MAC (%) at sea level barometric pressure (760 mmHg); (altitude-adjusted MAC) = measured MAC (%) x measured ambient barometric pressure (700 mmHg in Madrid)/barometric pressure at sea level (760 mmHg).20

Behavioural tests of nociception

To minimise the influence of stress during the experimental procedure, animals were acclimatised for 3 days to the testing procedure before baseline values were obtained. Two behavioural tests of mechanical nociception were employed: von Frey and Randall–Selitto.

Von Frey test

The hind paw withdrawal response to von Frey filament stimulation was evaluated.21 Animals were placed in a methacrylate cage with a wire grid bottom (1 cm2 perforations), through which the electronically calibrated von Frey anaesthesiometer filaments were applied (Electronic von Frey Anesthesiometer Model 2393; IITC Life Science Inc., Woodland Hills, California, USA). Before starting the test, the animals were left in the cage for 15 to 30 min for acclimatisation, that is, the rats remained calmed and still, without exploratory movements or defaecation and not resting.21 Eight von Frey filaments were used and bending force ranged from 0.1 to 60 g. The filaments (from the thinnest to the thickest) were applied to the right foot paw for 7 s or until the rat withdrew its hind limb.21–23 Locomotion was not considered as a withdrawal response, and in that case, the stimulus was repeated.24 The stimulus was applied between three and five times with each filament with an interval of 5 s. The lowest force producing a withdrawal response was considered the threshold. Three positive responses determined the threshold for each measurement. The threshold in each animal was based on three separate measurements (5 min interval between two consecutive measurements). The mean of these threshold values at each time point was considered the mechanical nociceptive thresholds (MNT).25

Randall–Selitto test

Mechanical thresholds were quantified using the Randall–Selitto paw pressure device (Digital Randall-Selitto model 2600; IITC Life Science Inc.),26 which produces a linearly increasing mechanical force in grams. Before starting the test, the animal was handled for at least 5 min till it was calmed and still. The stimulus was applied to the plantar surface of the left hind paw by increasing the pressure until the rat withdrew its limb, which was considered the threshold.23 The threshold was measured two to five times until two consecutive values differing by no more than 15% were obtained and the mean of these two values was considered the threshold value at that time point. A minimum of 5 min was allowed between consecutive measures.24 A cut-off pressure of 300 g was used to avoid tissue damage.27 The threshold value at each time point was considered the mechanical nociceptive threshold.

Opioid-induced hyperalgesia was confirmed by a reduction in withdrawal thresholds in the Randall–Selitto or von Frey tests.

Experimental design and drug groups

Effects of opioids on minimum alveolar concentration (after 7 days)

Once the animals were anaesthetised, a baseline MAC was determined. Each rat was then randomly administered one of the treatments, and 30 min later, a new MAC determination was started. Seven days later, the MAC was again determined with the same treatment. A schematic view of the experimental design is shown in Fig. 1a. Finally, rats were euthanised with an overdose of potassium chloride given intravenously while still deeply anaesthetised.

Fig. 1
Fig. 1:
Experimental design. (a) Determination of the minimum alveolar concentration (MAC) at baseline and following opioid administration to determine (immediate) MAC reduction at day 0 and after 7 days. (b) Effects of remifentanil on mechanical nociceptive thresholds (MNT) and MAC over a 4-week period. After an adaptation period of the rats during 3 days, animals were assessed for MNT followed by MAC determinations at baseline (day 0) and 1, 3, 7, 14, 21 and 28 days after remifentanil administration. (c) Effects of remifentanil and ketamine on MNT and MAC after 7 days.

