Dr. Lloyd E. Davis1 wrote in 1983 that
[o]ne of the psychological curiosities of therapeutic decision making is the withholding of analgesic drugs, because the clinician is not absolutely certain that the animal is experiencing pain. Yet the same individual will administer antibiotics without documenting the presence of a bacterial infection. Pain and suffering constitute the only situation in which I believe that, if in doubt, one should go ahead and treat.
Yet, treatment of acute pain, particularly postoperative pain, in animals has historically received inadequate attention from veterinarians and their staff members. Between 1983 and 1989, for instance, only 40% of dogs hospitalized after surgery at a veterinary teaching hospital received analgesics, despite the extensive nature of some of the procedures performed, including limb amputation, limb-sparing surgery, thoracotomy, cervical vertebral stabilization, and fracture repair.2 Dogs undergoing so-called routine surgeries such as castration and OHE have traditionally been even less likely to receive pain medications. A survey of clinical use of analgesics by Canadian veterinarians published in 1996,3 for instance, reported that only 10.5% of dogs undergoing castration and 12.6% of dogs undergoing OHE received analgesics. An Australian survey also published in 19964 reported that analgesics were used in approximately 4% and 6% of dogs undergoing castration or OHE, respectively. In more recent years, additional attention has been focused on pain management in the veterinary curriculum and veterinary literature, which may have increased both awareness of the need for analgesia and the frequency with which analgesics are administered. A British survey reported in 1999,5 for example, found that approximately 32% and 53% of veterinarians would administer analgesics to dogs undergoing castration or OHE, respectively.5 However, a French survey published in 20046 indicated that only 17.2% and 36.3% of dogs received analgesics after castration and OHE, respectively.
Considering that veterinary patients cannot verbalize their feelings of discomfort, it is not surprising that many animals in pain go untreated. It is possible that certain veterinarians are not concerned about animal pain, but it is more likely that many veterinarians choose not to administer analgesics simply because they believe that their patients are not in pain. The authors have been told by well-meaning practitioners that these practitioners can complete routine surgical procedures such as castration and OHE so quickly and with so little tissue trauma that there is little inflammation and, consequently, little postoperative pain. However, the stated perception that dogs undergoing routine surgical procedures performed by experienced surgeons are not in as much pain as those undergoing the same procedures performed by inexperienced surgeons, such as veterinary students, requires investigation. Although it is likely that duration of surgery and degree of tissue trauma vary substantially with degree of surgeon experience, the effect these differences have on degree of postoperative pain is less clear and it may be that dogs undergoing castration or OHE by experienced surgeons are actually experiencing more pain than is being perceived by their human caregivers.
The study reported here was designed to determine whether claims that dogs do not experience pain following routine castration or OHE performed by an experienced surgeon can be justified. Specifically, the purposes of the study reported here were to evaluate the degree of postoperative pain in dogs undergoing elective castration or OHE at various veterinary hospitals; determine whether degree of postoperative pain varied significantly with surgeon experience or the amount of tissue trauma estimated on the basis of length of the skin incision and duration of the procedure; determine whether administration of morphine or nalbuphine prior to surgery or administration of ketoprofen after surgery decreased the incidence of postoperative pain-related behaviors; and determine whether amount of tissue trauma or analgesic administration (none, morphine, nalbuphine, or ketoprofen) correlated with owners' perceptions of their dogs' comfort during the first 3 days after surgery. Our hypotheses were that castration and OHE induced postoperative pain in dogs and that the degree of postoperative pain would be related to surgeon experience, length of the skin incision, and duration of surgery, regardless of whether analgesics were administered and individual patient characteristics.
Materials and Methods
The study protocol was approved by the Institutional Animal Care and Use Committee at Colorado State University. Study procedures were conducted at the Harrison Memorial Animal Hospital, Denver; the Humane Society of Boulder Valley, Boulder, Colo; and the Denver Dumb Friends League, Denver. Healthy dogs that underwent elective OHE or castration at any of the 3 study locations were eligible for inclusion in the study. Owner consent was obtained for all dogs enrolled in the study.
Sex, breed, and body weight of all dogs enrolled in the study were recorded, along with preoperative heart rate, respiratory rate, and body temperature. Additional preoperative evaluations were performed as dictated according to each study location's usual standard of practice.
For each of the study locations, previously established protocols were used for anesthetic induction and maintenance. All dogs were premedicated with acepromazine (0.025 mg/kg [0.011 mg/lb], SC). Anesthesia was induced with diazepam (0.28 mg/kg [0.13 mg/lb], IV) and ketamine (5.6 mg/kg [2.5 mg/lb], IV) and maintained with isoflurane in oxygen; all dogs were intubated. Because diazepam became unavailable for a short time during the study, in some dogs, anesthesia was induced with tiletamine and zolazepam (1 to 2 mg/kg [0.45 to 0.9 mg/lb], IV). Glycopyrrolate (0.01 to 0.02 mg/kg [0.0045 to 0.009 mg/lb], IV or SC) was administered during anesthesia at the discretion of the anesthetist to prevent or correct bradycardia.
