BACKGROUND
postoperative pain management is a major concern and influences recovery time, quality of life, and surgical outcome  <cit> .
acute pain alters metabolic and hemodynamic systems, facilitates the release of the inflammatory cascade, and triggers a sympathetic stress response  <cit> .
serum values of cortisol, glucose, and interleukins may provide important information, in association with clinical parameters and pain scales, in the study of different pain conditions and proposed treatments in dogs  <cit> .
pro-inflammatory cytokines, such as interleukin- <dig> , show a high association with acute pain because they sensitise the nervous system.
il- <dig> levels in plasma are detectable 60 minutes after injury, peaking at 4- <dig> hours, and are proportional to tissue injury  <cit> .
an adequate approach to postoperative pain control is preventive analgesia.
it has been suggested that the administration of non-steroidal anti-inflammatory drugs , opioids, or local anaesthetics prior to injury reduces central sensitisation, analgesic consumption, and the use of rescue analgesia  <cit> .
opioids are commonly used for sedation and the treatment of postoperative pain in small animals  <cit> .
although it is well known that they may cause immune suppression in humans by helping to control the release of inflammatory substances  <cit> , there is little information regarding opioid action on the canine immune system.
methadone is an opioid that has recently been introduced into routine veterinary medicine practice, and its analgesic effects have been of great interest.
it acts as a mu-opioid receptor agonist and as an n-methyl-d-aspartate  receptor antagonist  <cit> .
additionally, methadone is considered to be as powerful as morphine but causes fewer side effects, such as nausea, vomiting, and dysphoria  <cit> .
tramadol is a well-known synthetic opioid with low mu-opioid agonist affinity , and its efficacy is due to two different mechanisms, namely, it interacts with mu-opioid receptors and has effects on serotonin and norepinephrine reuptake.
o-desmethyl tramadol , a product of tramadol hepatic metabolism, is known to present a higher affinity for mu receptors  <cit> .
it is also believed to have an immunostimulatory effect, as it decreased il- <dig> levels in a model of incisional pain in rats  <cit> .
the aim of the present study was to evaluate the effects of methadone and tramadol on postoperative pain in dogs undergoing tibial plateau levelling osteotomy  and their influence on serum il- <dig> levels.
methods
animals
twenty-eight client-owned dogs admitted to the orthopedics and trauma laboratory that presented with cranial cruciate ligament ruptures were included in the study.
all the animals were judged as healthy based on physical examinations, complete blood cell counts, and serum biochemical analyses before surgery.
animals presenting with any alterations in these parameters were excluded from the study, as were those given any type of analgesic drug prior to the study.
owners of the eligible dogs agreed to their inclusion in the study by signing a consent agreement form.
food and water were withheld for 12 hours prior to surgery.
experimental design
a complete physical evaluation was performed for all animals before any other intervention.
pain scores were assessed by several scales , and the colorado university canine acute pain scale) at baseline for later comparison with the postoperative period.
blood samples were collected for the assessment of il- <dig> levels using a specific kit for canine serum  before premedication ) and after surgery .
animals were randomly assigned to three different groups as follows: the tra group  received 4 mg/kg of tramadol ; the met <dig>  group  received  <dig>  mg/kg of methadone ; and the met <dig>  group  received  <dig>  mg/kg of methadone.
all drugs were injected intramuscularly as premedication.
the study was approved by the ethical committee of faculdade de medicina  and faculdade de medicina veterinária e zootecnia  from the universidade de são paulo.
procedure
sixty minutes after premedication, the cephalic vein was catheterised for fluid administration  at a rate of 10 ml/kg/h and for drug infusions.
anaesthesia was induced with iv propofol   to achieve the desired effects , and intubation was then performed with an adequately sized endotracheal tube.
general anaesthesia was maintained with isoflurane  in 100% oxygen under positive pressure ventilation  in a circle rebreathing system, with the respiratory rate adjusted to maintain the end-tidal concentration of carbon dioxide between  <dig> and 45 mmhg.
heart and respiratory rates, direct arterial blood pressure, and oxygen saturation  were monitored by a multiparameter monitor .
a 20-gauge catheter was aseptically placed on the dorsal pedal artery and connected to the monitor’s pressure transducer with zero reference pressure at the level of the heart.