Drugs were selected because they are commonly used clinically and have been associated with the development of OIH.3,4,7,8 The dose of each selected opioid was based on the clinical dose range for the rat and a previous study determining its MAC-sparing action.28 Animals were randomly allocated to receive one out of five treatments (n = 7 per group): buprenorphine 150 μg kg−1 (Buprex; Esteve Laboratories, Barcelona, Spain); methadone 8 mg kg−1 (Metasedin; Esteve Laboratories); morphine 10 mg kg−1 (Morfina; Braun, Barcelona, Spain); remifentanil 120 μg kg−1 h−1 (Ultiva; Glaxo-Smith-Kline, Madrid, Spain); and 0.9% saline (control group). Remifentanil was administered as an intravenous infusion for approximately 2 h and the remaining opioids were diluted in 0.9% saline to a total volume of 1 ml and administered by the intraperitoneal (i.p.) route.

Duration of remifentanil-induced hyperalgesia and effects on minimum alveolar concentration

Rats were acclimatised over 3 days to the von Frey and Randall–Selitto tests. On day 0, rats were anaesthetised and instrumented and the baseline MAC was determined. Remifentanil was selected to determine the duration of the effects on hyperalgesia and MAC. Each rat was randomly administered the treatment (either remifentanil or 0.9% saline) intravenously as an infusion (n = 8 per group). The remifentanil dose was 120 μg kg−1 h−1, and in both groups, the rate of infusion was 4.8 ml kg−1 h−1 for 2 h. The rats were then allowed to recover from anaesthesia. The same procedures were performed at days 1, 3, 7, 14, 21 and 28 after treatment to evaluate MNT and MAC. Schematic view of the MAC determination is shown in Fig. 1b. As baseline values were regained on day 28 after treatment, the rats were euthanised with an overdose of potassium chloride given intravenously while still deeply anaesthetised.

Effects of ketamine on remifentanil-induced hyperalgesia and minimum alveolar concentration (after 7 days)

A similar study was performed wherein remifentanil was administered alone or combined with ketamine, and its effects determined at 7 days. A schematic view of the experimental design is shown in Fig. 1c. Remifentanil was selected to determine the blocking action of ketamine on OIH and MAC. Animals were randomly assigned to receive remifentanil [120 μg kg−1 h−1 intravenously for 2 h, remifentanil (same dose) as well as ketamine (Ketolar 500; Parke-Davis-Pfizer, Madrid, Spain] or 0.9% saline (n = 7 per group). Ketamine 10 mg kg−1 was administered intravenously 30 min before remifentanil infusion and maintained at 10 mg kg−1 h−1 for 2.5 h. The two lateral veins of the rat's tail were used for the infusions. Rats in the control group received the same volume of saline (4.8 ml kg−1 h−1) intravenously for 2 h.

Statistical analysis

An n value of between seven and eight animals per group was considered adequate on the basis of previous data from similar studies in our laboratory and from calculation of the n value (nQuery Advisor version 2.0; Statistical Solutions Ltd., Cork, Ireland).16 The Kolmogorov–Smirnov test was performed to verify normal distribution of the data. Differences in MAC and the von Frey and Randall–Selitto tests produced by treatments were compared with one-way analysis of variance (ANOVA). The Bonferroni posthoc test was used to compare differences among groups. An ANOVA for repeated measures was conducted (drug x time) by comparing the percentage of variation in the MAC, von Frey and Randall–Selitto tests. A one-way ANOVA was employed to compare both groups at equal times. A 95% confidence interval was established and a P value of less than 0.05 was set to indicate statistical significance. Results are presented as mean (SD). Statistical analyses were performed using SPSS (version19; IBM, Armonk, New York, USA). All data are presented as mean (SD).


Buprenorphine, methadone, morphine and remifentanil, but not 0.9% saline, reduced the MAC immediately after administration by 29 (7), 32 (9), 34 (5) and 22 (12)%, respectively (P < 0.005, all groups).