In each dog, OHE or castration was performed by an experienced clinical veterinarian or a fourth-year veterinary student working under the direct supervision of a veterinarian. Experienced clinical veterinarians who participated in the study were not board-certified surgeons, but all had been practicing for at least 5 years at the time of the study, and each typically performed between 30 and 35 surgical procedures each week. No attempts were made to further quantify the skill or expertise of experienced clinical veterinarians who participated in the study. Duration of anesthesia, length of the skin incision, and duration of surgery from initial skin incision to final closure were recorded.
Dogs were randomly assigned to 1 of 4 groups. Dogs in the control group did not receive any perioperative analgesics. Dogs in the morphine group were given a single dose of morphine (0.5 mg/kg [0.23 mg/ lb], SC) 30 minutes prior to the induction of anesthesia. Dogs in the nalbuphine group were given a single dose of nalbuphine (0.5 mg/kg, SC) 30 minutes prior to induction of anesthesia. Dogs in the ketoprofen group were given a single dose of ketoprofen (2 mg/kg, SC) at the time of extubation. For 1 of the 3 participating study locations, the standard protocol at the time of the present study was to not administer analgesics to dogs undergoing routine OHE or castration, and only those dogs that underwent surgery at this location were eligible for assignment to the control group. None of the dogs at the other 2 study locations were assigned to the control group, and analgesics were not withheld from any dog for purposes of the present study.
A pain evaluation form modified from the University of Melbourne Pain Scale7 and the Glasgow Composite Pain Tool8 was used to assess perioperative pain (Appendix). For all dogs at all 3 study locations, all pain scores were assigned by a single individual (GAW) who was blinded to analgesic treatment. Pain scores were assigned prior to anesthesia (baseline), at the time of extubation following surgery (time 0), and 30, 60, 90, 120, 180, and 240 minutes after extubation. Potential overall pain scores ranged from 0 (least painful) to 20 (most painful). Because most dogs enrolled in the study were discharged on the day of surgery, 4 hours was the practical limit for assignment of pain scores after surgery. Ketoprofen (2 mg/kg, IM) was administered to any dog with a pain score ≥ 10 at any of the postoperative evaluation times. In addition, ketoprofen was administered at the discretion of the attending veterinarians. Owners of dogs enrolled in the study were asked to record a score for their dogs' appetite, activity level, and degree of wound soreness daily for the first 3 days after surgery.
Statistical analysis—To test for possible preexisting differences among groups, baseline heart rate, respiratory rate, temperature, and body weight were analyzed by means of a restricted, maximum likelihood– based, mixed-effect model that included the categoric fixed effects of treatment, sex, and study location (individual hospital) and the random effect of individual dog. The Kenward-Roger approximation was used to estimate denominator degrees of freedom for tests of fixed effects. To assess pain score outcomes as evaluated by the single blinded observer, pain score data for each time period were entered as the change (increase or decrease) in score from the baseline (preanesthesia) value, and the significance and magnitude of treatment group differences were assessed by means of a restricted, maximum likelihood–based, mixed-effect model for repeated measures. Analyses of pain scores included the categoric fixed effects of group, observation time, surgeon (experienced veterinarian vs student), and sex (OHE vs castration); the continuous fixed effects of anesthesia time, surgery time, incision length, and baseline pain score; and the interactions of group by observation time and baseline pain score by observation time. An unstructured correlation was used to model within-subject errors. The Kenward-Roger approximation was used to estimate denominator degrees of freedom for all analyses. Analyses of scores for comfort, vocalization, movement, and response to palpation included overall comparisons (main and interaction effects), contrasts at each time point within each treatment group to test whether the within-group change from baseline was significant, and contrasts among the treatment groups at each time point. The main effect of treatment used data from all time points to assess whether a consistent difference existed among treatment groups. The treatment-by-time interaction assessed whether differences among treatment groups varied over time. A subset of the data was used to evaluate the categoric fixed effects of study site and anesthesia type (ketamine and diazepam vs tiletamine and zolazepam) on outcome variables. A subset was used because not all treatment groups were represented at each site, and both anesthesia methods were not used at all locations. Breed effects were evaluated with another subset of the data and contained the same fixed effects as those used in the analysis of site; breeds represented by at least 2 dogs were included in this analysis. To assess owner evaluation outcomes, the significance and magnitude of treatment group differences on client survey outcomes were assessed by means of a restricted, maximum likelihood– based, mixed-effect model. Analyses of owner evaluations included the categoric fixed effects of group, day after surgery, sex, and site; the continuous fixed effects of anesthesia time, surgery time, and incision length; and the interaction of treatment group by day after surgery. An unstructured correlation was used to model within-subject errors and to account for relationships of measurements over time. The Kenward-Roger approximation was used to estimate denominator degrees of freedom. All analyses were performed with standard software9,a and were performed as 2-tailed tests. Values of P ≤ 0.05 were considered significant.