end-tidal concentrations of isoflurane  and carbon dioxide  were monitored by a gas analyser and capnograph , with the etiso initially set at  <dig> %.
the gas analyser was calibrated before each experiment with a standard gas mixture .
in the case of hypotension , ephedrine  was administered.
one bolus of fentanyl  was administered intravenously just before starting the osteotomy.
whenever there was a change in heart rate or blood pressure greater than 20% compared with pre-stimulation values, a  <dig> % increase in isoflurane was administered.
temperature was maintained between  <dig> and 38°c with the use of a warming blanket .
an 18-gauge catheter was aseptically placed in a jugular vein to facilitate subsequent blood sample collections.
all surgeries were performed by the same experienced surgeon .
sedation scores were assessed 30 minutes after premedication injection and at  <dig> and 3 hours after surgery.
a scale proposed by valverde et al.
<cit> , ranging from  <dig> to  <dig>  was used for this purpose, with 0 = no effect, 1 = mild effect , 2 = moderate effect , and 3 = severe effect .
physiological parameters were recorded at  <dig> ,  <dig> ,  <dig> ,  <dig> , and  <dig>  hours after premedication.
pain was assessed at the same time points after analgesic injection, except at t <dig>  using the vas and modified glasgow composite measure pain scale  <cit> , each of which ranged from  <dig> to  <dig>  where  <dig> indicates no pain and  <dig> indicates the worst pain possible.
the colorado university canine acute pain scale was also used and ranged from  <dig> to  <dig>  with  <dig> indicating minimal pain and  <dig> indicating severe pain .
all parameters were assessed by the same observer , who was unaware of the analgesic treatment and has experience in pain evaluation.
if scores for the vas, colorado, and glasgow scales were higher than  <dig>   <dig>  and  <dig>  respectively, rescue analgesia was administered as follows to reduce the influence of other analgesic drugs on pain and il- <dig> levels: 1 mg/kg of tramadol for the tra group and  <dig>  mg/kg of methadone for the met <dig>  and met <dig>  groups.
animals were re-evaluated 20 minutes after the administration of rescue medication, and if signs of pain persisted,  <dig>  mg/kg of morphine was administered and the animal was withdrawn from the study.
the animals were observed after surgery, sent home, and then returned 24 hours later for evaluation.
the occurrence of any adverse effects, such as nausea, vomiting, dysphoria, mydriasis, or intense salivation, was recorded, as well as the number of animals requiring rescue analgesia.
nsaids were not administered during the evaluation period, but carprofen  was prescribed for 10 days postoperatively, as is usually recommended by surgeons.
blood collection was performed at tbl, t <dig>  t <dig>  and t <dig> for measurement of serum il- <dig> concentrations using a commercial elisa kit with canine-specific monoclonal antibodies.
blood collection, centrifugation to obtain sera, and laboratory techniques for il- <dig> measurement were performed as previously described by martins et al.
<cit> .
statistical analysis
data were analysed with the kolmogorov-smirnov test for normally distributed data using the software package sigma plot v <dig>  .
physiological parameters were analysed by analysis of variance  for repeated measurements, followed by the tukey test.
pain and sedation scores were compared at different times by friedman’s test, followed by the tukey test.
groups were compared by the non-parametric kruskal-wallis test, followed by a post-hoc dunn’s test.
a significance level of p <  <dig>  was used, and values are presented as the mean ± sd.
RESULTS
the animals ranged in age from  <dig> to 7 years, with a mean weight  of  <dig>   kg.
the mean surgery time and time to extubation did not differ significantly between groups .
a significant difference was found in heart rate , respiratory rate , and diastolic blood pressure  2 hours after premedication administration in all groups compared with baseline values .
this result occurred because this time point was evaluated during surgery when all animals were under general anaesthesia with a volatile anaesthetic agent .table  <dig>
mean surgery and extubation times by treatment group
group
surgery time*
extubation time*
tra
met  <dig>
met  <dig>
*measured in minutes.
phisyological parameters measure during time points by treatment group
#
time tbl
point t1
t2
t4
t6
t24
hr
tra
met  <dig>
met  <dig>
rr
tra
met  <dig>
met  <dig>
sap
tra
met  <dig>
met  <dig>
map
tra
met  <dig>
met  <dig>
dap
tra
met  <dig>
met  <dig>
#
hr: heart rate in beats per minute; rr: respiratory rate in breaths per minute; sap, map and dap: systolic, mean and diastolic arterial pressures in mmhg.
