The influence of pre-anaesthetic administration of buprenorphine on

The influence of pre-anaesthetic administration of buprenorphine on the anaesthetic effects of ketaminejmedetomidine and pentobarbitone in rats and the consequences of repeated anaesthesia J.

v.

Roughan 1,

o.

Burzaco Ojeda2 & P. A. Flecknell'

lComparative Biology Centre, Medical School, Framlington Place, University of Newcastle upon Tyne, Newcastle upon Tyne NE2 4HH, UK and 2Surgical Teaching Hospital, Faculty of Veterinary Medicine, University of Zaragoza, Spain

Summary Rats received pentobarbitone (60, 48 and 36mg/kg i.p.) or ketamine/medetomidine (75/100,60/80 and 45/60mg/llg/kg i.p.l alone, or one hour following buprenorphine (0.5 mg/kg s.c.). Animals were anaesthetized once per week for 6 weeks with one of three anaesthetic doses according to a randomized block design. In the pentobarbitone group, animals which received buprenorphine had longer sleep times (236 ± 22 d. 204 ± 21 min) and longer durations of surgical anaesthesia (83± 14 d. 27 ± 8 min) (P

~

Dumtion Surgical

S81inel60

Ii

e

0..

Salinel48 Saline136 50

100 150 200 Duration (minutes)

250

o

300

Fig 1 Mean duration of surgical anaesthesia, latency to first movement and total sleep time for animals which received buprenorphine or saline prior to one of three doses of pentobarbitone (60, 48 and 36 mg/kg). Data were averaged over all anaesthesia days. Note that in comparison with animals that received saline, those pre-medicated with buprenorphine were more deeply anaesthetized at equivalent doses of pentobarbitone (saline group, n = 12, buprenorphine group, n = 12)

thesia was a significant increase in total sleep time, the effect again being co-dependant on the dose of pentobarbitone (PO.OS). Table 3 indicates the numbers of occasions on which animals attained surgical planes of

p'barb (mglkg) Pre-treat

36

48 Saline

60

48

36

60

Buprenorphine

Fig 2 Mean pedal withdrawal scores ( + SEM)averaged over the first 14 time samples during anaesthesia and all anaesthesia days for each combination of pre-treatment with saline/buprenorphine and each of three doses of pentobarbitone (60, 48 and 36 mg/kg). Note that animals which received buprenorphine were less responsive, and also that mean pedal withdrawal scores were progressively reduced with increasing pentobarbitone dose (saline group, n = 12, buprenorphine group, n = 12)

anaesthesia for each combination of pretreatment and pentobarbitone dose (data pooled over all anaesthesia days). Animals which received buprenorphine attained a surgical plane of anaesthesia on a greater number of occasions. None of the animals required assisted ventilation or oxygen administration.

Table 1 The mean total sleep time, latency to first spontaneous movement, duration of surgical anaesthesia, recovery time and respiratory rate (± SEM)for animals receiving each combination of pre-treatment with saline/buprenorphine and subsequent pentobarbitone dose (60, 48 and 36 mg/kg) Buprenorphinepre-treated (0.05mg/kg s.c.) Salinepre-treated (0.2ml/100g s.c.) Pentobarbitone dose

Total sleep time (min) Latencyto movement (min) 5urgicalanaesthesia (min) Recoverytime Respiratoryrate

High (60mg/kg)

Medium (48mg/kg)

Low (36mg/kg)

High (60mg/kg)

Medium (48mg/kg)

Low (36mg/kg)

293±42 247±42 122±30 100±24 75±5

236±40 196±37 68±25 76±22 81±6

180±24 149±25 60±14 51±18 73±5

252±39 152±36 46±14 46±18 92±7

198±37 122±24 35±16 40± 12 87±7

161±29 111±20 O±O 31±12 82±5

Data are meanscalculatedover all anaesthesiadays (recoverytime and respiratoryrate used samples1-14)(salinegroup,

n = 12,buprenorphinegroup, n= 12) Laboratory Animals

(1999)33

Pre-anaesthetic use of opioids in combination Table 2 The effect of repeated anaesthesia anaesthetic treatment (buprenorphine/saline)

with ketamine/medetomidine

239

and pentobarbitone

on total sleep time for animals receiving each combination and dose of pentobarbitone (60, 48 and 36 mg/kg