Results
A total of 426 dogs were enrolled in the study, including 44 dogs in the control group, 144 dogs in the morphine group, 119 dogs in the nalbuphine group, and 119 dogs in the ketoprofen group. Of the 426 dogs, 186 (43.6%) underwent castration and 240 (56.4%) underwent OHE. Experienced clinical veterinarians performed 337 (79.1%) of the procedures, and fourth-year veterinary students performed 89 (20.9%). There was no significant effect of study location (individual hospital) on any outcome. There were no significant differences among treatment groups in regard to baseline heart rate, respiratory rate, temperature, or body weight. Labrador Retrievers, Golden Retrievers, Labrador Retriever crosses, Australian Shepherd crosses, Beagles, Chow Chow crosses, German Shepherd Dog crosses, and terrier crosses were the most common; there was no significant effect of breed on any outcome.
Overall pain scores were not significantly different between dogs that underwent castration and dogs that underwent OHE. Therefore, data for all dogs were combined for subsequent statistical analyses. For the experienced clinical veterinarians, mean ± SD anesthesia time was 28 ± 11 minutes and mean surgery time was 16 ± 7 minutes. For the fourth-year veterinary students, mean anesthesia time was 43 ± 24 minutes and mean surgery time was 31 ± 23 minutes. There was no significant effect of duration of surgery or surgeon experience (experienced veterinarian vs veterinary student) on any outcome. Mean ± SD incision length was 3.2 ± 1.3 cm for the experienced clinical veterinarians and 4.1 ± 1.7 cm for the veterinary students (values for dogs that underwent castration were combined with values for dogs that underwent OHE). Incision length had a significant (P = 0.006) effect on scores for response to palpation, but did not have a significant effect on overall pain score or scores for movement, comfort, or vocalization. Anesthetics used for induction (ketamine and diazepam vs tiletamine and zolazepam) had no effect on comparisons among treatment groups.
In all 4 treatment groups, overall pain scores were significantly higher after surgery, compared with baseline (ie, preanesthesia) scores (Table 1). There were also significant differences among groups in regard to the magnitude of the increase in overall pain score after surgery. At all time points, dogs in the control group had significantly greater increases in overall pain score than did dogs in the other 3 treatment groups. The increase in overall pain score at time 0 (ie, at the time of extubation) was significantly lower for dogs in the morphine group than for dogs in the other 3 groups, and mean increases in overall pain score for dogs in the morphine group were among the lowest at all time points. Mean increases in overall pain score for dogs in the ketoprofen group were among the lowest from 30 through 240 minutes after surgery, and mean increase in overall pain score for dogs in the nalbuphine group typically were between mean values for dogs in the control group and values for dogs in the morphine and ketoprofen groups.
Mean ± SD increase in overall pain score, compared with baseline (ie, preanesthesia) score, in 426 dogs undergoing routine OHE or castration that did not receive perioperative analgesic treatment (n = 44), received morphine (0.5 mg/kg [0.23 mg/lb], SC; 144) 30 minutes prior to the induction of anesthesia, received nalbuphine (0.5 mg/kg, SC; 119) 30 minutes prior to induction of anesthesia, or received ketoprofen (2 mg/kg [0.9 mg/lb], SC; 119) at the time of extubation.
| Analgesic treatment | Time after extubation (min) | ||||||
|---|---|---|---|---|---|---|---|
| 0 | 30 | 60 | 90 | 120 | 180 | 240 | |
| None | 8.04 ± 3.61a | 6.72 ± 2.79a | 5.31 ± 2.1a | 4.74 ± 2.14a | 4.30 ± 2.24a | 3.93 ± 2.39a | 2.66 ± 2.17a,b |
| Morphine | 3.77 ± 3.27c | 4.62 ± 2.55c | 4.26 ± 2.60b | 4.02 ± 2.33a,b | 3.35 ± 2.20b,c | 2.62 ± 1.96c | 2.18 ± 1.38b,c |
| Nalbuphine | 5.18 ± 3.96b | 5.64 ± 3.42b | 4.64 ± 2.61a,b | 4.30 ± 2.59a | 3.69 ± 2.02a,b | 3.14 ± 1.97b | 2.70 ± 1.97a |
| Ketoprofen | 5.69 ± 4.01b | 4.32 ± 2.43c | 4.04 ± 2.47b | 3.57 ± 2.22b | 3.02 ± 2.15c | 2.15 ± 1.61c | 2.01 ± 1.52c |
Potential overall pain scores ranged from 0 (least painful) to 20 (most painful; Appendix). All values were significantly (P < 0.001) different from 0 (ie, mean assigned scores were significantly different from baseline values).