*:differs from tbl; a: differs from met  <dig> , b: differs from met  <dig> ; p <  <dig> .
none of the animals required ephedrine treatment for hypotension.
no values were lower than those considered normal under these conditions, except for the value in one animal in the met <dig>  group that had a hr ≤  <dig> beats per minute  and required an intravenous atropine sulphate injection .
groups that received methadone treatment had lower mean hr values compared with those in the tramadol treatment group, although these differences were not significant.
the mean rr values decreased after premedication compared with those at baseline in all groups .
sedation scores did not differ between groups or time points, with mean values lower than  <dig> at every evaluation .
there were no significant differences between groups in the colorado pain scale scores, and there were no mean values greater than  <dig> .
in the tra group, vas scores were increased at t <dig> compared with those at tbl, t <dig>  and t <dig> .
in the met <dig>  group, there was a significant increase at t <dig> compared with at t <dig> .
no significant difference was found within the met <dig>  group, which presented the lowest mean vas scores at t <dig> , while the mean scores in the tra and met <dig>  groups were  <dig>  ±  <dig>  and  <dig>  ±  <dig> , respectively  .table  <dig>
sedation scores* by treatmetn group - median
group
tbl
t1
t4
t6
t24
tra
met  <dig>
met  <dig>
*according to valverde et al.
<cit> .
colorado university pain scale scores by treatment group - median
group
tbl
t1
t4
t6
t24
tra
met  <dig>
met  <dig> mean ± sd vas scores measured at timepoints by treatment groups.
dogs in the met <dig>  group also had lower scores on the modified gcps , indicating better pain control compared with that provided by the other analgesic treatments  at t <dig> .
six animals in the tra group required rescue analgesia , as did one animal in the met <dig>  group at t <dig>  pain control was found to be adequate thereafter, with no animal requiring further medication .
treatment groups did not differ significantly with regard to serum il- <dig> levels.
an increase was observed in all groups compared with baseline values, although this increase was not significant.
discussion
this study evaluated and compared the effects of methadone and tramadol on postoperative pain and il- <dig> levels in dogs undergoing orthopaedic surgery.
vas and modified glasgow composite measure pain scale scores showed that methadone yielded better pain control.
analgesia improved with an increased dosage increment, suggesting that methadone-induced analgesia is dose-dependent.
nevertheless, all treatments had effects on il- <dig> by controlling its systemic release after surgery.
adequate pain control is a major concern in the perioperative period.
acute pain may delay recovery and wound healing, reduce mobilisation and food and water consumption, and evolve to chronic postsurgical pain if not treated properly.
it is well known that opioid administration prior to surgery reduces postoperative pain scores and analgesic drug requirements.
methadone was recently introduced into routine hospital use as a treatment for heroin addicts.
however, it has been shown that it may also have significant effects on acute, chronic, oncologic, and neuropathic pain, making it an excellent alternative to the use of morphine, as methadone causes fewer side effects and has a multi-action profile, acting as a mu-opioid receptor agonist and an nmda receptor antagonist  <cit> .
a control  group was not included in this study because it is unethical to perform orthopaedic surgery that has the possibly of causing a high degree of pain without analgesic administration.
the authors are aware that the absence of a control group may decrease the confidence in the results.
tramadol was chosen as a positive control for methadone because it is a well-known drug for acute pain control in small animals, providing the same analgesic effect as morphine in equipotent doses, and because of its immunomodulatory properties  <cit> .
animals in the groups treated with methadone exhibited higher sedation scores than those treated with tramadol, in accordance with previous data  <cit> .
in this study, tranquilisers were not administered as part of the premedication regimen due to the aim of achieving a lighter degree of sedation to reduce interference with the pain evaluation.
although sedation scores were higher in the methadone groups, no significant differences in these scores were observed.
methadone is claimed to promote a more intense degree of sedation than tramadol.
the degree of sedation resulting from tramadol administration is less intense, even when acepromazine is co-administered  <cit> , but this result has already been described in dogs  <cit> .
however, isolated use of opioids is recommended, especially in animals that can be easily handled, because the degree of sedation in this study did not exceed  <dig>  on the sedation scale.
respiratory depression may be induced by the use of opioids, depending on the dosage and administration route  <cit> .
importantly, this effect was not observed in this study, which may be related to the intramuscular administration employed, which alters absorption kinetics and leads to fewer side effects, representing an advantage over morphine.
dysphoria was not observed in any of the groups.
this is a very interesting finding because dysphoria is a common side effect of opioids in dogs and also an effect observed in other species after tramadol administration  <cit> .
this result may be related to the dosage employed in this study or the presence of injury/pain.
in pain-free conscious dogs, garofalo et al.