of pre-

Anaesthesia day Pentobarbitone

2

4

3 Buprenorphine

60mg/kg 48mg/kg 36mg/kg

232±29 187±60 x±x

292±92 x±x 161 ±46

x±x 173±5 190±4

5

6

421±11 X±X 159±71

X±X 240±85 210±52

240±87 x±x 187±69

X±X 280±87 143±83

pre-treated 226±63 341 ± 11 X±X

Saline pre-treated 60mg/kg 48mg/kg 36mg/kg

181 ±85 93±22 X±X

252±56 x±x 183±52

X±X 254±8 131 ±5

334±97 165±68 X±X

Data are averages for all animals anaesthetized each day (saline group, n= 12, buprenorphine group, n= 12). X denotes missing data (dose combination not administered that day) Table 3 The total number of occasions over all anaesthesia days when animals were judged to have attained a plane of surgical anaesthesia (pedal withdrawal score = 1) following administration of each combination of pre-anaesthetic treatment (buprenorphine/saline) and dose of pentobarbitone (60, 48 and 36 mg/kg) Buprenorphine pre-treated (0.05 mg/kg s.c.)

Saline pre-treated (0.2 ml/1 00 9 s.c.) Pentobarbitone

Surgical anaesthesia No surgical anaesthesia

dose

High (60mg/kg)

Medium (48mg/kg)

Low (36mg/kg)

High (6Omg/kg)

Medium (48mg/kg)

Low (36mg/kg)

9 3

8 4

9 3

9 3

5 7

0 12

Ketamine / medetomidine Rats were anaesthetized without incident on two occasions. During the third anaesthetic, one animal died (buprenorphine/low-dose ketamine/medetomidine), and two rats in the same dose group became cyanotic and received atipamezole. Three animals were added to the ketamine/medetomidine group to replace these animals. Two of these animals died, one after being anaesthetized with low-dose ketamine/medetomidine and buprenorphine, the other after high-dose anaesthetic and no buprenorphine. The other rat received low-dose ketamine/medetomidine and no buprenorphine, and required reversal with atipamezole. One of the original group died after receiving high-dose ketamine/ medetomidine and buprenorphine, and another which received low-dose anaesthetic and no buprenorphine requil'ed reversal with

atipamezole. During the fifth anaesthetic session, five animals (two high-dose with buprenorphine, two high-dose with no buprenorphine, and one low-dose with buprenorphinel required reversal with atipamezole. One animal (low-dose, no buprenorphine) died. Animals which died despite attempts at resuscitation developed dyspnoea associated with clinical signs of acute pulmonary oedema, the presence of which was confirmed on postmortem examination.

Discussion Ketamine has been widely used for anaesthesia of laboratory rats, most frequently in combination with xylazine. Ketamine combined with medetomidine, a more selective alpharadrenoreceptor agonist, has become Laboratory Animals (1999) 33

240

established as a popular anaesthetic regimen, primarily because of the availability of atipamezole, a highly specific alpha2-adrenoreceptor antagonist lFleckneIl19961. Since ketamine has minimal anaesthetic effects when used alone in rodents, the reversal of medetomidine results in a rapid, marked reduction in anaesthetic depth and the shortening of recovery times. Pentobarbitone has long been used as an anaesthetic for rodents, despite its poor analgesic qualities and narrow safety margin lField et ai. 1993). The dose rates of the anaesthetics selected for study' were intended to provide light to medium planes of anaesthesia in the strain of rat used, and this was confirmed in the pilot study. Noxious stimuli have been shown to cause C-fibre induced excitability changes in the spinal cord. These changes result in a lowering of the pain threshold. If morphine is administered before noxious stimulation, the dose required to block these changes is an order of magnitude lower than if morphine is given after the changes have occurred IWoolfe & Wall 1986). The prevention of sensitization by surgical stimuli is the basis for the clinical application of pre-emptive analgesia, and although opinion is currently divided concerning the practical relevance of this technique in man (McQuay et al. 1988, Dahl et al. 1992, 1993, Ejlerson et al. 1992, Zacharias et al. 19931, it has been strongly recommended in veterinary and laboratory animal anaesthetic practice (Lascelles et al. 1995, 1997). If preoperative administration of analgesics provides more effective pain relief than postoperative administration, then it is clearly desirable to adopt this regimen. However, it is important to establish 'whether clinically significant interactions occur between the analgesic agent used and the anaesthetic regimen. Of particular concern is the possibility of increased respiratory depression, and of prolongation of recovery times. Conversely, the use of preoperative analgesics may enable a reduction of the required dose of other anaesthetic agents, in effect contributing an analgesic component to a balanced anaesthetic regimen. In the present study, preoperative administration of the partial 11 opioid agonist, Laboratory Animals (1999) 33