Within each time period, groups that do not share at least 1 superscript letter were significantly (P < 0.05) different.
Scores for response to palpation, movement, vocalization, and comfort were also all significantly higher after surgery, compared with baseline values, in all 4 groups (Tables 2–5), and there were significant differences among groups in regard to increases in scores for response to palpation and movement. Dogs in the control group had significantly higher increases in scores for response to palpation of the surgical site than did dogs in the morphine and ketoprofen groups at nearly all time points and higher increases in scores for response to palpation than did dogs in the nalbuphine group at 0, 90, 180, and 240 minutes. Increases in scores for movement for dogs in the morphine group were significantly lower than increases in scores for all other groups at time 0 and for control dogs at 0, 30, 120, and 180 minutes. When sex differences were evaluated, females (OHE) were significantly more likely to vocalize (P = 0.001) and to respond to palpation of the surgical site (P = 0.028) than were males (castration). Only 10 of the 426 (2.3%) dogs were given rescue analgesia, including 6 dogs in the morphine group and 4 dogs in the nalbuphine group.
Mean ± SD increase in score for response to palpation of the surgical site, compared with baseline (ie, preanesthesia) score, in 426 dogs undergoing routine OHE or castration.
| Analgesic treatment | Time after extubation (min) | ||||||
|---|---|---|---|---|---|---|---|
| 0 | 30 | 60 | 90 | 120 | 180 | 240 | |
| None | 1.56 ± 1.23a | 1.52 ± 1.04a | 1.73 ± 1.12a | 1.88 ± 1.17a | 1.62 ± 1.28a | 1.54 ± 1.24a | 1.41 ± 1.24a |
| Morphine | 0.59 ± 0.95c | 0.98 ± 0.98b | 1.32 ± 1.13b | 1.27 ± 1.09b | 0.99 ± 1.00b | 0.84 ± 0.93c | 0.88 ± 0.89b |
| Nalbuphine | 1.11 ± 1.21b | 1.53 ± 1.18a | 1.38 ± 1.14ab | 1.25 ± 1.11b | 1.27 ± 1.08a | 1.11 ± 1.03b | 0.97 ± 0.93b |
| Ketoprofen | 1.04 ± 1.18b | 1.02 ± 0.97b | 0.96 ± 0.88c | 1.11 ± 1.00b | 1.16 ± 1.05a,b | 0.82 ± 0.89c | 1.05 ± 1.01b |
Potential scores ranged from 0 to 3, where 0 = no response to palpation; 1 = looking at wound or appearing anxious; 2 = crying or flinching; and 3 = snapping, growling, or guarding wound. All values were significantly (P < 0.001) different from 0 (ie, mean assigned scores were significantly different from baseline values).
See Table 1 for key.
Mean ± SD increase in score for movement, compared with baseline (ie, preanesthesia) score, in 426 dogs undergoing routine OHE or castration.
| Analgesic treatment | Time after extubation (min) | ||||||
|---|---|---|---|---|---|---|---|
| 0 | 30 | 60 | 90 | 120 | 180 | 240 | |
| None | 1.49 ± 1.28a | 0.77 ± 0.90a | 0.33 ± 0.61a | 0.20 ± 0.48a | 0.23 ± 0.53a | 0.16 ± 0.50a | 0.03 ± 0.00b |
| Morphine | 0.81 ± 1.19b | 0.33 ± 0.75c | 0.17 ± 0.51a | 0.10 ± 0.37a | 0.07 ± 0.36b | 0.04 ± 0.28b | 0.01 ± 0.15b |
| Nalbuphine | 1.19 ± 1.22a | 0.55 ± 0.87ab | 0.28 ± 0.64a | 0.14 ± 0.42a | 0.08 ± 0.32b | 0.15 ± 0.47a | 0.10 ± 0.36a |
| Ketoprofen | 1.41 ± 1.33a | 0.41 ± 0.74b,c | 0.31 ± 0.60a | 0.13 ± 0.42a | 0.06 ± 0.25b | 0.00 ± 0.09b | 0.02 ± 0.16b |
Potential scores ranged from 0 to 2, where 0 = no abnormalities of movement; 1 = stiff or ataxic; and 2 = slow or reluctant to rise or sit, orlame. All values were significantly (P < 0.001) different from 0 (ie, mean assigned scores were significantly different from baseline values).
See Table 1 for key.