<cit>  reported the presence of dysphoria in all animals after 1 mg/kg of methadone was intravenously administered.
in previous studies where tramadol was used for the control of postoperative pain after ovariohysterectomy, mandibulectomy, and maxillectomy, dysphoria was not observed  <cit> .
no other adverse effects were observed in any of the treatment groups during the observation period.
however, eight animals in the tra group exhibited intense salivation after surgery, possibly due to nausea  <cit> .
postoperative nausea and vomiting are a major concern associated with human anaesthesia and are described to be as limiting as pain by the patients  <cit> .
these symptoms are described in humans after the intravenous injection of tramadol and are related to prolonged administration.
none of the animals in this study presented any other side effects during the observation period.
the absence of vomiting after tramadol and methadone administration in the pre-anaesthetic period is considered, in our view, an important advantage of these agents.
morphine, oxymorphone, and hydromorphone are frequently utilised as premedications in dogs and cause a high incidence of vomiting  <cit> , which may lead to aspiration pneumonia  <cit> .
in humans, vomiting is considered one of the main anaesthetic complications and is associated with a high incidence of intubation and acute respiratory distress syndrome   <cit> .
in veterinary medicine, there are few published reports of this post-anaesthetic complication, although it has recently been shown to have a much higher incidence in animals  than in humans   <cit> .
regarding pain assessment, animals received rescue analgesia if any of the pain scale scores were increased.
the scores for all animals returned to baseline levels after opioid dose adjustment , demonstrating that the effects of opioid analgesia were dose-dependent under the study conditions.
there was a slight correlation between the three pain scales.
the modified glasgow composite measure pain scale showed a higher association with patients’ clinical statuses.
this result most likely occurred because this scale allows for a more detailed assessment of behaviours, making it suitable as a measure of pain in clinical situations  <cit> .
acute injuries, such as surgery, have been reported to cause an early response with cytokine production.
however, this response in not related to the duration of surgery and is localised to the site of injury.
il- <dig> levels show a high association with pain and are related to pain intensity  <cit> .
a recent study evaluating il- <dig> levels in dogs with oral cancer revealed a baseline il- <dig> value of 100 pg/ml and a mean peak level of 140 pg/ml after surgery during the observation period.
in other studies comparing the il- <dig> levels of healthy and leishmania-positive dogs, the authors found a higher value of  <dig>  pg/ml in infected dogs  <cit> .
in the present study, there was a similar increase in il- <dig> levels in all treatment groups, representing an immune response to surgical stimulation, with mean values under 185 pg/ml.
these findings are in accordance with studies evaluating il- <dig> levels in dogs undergoing laparoscopic surgery, in which values increased above baseline  <dig> to 6 hours after surgery, with mean values not exceeding 100 pg/ml  <cit> .
CONCLUSIONS
in conclusion, methadone administered as premedication at a dose of  <dig>  mg/kg had a better effect on pain control after orthopaedic surgery compared with a lower dosage of the same drug and with tramadol.
however, the dosage increases, even tramadol increases, administered as rescue analgesia promoted adequate pain control.
the influence of the two analgesics on il- <dig> release could not be demonstrated in the present data, but significantly high levels of il- <dig> were not found.
additional file
additional file 1:
colorado state university canine acute pain scale.
competing interests
the authors declare that they have no competing interests.
authors’ contributions
lbc, dao, and dtf conceived and designed the experiment.
lbc, lcc, and mfcir performed the experiment.
mapk performed all the laboratory analyses and analysed the data.
lbc and dtf prepared the manuscript.
craf performed all the surgeries.
all authors read and approved the final manuscript.