Roughan, Burzaco Ojeda & Flecknell

buprenorphlne, had an additive effect on pentobarbitone anaesthesia. Data indicated that recommended doses of pentobarbitone can be reduced by at least 20% when buprenorphine is used as a pre-anaesthetic medication. As a consequence of anaesthetic complications and the mortality in animals which received ketamine/medetomidine, formal statistical data analysis could not be undertaken, but inspection of the data suggested that an additive effect of buprenorphine, similar to that seen with pentobarbitone occurred. The administration of buprenorphine significantly improved the reliability of pentobarbitone anaesthesia. Over all occasions when pentobarbitone was administered alone, animals attained a surgical plane of anaesthesia (as judged by the complete loss of the pedal withdrawal response) on 14 occasions, in comparison to 26 in those that received buprenorphine (Table 3). The administration of buprenorphine had a small but significant effect on respiratory rate which was not judged to be of clinical significance. The most reliable means of measuring the effects on respiratory function would have been by arterial blood gas analysis, but this would have required invasive surgery, and was not considered justifiable in these initial investigations. Attempts to measure oxygen saturation using pulse oximetry were not successful, which in the case of animals anaesthetized with medetomidine, was probably due to the marked peripheral vasoconstriction produced (Golden et al. 1998). Although major effects were not detected, it should be noted that at the dose rate used, buprenorphine has only very minor respiratory depressant effects when administered alone to normal animals, and care should be taken if full 11 agonists, such as morphine, are used. Although buprenorphine prolonged recovery times, inspection of Fig 1 shows that in the absence of buprenorphine, a pentobarbitone dose of 60 mg/kg is effective in producing approximately 45 min of surgical anaesthesia, whereas if buprenorphine is given, a lower dose of 36 mg/kg is sufficient to provide at least one hour of surgical anaesthesia in an identical proportion of animals/number of occasions 19/12llTable

Pre-anaesthetic use of opioids in combination with ketaminejmedetomidine

3). Comparing these equi-anaesthetic groups also indicates that the duration of surgical anaesthesia and the times of latency to first movement are comparable (Fig 1 and Table 1).

A serious and unexpected effect that emerged was a high incidence of anaesthetic deaths and respiratory arrests in rats given ketamine/medetomidine. This did not occur until mid-way through the study, and initially appeared to be associated with repeated anaesthesia, however, at the next anaesthetic session, three rats being anaesthetized for the first time also developed anaesthetic complications. The rats died of respiratory failure, with animals developing acute pulmonary oedema. Female rats appeared more susceptible to the effects of the agents used. Females showed a small but insignificantly greater total sleep time and latency to first movement, but a significantly greater degree of respiratory depression. Although the latter does not necessarily imply deeper anaesthesia, we suggest that this be taken into consideration when using this regimen. This sex variation is opposite to that reported with pentobarbitone in mice (Lovell 19861. All dose rates were calculated on a mg/kg basis, and allometric scaling was not used (Morris 1995). Had dose rates based on a mg/kgO.75 been employed, this would have led to increased dose rates being used in the smaller female rats, and this would probably have further increased the differences between the two sexes. In conclusion, the administration of buprenorphine had a clear anaesthetic-sparing effect, which would allow a subsequently administered dose of pentobarbitone to be reduced by approximately 20%; although when adopting this regimen, dose titration studies should be undertaken in the particular strain of rat which is to be used. In our experience, ketamine used in combination with medetomidine provides a safe and effective means of anaesthetizing rats. The unexpected mortality seen in the ketamine/ medetomidine group in the present study suggests that caution should be exercised when these drugs are administered either in combination with pre-anaesthetic opioid

and pentobarbitone

241

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