Mean ± SD increase in score for vocalization, compared with baseline (ie, preanesthesia) score, in 426 dogs undergoing routine OHE or castration.
| Analgesic treatment | Time after extubation (min) | ||||||
|---|---|---|---|---|---|---|---|
| 0 | 30 | 60 | 90 | 120 | 180 | 240 | |
| None | 0.80 ± 1.24 | 0.12 ± 0.51 | 0.28 ± 0.70 | 0.15 ± 0.54 | 0.29 ± 0.74 | 0.33 ± 0.75 | 0.10 ± 0.35 |
| Morphine | 0.52 ± 1.07 | 0.43 ± 0.84 | 0.17 ± 0.63 | 0.27 ± 0.72 | 0.21 ± 0.74 | 0.16 ± 0.55 | 0.13 ± 0.48 |
| Nalbuphine | 0.55 ± 1.08 | 0.35 ± 0.89 | 0.32 ± 0.86 | 0.40 ± 0.81 | 0.32 ± 0.75 | 0.31 ± 0.71 | 0.26 ± 0.66 |
| Ketoprofen | 0.53 ± 1.19 | 0.22 ± 0.59 | 0.36 ± 0.73 | 0.23 ± 0.62 | 0.16 ± 0.53 | 0.09 ± 0.41 | 0.11 ± 0.41 |
Potential scores ranged from 0 to 4, where 0 = not vocalizing; 1 = barking (if abnormal); 2 = crying or whimpering; 3 = groaning; and 4 = screaming. All values were significantly (P < 0.001) different from 0 (ie, mean assigned scores were significantly different from baseline values). There were no significant (P = 0.121) differences among groups at any time point.
Mean ± SD increase in score for comfort, compared with baseline (ie, preanesthesia) score, in 426 dogs undergoing routine OHE or castration.
| Analgesic treatment | Time after extubation (min) | ||||||
|---|---|---|---|---|---|---|---|
| 0 | 30 | 60 | 90 | 120 | 180 | 240 | |
| None | 0.01 ± 0.00 | 0.86 ± 0.93 | 0.71 ± 0.95 | 0.37 ± 0.72 | 0.37 ± 0.75 | 0.33 ± 0.75 | 0.13 ± 0.49 |
| Morphine | 0.06 ± 0.35 | 0.47 ± 0.81 | 0.68 ± 0.86 | 0.79 ± 0.88 | 0.54 ± 0.84 | 0.28 ± 0.66 | 0.09 ± 0.40 |
| Nalbuphine | 0.07 ± 0.33 | 0.65 ± 0.85 | 0.84 ± 0.89 | 0.73 ± 0.89 | 0.54 ± 0.82 | 0.27 ± 0.64 | 0.21 ± 0.60 |
| Ketoprofen | 0.04 ± 0.24 | 0.68 ± 0.88 | 0.59 ± 0.79 | 0.41 ± 0.75 | 0.42 ± 0.83 | 0.17 ± 0.57 | 0.11 ± 0.46 |
Potential scores ranged from 0 to 3, where 0 = at rest or comfortable; 1 = restless; 2 = uncomfortable; and 3 = rolling or thrashing. All values were significantly (P < 0.001) different from 0(ie, mean assigned scores were significantly different from baseline values). There were no significant (P = 0.104) differences among groups at any time point.
Owners of 151 of the 426 (35.4%) dogs completed and returned the owner evaluation form. There were significant differences among treatment groups in regard to owner-assigned scores for appetite (Table 6) and activity (Table 7) but not in regard to scores for wound soreness (Table 8). On the first day at home after surgery, dogs in the ketoprofen group ate more normally than did dogs in the other groups and were more active than dogs in the control or morphine groups. Dogs in the nalbuphine group were more active than dogs in the control group. On the third day after surgery, dogs in the morphine, nalbuphine, and ketoprofen groups had lower (ie, more normal) activity scores than did dogs in the control group. There were significant treatment-by-time interactions for appetite and activity, such that dogs in the ketoprofen group ate more normally than did dogs in the other treatment groups on day 1, but by day 3, dogs in the morphine group ate more normally than did dogs in the control group, with appetite scores for dogs in the nalbuphine and ketoprofen groups being intermediate and not significantly different from scores for either the control or the morphine group.
Mean ± SD owner-assigned scores for appetite during the first 3 days after surgery in 151 dogs undergoing routine OHE or castration.
| Analgesic treatment | Time after surgery (d) | ||
|---|---|---|---|
| 1 | 2 | 3 | |
| None | 3.65 ± 1.11a | 2.53 ± 1.12a | 2.41 ± 0.82a |
| Morphine | 3.28 ± 1.20a | 2.54 ± 0.98a | 2.00 ± 0.42b |
| Nalbuphine | 3.55 ± 1.24a | 2.50 ± 1.08a | 2.22 ± 0.78a,b |
| Ketoprofen | 2.65 ± 1.24b | 2.30 ± 0.75a | 2.18 ± 0.67a,b |
Potential scores ranged from 1 to 5, where 1 = ate more than normal; 2 = ate about normal; 3 = ate about normal with coaxing; 4 = ate less than normal but did eat something (may include something other than normal food); and 5 = did not eat at all. Scores decreased significantly (P < 0.001) over time.