 postoperative pain management is a major concern and influences recovery time, quality of life, and surgical outcome. acute pain alters metabolic and hemodynamic systems, facilitates the release of the inflammatory cascade, and triggers a sympathetic stress response. serum values of cortisol, glucose, and interleukins may provide important information, in association with clinical parameters and pain scales, in the study of different pain conditions and proposed treatments in dogs. pro-inflammatory cytokines, such as interleukin-, show a high association with acute pain because they sensitise the nervous system. il- levels in plasma are detectable 60 minutes after injury, peaking at 4- hours, and are proportional to tissue injury. an adequate approach to postoperative pain control is preventive analgesia. it has been suggested that the administration of non-steroidal anti-inflammatory drugs, opioids, or local anaesthetics prior to injury reduces central sensitisation, analgesic consumption, and the use of rescue analgesia. opioids are commonly used for sedation and the treatment of postoperative pain in small animals. although it is well known that they may cause immune suppression in humans by helping to control the release of inflammatory substances, there is little information regarding opioid action on the canine immune system. methadone is an opioid that has recently been introduced into routine veterinary medicine practice, and its analgesic effects have been of great interest. it acts as a mu-opioid receptor agonist and as an n-methyl-d-aspartate receptor antagonist. additionally, methadone is considered to be as powerful as morphine but causes fewer side effects, such as nausea, vomiting, and dysphoria. tramadol is a well-known synthetic opioid with low mu-opioid agonist affinity, and its efficacy is due to two different mechanisms, namely, it interacts with mu-opioid receptors and has effects on serotonin and norepinephrine reuptake. o-desmethyl tramadol, a product of tramadol hepatic metabolism, is known to present a higher affinity for mu receptors. it is also believed to have an immunostimulatory effect, as it decreased il- levels in a model of incisional pain in rats. the aim of the present study was to evaluate the effects of methadone and tramadol on postoperative pain in dogs undergoing tibial plateau levelling osteotomy and their influence on serum il- levels. methods animals twenty-eight client-owned dogs admitted to the orthopedics and trauma laboratory that presented with cranial cruciate ligament ruptures were included in the study. all the animals were judged as healthy based on physical examinations, complete blood cell counts, and serum biochemical analyses before surgery. animals presenting with any alterations in these parameters were excluded from the study, as were those given any type of analgesic drug prior to the study. owners of the eligible dogs agreed to their inclusion in the study by signing a consent agreement form. food and water were withheld for 12 hours prior to surgery. experimental design a complete physical evaluation was performed for all animals before any other intervention. pain scores were assessed by several scales, and the colorado university canine acute pain scale) at baseline for later comparison with the postoperative period. blood samples were collected for the assessment of il- levels using a specific kit for canine serum before premedication ) and after surgery. animals were randomly assigned to three different groups as follows: the tra group received 4 mg/kg of tramadol; the met group received  mg/kg of methadone; and the met group received  mg/kg of methadone. all drugs were injected intramuscularly as premedication. the study was approved by the ethical committee of faculdade de medicina and faculdade de medicina veterinária e zootecnia from the universidade de são paulo. procedure sixty minutes after premedication, the cephalic vein was catheterised for fluid administration at a rate of 10 ml/kg/h and for drug infusions. anaesthesia was induced with iv propofol to achieve the desired effects, and intubation was then performed with an adequately sized endotracheal tube. general anaesthesia was maintained with isoflurane in 100% oxygen under positive pressure ventilation in a circle rebreathing system, with the respiratory rate adjusted to maintain the end-tidal concentration of carbon dioxide between and 45 mmhg. heart and respiratory rates, direct arterial blood pressure, and oxygen saturation were monitored by a multiparameter monitor. a 20-gauge catheter was aseptically placed on the dorsal pedal artery and connected to the monitor’s pressure transducer with zero reference pressure at the level of the heart. end-tidal concentrations of isoflurane and carbon dioxide were monitored by a gas analyser and capnograph, with the etiso initially set at %. the gas analyser was calibrated before each experiment with a standard gas mixture. in the case of hypotension, ephedrine was administered. one bolus of fentanyl was administered intravenously just before starting the osteotomy. whenever there was a change in heart rate or blood pressure greater than 20% compared with pre-stimulation values, a % increase in isoflurane was administered. temperature was maintained between and 38°c with the use of a warming blanket. an 18-gauge catheter was aseptically placed in a jugular vein to facilitate subsequent blood sample collections. all surgeries were performed by the same experienced surgeon. sedation scores were assessed 30 minutes after