Mean baseline MAC determined in all rats was 2.3 (0.3)% (n = 35) with no differences among groups (P > 0.40 for all groups). An increase in the MAC was observed 7 days later in all groups administered opioids (P ≤ 0.03) but not 0.9% saline [MAC 2.3 (0.2)% vs. baseline, P = 0.530] (Fig. 2). The increase in the MAC [16 (10)%, P ≤ 0.001] was not different between groups receiving opioids 7 days earlier, with a mean value of 2.8 (0.3)%, (P > 0.05) (n = 28). This time-related increase was not observed in the control group [2 (7)%, P > 0.05] (Fig. 3).

Fig. 2
Fig. 2:
Minimum alveolar concentration at baseline and 7 days after the administration of a single dose of buprenorphine, methadone, morphine and remifentanil in rats. Data are expressed as mean (SD). Then value is always 7 rats per group. *Difference vs. control group at the same time point (P < 0.05).
Fig. 3
Fig. 3:
Mechanical nociceptive thresholds in rats [determined with the von Frey (a) and Randall–Selitto (b) tests] and minimum alveolar concentration (MAC) (c) values from rats determined at baseline and after the administration of either remifentanil or 0.9% saline (control). Data are expressed as mean (SD). Then value is always 8 rats per group. aDifferences vs. control group at same time point (P < 0.05).

The immediate MAC reduction produced by the studied opioids (buprenorphine, methadone, morphine and remifentanil) was not modified by a previous sevoflurane anaesthesia using opioids 7 days earlier (P > 0.05 for all groups). The MAC reduction observed in the second MAC determination following buprenorphine, methadone, morphine and remifentanil administration was 32 (8), 33 (6), 34 (7) and 19 (11)%, respectively.

Remifentanil produced a significant increase in absolute and relative MNT (von Frey test) (P = 0.010 and P < 0.001, respectively; Fig. 4), which was observed until day 21 (P ≤ 0.017) but not at day 28 (P = 0.449). A maximum decrease was observed on day 7 with a reduction of 48 (5)%. A similar reduction was observed with the Randall–Selitto test (absolute and relative thresholds P = 0.007 and P < 0.001, respectively; Fig. 4). The maximum decrease was observed on day 3 with a reduction of 50 (20)% and differences between groups were observed from day 1 to day 21 (P ≤ 0.015), but not at day 28 (P = 0.149).

Fig. 4
Fig. 4:
Changes in mechanical nociceptive thresholds in rats [determined with von Frey (a) and Randall–Selitto (b) tests] and minimum alveolar concentration (MAC) (c) produced by remifentanil, remifentanil and ketamine or 0.9% saline (control). Then value is always 7 rats per group. Data are expressed as mean (SD). aDifferences vs. control group at same time point (P < 0.05).

The MAC was increased in rats administered remifentanil (absolute and relative values P = 0.042 and P < 0.001, respectively; Fig. 4) from day 1 to day 21 (P ≤ 0.013), but not at day 28 (P = 0.110). The maximum increase [19 (7)%] was observed on day 7.

Remifentanil produced an increase in the MAC of 20 (3)% (P = 0.007) and a decrease in MNT of 42 (6)% (von Frey; P = 0.001) and 45 (7)% (Randall–Selitto; P = 0.001) on day 7 after drug administration. However, a preventive effect of ketamine on remifentanil-induced effects on MAC and MNT (von Frey and Randall–Selitto tests) was observed and no significant differences from baseline values were determined. The percentage of change from baseline values was 2 (6)% in the MAC value and 3 (7)% and 6 (3)% with the von Frey and Randall–Selitto tests (vs. baseline P = 1.000 for both MAC and MNT tests; Fig. 3). Similarly, no differences in the MAC and MNT tests were observed in the remifentanil plus ketamine group compared with control (P = 1.000; Fig. 3).