Within each day, groups that do not share at least 1 superscript letter were significantly (P < 0.05) different.
Mean ± SD owner-assigned scores for activity during the first 3 days after surgery in 151 dogs undergoing routine OHE or castration.
| Analgesic treatment | Time after surgery (d) | ||
|---|---|---|---|
| 1 | 2 | 3 | |
| None | 3.65 ± 0.86a | 2.65 ± 0.81a | 2.29 ± 0.74a |
| Morphine | 3.28 ± 0.89a,b | 2.42 ± 0.78a | 1.96 ± 0.38b |
| Nalbuphine | 3.01 ± 0.99b,c | 2.48 ± 0.91a | 1.99 ± 0.62b |
| Ketoprofen | 2.82 ± 0.98c | 2.27 ± 0.71a | 2.02 ± 0.39b |
Potential scores ranged from 1 to 5, where 1 = more aggressive or restless than normal; 2 = about normal; 3 = slightly less active than normal; 4= less active than normal and appears somewhat depressed; and 5 = appears depressed, unresponsive, or avoids contact. Scores decreased significantly (P < 0.001) over time.
Within each day, groups that do not share at least 1 superscript letter were significantly (P < 0.05) different.
Mean ± SD owner-assigned scores for degree of wound soreness during the first 3 days after surgery in 151 dogs undergoing routine OHE or castration.
| Analgesic treatment | Time after surgery (d) | ||
|---|---|---|---|
| 1 | 2 | 3 | |
| None | 1.65 ± 0.64 | 1.41 ± 0.58 | 1.17 ± 0.37 |
| Morphine | 1.39 ± 0.60 | 1.17 ± 0.38 | 1.11 ± 0.32 |
| Nalbuphine | 1.57 ± 0.70 | 1.37 ± 0.49 | 1.20 ± 0.41 |
| Ketoprofen | 1.48 ± 0.58 | 1.23 ± 0.42 | 1.10 ± 0.31 |
Potential scores ranged from 1 to 3, where 1 = does not appear particularly sore or protective of wound; 2 = seems slightly sore or avoids touch if possible; and 3 = seems very sore or vocalizes or tries to bite if area is touched. Scores decreased significantly (P < 0.001) overtime. There were no significant (P = 0.199) differences among groups.
Discussion
Results of the present study suggested that dogs had evidence of behaviors suggestive of pain following OHE or castration. Analgesic treatment mitigated the expression of pain-related behaviors, with morphine and ketoprofen providing better pain relief than nalbuphine. Factors that did not have any significant effect on the frequency or severity of pain-related behaviors included dog breed, study location, anesthetic induction protocol, duration of surgery, or surgeon experience. Although length of the surgical incision had a significant effect on scores for response to palpation of the surgical site, it did not significantly affect overall pain score or scores for movement, comfort, or vocalization.
A weakness of the present study was that a nonsurgical control group that received anesthesia only without surgery was not included. Because the study involved client-owned animals brought to the participating veterinary hospitals for neutering, inclusion of such a negative control group was not possible. However, the physiologic and behavioral characteristics of dogs undergoing anesthesia with or without OHE have been evaluated previously.10,11 One of these previous studies11 reported that OHE resulted in an increase in pain score, an increase in time spent asleep, and a decrease in greeting behaviors, compared with anesthesia without surgery. Additionally, although behavioral signs were different between dogs that underwent OHE with or without analgesic treatment, routinely monitored physiologic signs (heart rate and body temperature) failed to indicate either a stress response to OHE or an analgesic effect of oxymorphone.10,11 These findings suggest that evaluation of behaviors is more helpful than recording routine vital signs in assessing patient comfort after surgery.
Many publications have described pain in animals and its treatment; however, determining whether any individual animal is in pain is still difficult. In dogs, signs of pain can range from subtle changes (eg, reduced activity or dilated pupils) to extreme, overt demonstrations (eg, vocalizing and thrashing). Subtle pain-related signs are easily overlooked, particularly in a busy practice setting. If there are no overt demonstrations of pain that compel a practitioner to administer treatment, analgesics may be withheld. If, on the other hand, practitioners could be convinced that measurable levels of pain occur in dogs following routine surgical procedures in private practice, they may be more likely to adopt an approach of treating pain regardless of whether overt clinical signs are apparent. The present study provided evidence that behaviors suggestive of pain occur following neutering in dogs and can be detected by an observer with the help of a relatively simple evaluation form.