premedication injection and at and 3 hours after surgery. a scale proposed by valverde et al., ranging from to was used for this purpose, with 0 = no effect, 1 = mild effect, 2 = moderate effect, and 3 = severe effect. physiological parameters were recorded at,,,, and hours after premedication. pain was assessed at the same time points after analgesic injection, except at t using the vas and modified glasgow composite measure pain scale, each of which ranged from to where indicates no pain and indicates the worst pain possible. the colorado university canine acute pain scale was also used and ranged from to with indicating minimal pain and indicating severe pain. all parameters were assessed by the same observer, who was unaware of the analgesic treatment and has experience in pain evaluation. if scores for the vas, colorado, and glasgow scales were higher than and respectively, rescue analgesia was administered as follows to reduce the influence of other analgesic drugs on pain and il- levels: 1 mg/kg of tramadol for the tra group and  mg/kg of methadone for the met and met groups. animals were re-evaluated 20 minutes after the administration of rescue medication, and if signs of pain persisted,  mg/kg of morphine was administered and the animal was withdrawn from the study. the animals were observed after surgery, sent home, and then returned 24 hours later for evaluation. the occurrence of any adverse effects, such as nausea, vomiting, dysphoria, mydriasis, or intense salivation, was recorded, as well as the number of animals requiring rescue analgesia. nsaids were not administered during the evaluation period, but carprofen was prescribed for 10 days postoperatively, as is usually recommended by surgeons. blood collection was performed at tbl, t t and t for measurement of serum il- concentrations using a commercial elisa kit with canine-specific monoclonal antibodies. blood collection, centrifugation to obtain sera, and laboratory techniques for il- measurement were performed as previously described by martins et al.. statistical analysis data were analysed with the kolmogorov-smirnov test for normally distributed data using the software package sigma plot v. physiological parameters were analysed by analysis of variance for repeated measurements, followed by the tukey test. pain and sedation scores were compared at different times by friedman’s test, followed by the tukey test. groups were compared by the non-parametric kruskal-wallis test, followed by a post-hoc dunn’s test. a significance level of p <  was used, and values are presented as the mean ± sd.  the animals ranged in age from to 7 years, with a mean weight of kg. the mean surgery time and time to extubation did not differ significantly between groups. a significant difference was found in heart rate, respiratory rate, and diastolic blood pressure 2 hours after premedication administration in all groups compared with baseline values. this result occurred because this time point was evaluated during surgery when all animals were under general anaesthesia with a volatile anaesthetic agent.table mean surgery and extubation times by treatment group group surgery time* extubation time* tra met met *measured in minutes. phisyological parameters measure during time points by treatment group # time tbl point t1 t2 t4 t6 t24 hr tra met met rr tra met met sap tra met met map tra met met dap tra met met # hr: heart rate in beats per minute; rr: respiratory rate in breaths per minute; sap, map and dap: systolic, mean and diastolic arterial pressures in mmhg. *:differs from tbl; a: differs from met, b: differs from met; p < . none of the animals required ephedrine treatment for hypotension. no values were lower than those considered normal under these conditions, except for the value in one animal in the met group that had a hr ≤  beats per minute and required an intravenous atropine sulphate injection. groups that received methadone treatment had lower mean hr values compared with those in the tramadol treatment group, although these differences were not significant. the mean rr values decreased after premedication compared with those at baseline in all groups. sedation scores did not differ between groups or time points, with mean values lower than at every evaluation. there were no significant differences between groups in the colorado pain scale scores, and there were no mean values greater than. in the tra group, vas scores were increased at t compared with those at tbl, t and t. in the met group, there was a significant increase at t compared with at t. no significant difference was found within the met group, which presented the lowest mean vas scores at t, while the mean scores in the tra and met groups were  ±  and  ± , respectively.table sedation scores* by treatmetn group - median group tbl t1 t4 t6 t24 tra met met *according to valverde et al.. colorado university pain scale scores by treatment group - median group tbl t1 t4 t6 t24 tra met met mean ± sd vas scores measured at timepoints by treatment groups. dogs in the met group also had lower scores on the modified gcps, indicating better pain control compared with that provided by the other analgesic treatments at t. six animals in the tra group required rescue analgesia, as did one animal in the met group at t pain control was found to be adequate thereafter, with no animal requiring further medication. treatment groups did not differ significantly with regard to serum il- levels. an increase was observed in all groups compared with baseline values, although this increase