As expected, remifentanil decreased the MAC immediately following its administration but induced a delayed increase observable 24 h later. This increase might be a consequence of OIH, as the decrease in mechanical thresholds and the increase in MAC were simultaneously observed when remifentanil was administered; ketamine blocked both phenomena. The magnitude of the MAC increase was relatively moderate (16%) and consistently lasted up to 21 days following remifentanil administration. Other commonly used opioids, namely buprenorphine, methadone and morphine produced a similar increase in the MAC following administration and although the duration of the effect was only determined with remifentanil administration, a similar long-lasting effect of the studied opioids cannot be ruled out.

Remifentanil use has been associated with OIH in anaesthetised patients. Total doses of remifentanil of 1.55 and 60 μg kg−14 produced hyperalgesia in healthy volunteers (as determined through electrical stimulation) lasting 90 min. In children, a total dose of 115 μg kg−1 administered over 7 h produced an increase in pain scores and postoperative morphine consumption. Lower total doses of between 23 and 27 μg kg−1,29,30 however, failed to modify pain scores or morphine consumption in the postoperative period. These apparently contradictory results may be related to the total dose administered, the rate and duration of the infusion and the evaluation method.31

High doses of opioids have been suggested to promote OIH and relatively large doses are actually employed in rats compared with those administered to patients. Drug dosage should, however, be adjusted according to allometric escalation, which is based on the metabolic rate of different species.32 Then the doses employed in rats were actually those producing clinically meaningful effects on MAC and were, therefore, adequate for this species. In mice, a remifentanil infusion (total dose 80 to 100 μg kg−1 over 30 min) produced OIH 2 days later with a 40% reduction in mechanical thresholds that lasted for 10 days. Similar to that which has been observed in patients, lower doses (20 and 40 μg kg−1) of remifentanil failed to produce hyperalgesia in rodents,10 suggesting that a minimum opioid dose may be required. A similar effect was observed with the paw pressure (Randall–Selitto) test, although the minimum dose of remifentanil required to reduce mechanical thresholds was at least 160 μg kg−1.33 This dose produced hyperalgesia lasting 2 days and increasing the doses to between 240 and 400 μg kg−1 determined not only a further reduction in mechanical thresholds but also a more prolonged reduction lasting 3 to 5 days.33 A total dose of 240 μg kg−1 (120 μg kg−1 h−1) has been administered to rats in the current study producing a maximum reduction in mechanical thresholds of 50% between days 3 and 7, returning to baseline levels by day 28. A similar response was obtained with both behavioural tests of mechanical nociception, the von Frey and the Randall–Selitto tests, indicating the consistency of the observed effect.

Buprenorphine, methadone and morphine have also been associated with the development of hyperalgesia, both in human and animal studies. Patients treated with buprenorphine and methadone had similarly shorter withdrawal latencies with a cold pressure trial.9 Methadone also induced hyperalgesia to a cold pressure test, but not electric stimulation, and was associated with increased morphine requirements.6 In rats and mice, morphine-induced hyperalgesia has been demonstrated following both acute and chronic3 administration. Methadone and buprenorphine have also produced hyperalgesia at subanalgesic doses,7,8 although at analgesic doses, hyperalgesia was not observed with methadone7 and repeated administrations of buprenorphine were required to produce hyperalgesia.8

Opioid-induced hyperalgesia has been consistently demonstrated in rodent studies using a variety of opioids, dosing schedules, routes of administration and nociceptive paradigms, although data from human studies are far more limited. The few human studies suggesting a cause and effect relationship only demonstrated an aggravation of preexisting hyperalgesia by opioids.34 It should be noted that the rats employed in the current study were not submitted to a clinically relevant painful stimulus such as surgery.

Overall, evidence suggests a hyperalgesic response to opioids in both humans and rats, although the consequences of this phenomenon during the intraoperative period have not been established. Our results support a relationship between the hyperalgesia produced by remifentanil and an increase in the MAC of sevoflurane. A similar increase in the MAC produced by all studied opioids would also support a similar mechanism of action consistent with the presence of OIH.