Some veterinarians do not administer drugs for pain relief following routine surgical procedures, such as OHE and castration, partly because they do not believe the dogs are in pain and partly because of concerns about the adverse effects of analgesics, particularly opioids.12 At the time of the present study, one of the participating hospitals did not routinely administer analgesics to dogs undergoing OHE or castration, and all dogs in the control group underwent surgery at this hospital. None of the control dogs received rescue analgesia, indicating that the threshold for rescue analgesia in the present study (overall pain score ≥ 10 on a scale from 0 to 20) was too high. Nevertheless, the study protocol allowed for rescue analgesia to be administered at the discretion of the attending veterinarian even if overall pain score was < 10. Because it was not routine for veterinarians at the hospital where control dogs underwent surgery to prescribe analgesics for animals undergoing OHE or castration, it was not surprising that rescue analgesia was not prescribed for any of the study dogs. Subsequent to the present study, this hospital modified its policy, such that all animals undergoing surgical procedures now receive perioperative analgesics.
Inadequate treatment of acute postoperative pain is a major problem in both human and veterinary medicine. In 2001, all accredited human health-care organizations were required by the Joint Commission on Accreditation of Healthcare Organizations to comply with the organization's pain assessment and management standards.13 Several veterinary organizations, including the American College of Veterinary Anesthesiologists14 and the American Animal Hospital Association,15 have recommended that analgesics be administered whenever there is even a possibility that an animal might be experiencing pain, which includes all surgical procedures.
Results of the present study suggested that OHE and castration resulted in similar degrees of pain regardless of whether the procedure was performed by an experienced clinical veterinarian or a fourth-year veterinary student and irrespective of the duration of surgery. Length of incision did have a significant effect on scores for response to palpation of the surgical site, but did not significantly affect any other pain-related behaviors or overall pain score. It is certainly possible that an experienced surgeon causes less tissue trauma and inflammation than does an inexperienced one. However, our findings suggested that this is not reflected as a difference in behaviors associated with postoperative pain. Therefore, advanced surgical experience should not be used as an excuse for lack of analgesic treatment.
The present study also confirmed that the frequency or severity of pain-related behaviors associated with neutering in dogs could be mitigated by administration of analgesics before or after surgery. At most of the time points evaluated, dogs that did not receive any analgesics (control group) had significantly higher increases in overall pain scores and scores for response to palpation of the surgical site than did dogs that received analgesics. The analgesics used in the present study were selected because they are readily available and relatively inexpensive and therefore likely to be acceptable for use in small animal veterinary practices and low-cost neutering clinics. Morphine is the prototype mu-receptor agonist opioid and is one of the most common analgesics used in small animal practices. A dose of 0.5 mg/kg, SC, was selected because this is considered to be a mid-range dose that should be effective for mild to moderate pain.16 Nalbuphine is a mixed kappa-receptor agonist and mureceptor antagonist opioid that, in the United States, is generally less expensive than butorphanol and, unlike many other opioids, is not considered a scheduled substance by the US Drug Enforcement Agency. The dose of 0.5 mg/kg, SC, was selected because nalbuphine is reported to be similar in potency to morphine.16 However, despite its potential advantages of low cost and lack of Drug Enforcement Agency regulation, nalbuphine is not considered to be as efficacious for relief of moderate to severe pain as is morphine.b For the present study, morphine and nalbuphine were administered prior to anesthesia and surgery, as is typical in many practices, to augment preanesthetic sedation and provide preemptive analgesia. Ketoprofen, a nonsteroidal anti-inflammatory drug that inhibits both cyclooxygenase 1 and 2, was administered at the end of anesthesia, as has been recommended by some authors,17 to avoid potential adverse effects associated with nonsteroidal anti-inflammatory drugs, such as increased hemorrhage and altered renal vascular tone. The dose of ketoprofen used (2 mg/kg, SC) is in the recommended dose range.18 Waiting until extubation to administer ketoprofen delayed the onset of its analgesic effects. Nevertheless, by 30 minutes after extubation, dogs treated with ketoprofen had overall pain scores similar to those of dogs in the morphine group and significantly lower than scores for control dogs. Under the conditions of the present study, preoperative administration of morphine and postoperative administration of ketoprofen seemed most efficacious at reducing pain-related behaviors, but preoperative administration of nalbuphine was better than no treatment at all. There may be other analgesic drugs or multiple drug combinations that would be even more effective than these.
The present study confirms that dogs undergoing socalled routine OHE or castration do evidence pain-related behaviors that can be identified both in a practice setting and in the home environment; that the degree of pain is not dependent on the surgeon's experience, incision length, or duration of surgery; and that analgesic treatment reduces the severity of pain. Therefore, all dogs undergoing these surgical procedures should receive medications for pain, regardless of the experience or speed of the surgeon.