was not significant. discussion this study evaluated and compared the effects of methadone and tramadol on postoperative pain and il- levels in dogs undergoing orthopaedic surgery. vas and modified glasgow composite measure pain scale scores showed that methadone yielded better pain control. analgesia improved with an increased dosage increment, suggesting that methadone-induced analgesia is dose-dependent. nevertheless, all treatments had effects on il- by controlling its systemic release after surgery. adequate pain control is a major concern in the perioperative period. acute pain may delay recovery and wound healing, reduce mobilisation and food and water consumption, and evolve to chronic postsurgical pain if not treated properly. it is well known that opioid administration prior to surgery reduces postoperative pain scores and analgesic drug requirements. methadone was recently introduced into routine hospital use as a treatment for heroin addicts. however, it has been shown that it may also have significant effects on acute, chronic, oncologic, and neuropathic pain, making it an excellent alternative to the use of morphine, as methadone causes fewer side effects and has a multi-action profile, acting as a mu-opioid receptor agonist and an nmda receptor antagonist. a control group was not included in this study because it is unethical to perform orthopaedic surgery that has the possibly of causing a high degree of pain without analgesic administration. the authors are aware that the absence of a control group may decrease the confidence in the results. tramadol was chosen as a positive control for methadone because it is a well-known drug for acute pain control in small animals, providing the same analgesic effect as morphine in equipotent doses, and because of its immunomodulatory properties. animals in the groups treated with methadone exhibited higher sedation scores than those treated with tramadol, in accordance with previous data. in this study, tranquilisers were not administered as part of the premedication regimen due to the aim of achieving a lighter degree of sedation to reduce interference with the pain evaluation. although sedation scores were higher in the methadone groups, no significant differences in these scores were observed. methadone is claimed to promote a more intense degree of sedation than tramadol. the degree of sedation resulting from tramadol administration is less intense, even when acepromazine is co-administered, but this result has already been described in dogs. however, isolated use of opioids is recommended, especially in animals that can be easily handled, because the degree of sedation in this study did not exceed on the sedation scale. respiratory depression may be induced by the use of opioids, depending on the dosage and administration route. importantly, this effect was not observed in this study, which may be related to the intramuscular administration employed, which alters absorption kinetics and leads to fewer side effects, representing an advantage over morphine. dysphoria was not observed in any of the groups. this is a very interesting finding because dysphoria is a common side effect of opioids in dogs and also an effect observed in other species after tramadol administration. this result may be related to the dosage employed in this study or the presence of injury/pain. in pain-free conscious dogs, garofalo et al. reported the presence of dysphoria in all animals after 1 mg/kg of methadone was intravenously administered. in previous studies where tramadol was used for the control of postoperative pain after ovariohysterectomy, mandibulectomy, and maxillectomy, dysphoria was not observed. no other adverse effects were observed in any of the treatment groups during the observation period. however, eight animals in the tra group exhibited intense salivation after surgery, possibly due to nausea. postoperative nausea and vomiting are a major concern associated with human anaesthesia and are described to be as limiting as pain by the patients. these symptoms are described in humans after the intravenous injection of tramadol and are related to prolonged administration. none of the animals in this study presented any other side effects during the observation period. the absence of vomiting after tramadol and methadone administration in the pre-anaesthetic period is considered, in our view, an important advantage of these agents. morphine, oxymorphone, and hydromorphone are frequently utilised as premedications in dogs and cause a high incidence of vomiting, which may lead to aspiration pneumonia. in humans, vomiting is considered one of the main anaesthetic complications and is associated with a high incidence of intubation and acute respiratory distress syndrome. in veterinary medicine, there are few published reports of this post-anaesthetic complication, although it has recently been shown to have a much higher incidence in animals than in humans. regarding pain assessment, animals received rescue analgesia if any of the pain scale scores were increased. the scores for all animals returned to baseline levels after opioid dose adjustment, demonstrating that the effects of opioid analgesia were dose-dependent under the study conditions. there was a slight correlation between the three pain scales. the modified glasgow composite measure pain scale showed a higher association with patients’ clinical statuses. this result most likely occurred because this scale allows for a more detailed assessment of behaviours, making it suitable as a measure