The NMDA receptor has been implicated in the development of OIH11 and the administration of NMDA antagonists such as ketamine may prevent or limit OIH both in humans35 and animals. In the current study, MNT and the MAC increase produced by remifentanil were blocked by ketamine administration and this suggests a relationship between both phenomena, most likely related to the actual nociceptive status (hyperalgesia vs. hypoalgesia) and inhalational anaesthetic requirements.

Tolerance and OIH are not necessarily linked phenomena.36 Tolerance, but not OIH, has been demonstrated in patients receiving chronic opioid therapy,37 whereas OIH has been observed in patients exposed to perioperative opioids,37,38 former opioid addicts on methadone therapy6 and healthy human volunteers.39,40 Reasons for this may include the more moderate doses employed in patients receiving long-term therapy than the higher opioid doses acutely administered in other clinical situations such as the perioperative period. In addition, the human laboratory models may only measure secondary hyperalgesia, as this is clinically relevant for chronic pain patients.37

Although not a measure of analgesia, MAC reduction is a useful tool to mimic the intraoperative period wherein opioids reduce the MAC in a dose-dependent fashion.41 Recent evidence suggests that opioid efficacy may be reduced with time and an acute tolerance effect has been consistently determined in rats during inhalational anaesthesia, wherein the MAC method was applied to describe intraoperative opioid tolerance.14,42 In rats, the MAC reduction initially produced by a remifentanil infusion can be decreased by 50% only 1.5 h later, so the infusion dose has to be doubled to regain the initial MAC reduction.14 However, other factors, such as OIH, may also account for a reduction in opioid efficacy. The results from the present study suggest a similar tolerance effect to remifentanil, determined in MAC reduction terms both in normal and hyperalgesic states, indicating that hyperalgesia does not reduce opioid efficacy.

The observed results should not be extrapolated directly between species, as great variations in the effects of opioids on MAC may apply.43,44 In addition, the actual impact of the observed results in the clinical setting should be explored, as relevant differences other than interspecies variations may have an impact, such as the use of the surgical stimuli or the interference of other nonopioid analgesic drugs commonly employed in the perioperative period. A potential limitation of the study is the observed, although slight, increase in baseline MAC during the study period, as a nonsignificant trend towards MAC increases in animals not treated with opioids was observed. This may reflect a response to repeated tail clamping during MAC determination, although this would have affected all the experimental groups.41 The use of the repeated MAC method may have biased the impact of the studied opioids, and the actual effect of these drugs might have been reduced. Finally, hyperalgesia was not demonstrated with the opioids buprenorphine, methadone and morphine, and accordingly, a similar study should be performed wherein these are also determined.

In conclusion, remifentanil, buprenorphine, methadone and morphine all produced a similar increase in the MAC in rats. When remifentanil was used, the increase in the MAC and hyperalgesia was observed 24 h after administration and lasted 21 days. Overall, these data suggest that although opioid treatment typically decreases anaesthetic requirements within minutes or hours after its administration in the normal nonoperated rat, it may be followed by a prolonged increase in inhalational anaesthetic requirements associated with hyperalgesia. The blockade of both phenomena by ketamine suggests a relationship between an increase in the MAC and hyperalgesia.

Acknowledgements relating to this article

Assistance with the study: the authors thank Dr E de Miguel, MD, and C Largo, DVM, Experimental Surgery Department, La Paz University Hospital, Madrid, Spain, and Dr EP Steffey, School of Veterinary Medicine, University of California, Davis, California, USA, for expert manuscript review.

Financial support and sponsorship: this work was supported by a grant from the Fondo de Investigaciones Sanitarias. Spanish Health Ministry. Grant number FIS 08/0422.

Conflict of interests: none.

Presentation: preliminary data from this study were presented at the meeting of the Association of Veterinary Anaesthesists, 13 to 15 April 2011, Bari, and the World Congress of Veterinary Anaesthesia, 23 to 27 September 2012, Cape Town.


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