ABBREVIATIONS
| OHE | Ovariohysterectomy |
SAS, version 8, SAS Institute Inc, Cary, NC.
Sawyer DC, Rech RH. Ceiling effect for analgesia by butorphanol and nalbuphine (abstr). Vet Surg 1986;15:462.
References
- 1.↑
Davis LE. Species differences in drug disposition as factors in alleviation of pain. In: Kitchell RL, Erickson HH, eds. Animal pain. Bethesda, Md: American Physiological Society, 1983;175.
- 2.↑
Hansen B, Hardie E. Prescription and use of analgesics in dogs and cats in a veterinary teaching hospital: 258 cases (1983–1989). J Am Vet Med Assoc 1993;202:1485–1494.
- 3.↑
Dohoo SE, Dohoo IR. Postoperative use of analgesics in dogs and cats by Canadian veterinarians. Can Vet J 1996;37:546–551.
- 4.↑
Watson ADJ, Nicholson A, Church DB, et al. Use of anti-inflammatory and analgesic drugs in dogs and cats. Aust Vet J 1996;74:203–210.
- 5.↑
Capner CA, Lascelles BDX, Waterman-Pearson AE. Current British veterinary attitudes to perioperative analgesia for dogs. Vet Rec 1999;145:95–99.
- 6.↑
Hugonnard M, Leblond A, Keroack S, et al. Attitudes and concerns of French veterinarians towards pain and analgesia in dogs and cats. Vet Anaesth Analg 2004;31:154–163.
- 7.↑
Firth AM, Haldane SL. Development of a scale to evaluate postoperative pain in dogs. J Am Vet Med Assoc 1999;214:651–658.
- 8.↑
Holton L, Reid J, Scott EM, et al. Development of a behaviour-based scale to measure acute pain in dogs. Vet Rec 2001;148:525–531.
- 10.
Hansen BD, Hardie EM, Carroll GW. Physiological measurements after ovariohysterectomy in dogs: what's normal? Appl Anim Behav Sci 1997;51:101–109.
- 11.↑
Hardie EM, Hansen BD, Carroll GS. Behavior after ovariohysterectomy in the dog: what's normal? Appl Anim Behav Sci 1997;51:111–128.
- 12.↑
Dohoo SE, Dohoo IR. Factors influencing the postoperative use of analgesics in dogs and cats by Canadian veterinarians. Can Vet J 1996;37:552–556.
- 13.↑
Gilbert HC, Green CR. JCAHO's pain initiative—new opportunities/new risks. Am Soc Anesthesiologists Newsl 2000;64:6–7.
- 14.↑
American College of Veterinary Anesthesiologists' position paper on the treatment of pain in animals. J Am Vet Med Assoc 1998;213:628–630.
- 15.↑
American Animal Hospital Association pain management standards, 2003. Lakewood, Colo: American Animal Hospital Association, 2003.
- 16.↑
Wagner AE. Opioids. In: Gaynor JS, Muir WW, eds. Handbook of veterinary pain management. St Louis: Mosby, 2002;164–183.
- 17.↑
Deneuche AJ, Dufayet C, Goby L, et al. Analgesic comparison of meloxicam or ketoprofen for orthopedic surgery in dogs. Vet Surg 2004;33:650–660.
Criteria used to assign pain scores in dogs undergoing routine OHE or castration.
| Variable | Score | Criteria |
|---|---|---|
| Posture | 0 | Normal |
| 1 | Rigid | |
| 2 | Hunched or tense | |
| 3 | Abnormal | |
| 4 | Guarding affected area | |
| Comfort level | 0 | At rest or comfortable |
| 1 | Restless | |
| 2 | Uncomfortable | |
| 3 | Rolling or thrashing | |
| Pupils | 0 | Normal |
| 1 | Dilated | |
| Salivation | 0 | No |
| 1 | Yes | |
| Vomiting | 0 | No |
| 1 | Yes | |
| Vocalization | 0 | Not vocalizing |
| 1 | Barking (if abnormal) | |
| 2 | Crying or whimpering | |
| 3 | Groaning | |
| 4 | Screaming | |
| Mental status | 0 | Unchanged |
| 1 | Changed | |
| Movement | 0 | No abnormalities |
| 1 | Stiff or ataxic | |
| 2 | Slow or reluctant to rise or sit, or lame | |
| Response to palpation | 0 | No response |
| 1 | Looking at wound or appearing anxious | |
| 2 | Crying or flinching | |
| 3 | Snapping, growling, or guarding wound |
Overall pain score was calculated by summing scores for the 9 individual pain-related behaviors. Potential overall pain scores ranged from 0 (least painful) to 20 (most painful).