of pain in clinical situations. acute injuries, such as surgery, have been reported to cause an early response with cytokine production. however, this response in not related to the duration of surgery and is localised to the site of injury. il- levels show a high association with pain and are related to pain intensity. a recent study evaluating il- levels in dogs with oral cancer revealed a baseline il- value of 100 pg/ml and a mean peak level of 140 pg/ml after surgery during the observation period. in other studies comparing the il- levels of healthy and leishmania-positive dogs, the authors found a higher value of  pg/ml in infected dogs. in the present study, there was a similar increase in il- levels in all treatment groups, representing an immune response to surgical stimulation, with mean values under 185 pg/ml. these findings are in accordance with studies evaluating il- levels in dogs undergoing laparoscopic surgery, in which values increased above baseline to 6 hours after surgery, with mean values not exceeding 100 pg/ml.  in conclusion, methadone administered as premedication at a dose of  mg/kg had a better effect on pain control after orthopaedic surgery compared with a lower dosage of the same drug and with tramadol. however, the dosage increases, even tramadol increases, administered as rescue analgesia promoted adequate pain control. the influence of the two analgesics on il- release could not be demonstrated in the present data, but significantly high levels of il- were not found. additional file additional file 1: colorado state university canine acute pain scale. competing interests the authors declare that they have no competing interests. authors’ contributions lbc, dao, and dtf conceived and designed the experiment. lbc, lcc, and mfcir performed the experiment. mapk performed all the laboratory analyses and analysed the data. lbc and dtf prepared the manuscript. craf performed all the surgeries. all authors read and approved the final manuscript. 
blood samples for measurement of interleukin - were collected at tbl, t t and t this was a prospective, randomised investigation to evaluate the efficacy of tramadol and methadone as premedications in dogs undergoing osteotomies. heart rate, respiratory rate, and blood pressure values were maintained within acceptable ranges, and a reduction was observed at t in met and met compared with tbl. tra showed greater demand of rescue analgesia. however, dosage increases, administered as rescue analgesia, promoted adequate pain control even in tramadol group. there were no statistically significant differences between groups with respect to age, weight, gender, surgery time, and time to extubation. there were no statistically significant differences in sedation scores, colorado scale scores, or interleukin levels between groups and time points. increases in vas scores were observed in tra at t compared with tbl, t and t and between t and t. tra and met showed significantly higher mean ± sd vas scores than met at t. pain scores were described by visual analogue scale, modified glasgow composite, and colorado university acute pain scales. acute postsurgical pain is of great interest due to potential risk of becoming chronic if not treated properly, worsening patient’s recovery and quality of life. physiological parameters were evaluated at specified times: baseline,,,,, and hours after premedication. in met, there was significant increase in vas score at t compared with t.
 acute postsurgical pain is of great interest due to potential risk of becoming chronic if not treated properly, worsening patient’s recovery and quality of life. twenty-eight dogs with ruptured cruciate ligaments were divided into three groups that received intramuscular injections of 4 mg/kg of tramadol,  mg/kg of methadone, or  mg/kg of methadone. physiological parameters were evaluated at specified times: baseline,,,,, and hours after premedication. pain scores were described by visual analogue scale, modified glasgow composite, and colorado university acute pain scales. blood samples for measurement of interleukin - were collected at tbl, t t and t this was a prospective, randomised investigation to evaluate the efficacy of tramadol and methadone as premedications in dogs undergoing osteotomies.  there were no statistically significant differences between groups with respect to age, weight, gender, surgery time, and time to extubation. heart rate, respiratory rate, and blood pressure values were maintained within acceptable ranges, and a reduction was observed at t in met and met compared with tbl. increases in vas scores were observed in tra at t compared with tbl, t and t and between t and t. in met, there was significant increase in vas score at t compared with t. tra and met showed significantly higher mean ± sd vas scores than met at t. tra showed greater demand of rescue analgesia. there were no statistically significant differences in sedation scores, colorado scale scores, or interleukin levels between groups and time points.  methadone given as premedication in doses of  mg/kg was better at controlling pain compared with lower doses and tramadol. however, dosage increases, administered as rescue analgesia, promoted adequate pain control even in tramadol group. influence of these analgesics on il- release could not be demonstrated, but significant levels were not found. electronic supplementary material the online version of this article contains supplementary material, which is available to authorized users. issue-copyright-statement© the author 2014 