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medetomidine in the rabbit: influence of route of administration and the effect of combination with butorphanol
Veterinary Anaesthesia and Analgesia, 2002, 29, 14^19
R E S E A R C H PA P E R
Anaesthesia with ketamine/medetomidine in the rabbit: influence of route of administration and the effect of combination with butorphanol P Hedenqvist DVM, HE Orry BVMS, Cert LAS, JV Roughany & PA Flecknelly M.A Vet MB, PhD, DLAS, Dip ECVA, MRCVS
BSc, PhD,
LM Antunesy
MSc DVM
Veterinary Resources, Karolinska Institute, Stockholm, Sweden
yComparative Biology Centre, Medical School, Newcastle upon Tyne, UK
Correspondence: JV Roughan, Comparative Biology Centre, Medical School, Framlington Place, Newcastle upon Tyne NE2 4HH, UK.
Abstract Objective To compare the characteristics of anaesthesia induced with ketamine/medetomidine administered by the subcutaneous and intramuscular routes and to assess the e¡ects of the addition of butorphanol to this combination. Study design Prospective randomised study. Animals Six female New Zealand White rabbits. Methods Rabbits were given one of four combinations of ketamine and medetomidine (K/M) either subcutaneously (SC) or intramuscularly (IM) on four successive occasions with a 7-day interval between treatments. The dose combinations were; 15/0.25 mg kg1 SC; 15/0.25 mg kg1 IM; 15/0.5 mg kg1 SC, and 15/0.25 mg kg1 together with 0.4 mg kg1 butorphanol (K/M/B) SC. The e¡ects of anaesthesia on arterial blood gas values and cardiovascular variables were recorded at predetermined time points. Toe and ear pinch re£exes were judged to determine the duration of surgical anaesthesia. Loss of the righting re£ex was used to measure the duration of sleep time. Analyses used repeated measures analysis of variance. Results All groups lost the righting re£ex and ear pinch response. Three animals in the groups that received K/M alone lost their toe pinch re£ex, 14
whereas four lost this re£ex when given K/M/B. Time of onset of loss of the righting, toe and ear pinch re£exes did not di¡er signi¢cantly among the groups. The higher dose combination of medetomidine with ketamine and the combination of K/M/B produced a greater duration of loss of the ear pinch response than the lower dose of K/M administered by either route. No signi¢cant di¡erences were found among the groups in the duration of loss of the toe pinch re£ex. All animals developed a moderate bradycardia (mean heart rate <166 beats minute1) and moderatehypoxaemia(meanPaO2 < 6.0 kPa). Animalsgiven butorphanol showed the greatest reduction in respiratory rate (31 13 breaths minute1, p < 0.05) but this was not re£ected in any signi¢cant differences in arterial PCO2, PO2 or pH among the groups. Conclusions Administration of K/M by the SC route produced equivalent e¡ects in comparison to intramuscular administration. The addition of butorphanol increased the duration of anaesthesia, but produced a slight increase in the degree of respiratory depression. All dose rates resulted in hypoxaemia so oxygen should be administered when these combinations are used in rabbits. Clinical relevance Subcutaneous administration is both technically simpler and may cause less discomfort to the animal than IM injection, and so is preferred. The combination of K/M with butorphanol
Ketamine/medetomidine anaesthesia in the rabbit P Hedenqvist et al.
has relatively minor e¡ects on the depth and duration of anaesthesia, so o¡ers little advantage to the use of K/M alone.
rabbits were fed a commercial pelleted diet (Rabma pellets, SDS), autoclaved hay and water ad libitum.
Keywords anaesthesia, medetomidine, rabbit.
Anaesthesia
butorphanol,
ketamine,
Introduction Developing methods for safe and e¡ective anaesthesia has often been challenging in rabbits in which unacceptable intra- and postoperative mortality has occurred with a variety of anaesthetic techniques. The use of fentanyl/£uanisone/midazolam, although safe in comparison with some techniques, produces a relatively long recovery (Flecknell & Mitchell 1984). We recently evaluated the use of various dose combinations of ketamine and medetomidine (K/M) and found that it produces smooth induction and recovery in rabbits (Hedenqvist et al. 2001). The dose rates needed to produce surgical anaesthesia (15 mg kg1 ketamine, 0.25 mg kg1 medetomidine) require relatively large volumes of anaesthetic to be injected intramuscularly (IM) (0.4 mL kg1), which can cause discomfort to the animal. In addition, the combination produces marked respiratory depression. In other species, the dose of medetomidine and ketamine required to produce surgical anaesthesia can be reduced by combining these agents with butorphanol, a mixed agonist/antagonist opioid. This study compared subcutaneous (SC) administration of ketamine/medetomidine with IM injection, and also investigated the e¡ects of the addition of butorphanol.
Materials and methods All procedures were conducted in accordance with the guidelines set out in the Animals (Scienti¢c Procedures) Act 1986 (1989), under project license number PPL 60/2362. Animals Six female young adult New Zealand White (NZW) rabbits were used for the study (mean body mass SD 2391 71 g). They were obtained from a colony free from respiratory pathogens (Charles River UK) and group housed in £oor pens on dust-free shavings (‘Gold Shavings’, SDS, Whitham, UK) in a room with a 07:0019:00 hours light cycle. Room temperature was maintained at 20 38C, relative humidity at 50%, with 18 air changes per hour. The Veterinary Anaesthesia and Analgesia, 2002, 29, 14^19
Rabbits each received four anaesthetic combinations with a 1-week interval between successive anaesthetics, with the order of treatments allocated using a Latin square design. The combinations administered were low dose K/M (15/0.25 mg kg1) given either SC or IM, the low dose of ketamine combined with a higher dose of medetomidine given SC (15/ 0.5 mg kg1) or the lower dose K/M combination (15/ 0.25 mg kg1) given together with butorphanol (0.4 mg kg1) SC. All drugs were mixed in a single syringe for injection. Local anaesthetic cream (EMLA, Astra Pharmaceutical Ltd, Herts, UK) was applied to the rabbits’ ears 45 minutes before induction of anaesthesia to prevent pain during catheter placement (Flecknell et al. 1990). An ‘over-the-needle’ catheter (20 SWG, ‘Ven£on’ Jelco, Critikon, London, UK) was placed in the central ear artery and connected to a pressure transducer (Kontron ‘Supermon’, St Albans, Herts, UK) to monitor arterial blood pressure. The Kontron module was connected to a data capture system (1401 plus, CED, Cambridge, UK) for continuous computer recording. A thermostatically controlled heat-pad equipped with a rectal probe (Harvard Apparatus, Edinburgh, UK), set to 388C (the normal body temperature of the conscious rabbit at rest), was used to maintain body temperature during anaesthesia. After placement of the catheters, baseline (t ¼ 0) recordings of respiratory rate, mean arterial blood pressure (MAP) and heart rate were obtained for a 2 minute period. An arterial blood sample (0.5^ 1.0 mL) was then obtained for blood gas analysis (t ¼ 0 minutes) and the anaesthetic combination was administered. Depth of anaesthesia was assessed by recording the presence or absence of a righting response (the animal showing ability to spontaneously right itself after being placed on its back), an ear pinch response (shaking the head or vocalization when the pinna was pinched using the investigator’s ¢ngernails) and a pedal withdrawal response (PWR, withdrawal of the hind limb in response to pinching one digit with the investigator’s ¢ngernails). Simple eye ointment was administered in both eyes to prevent drying of the corneal surface. Recordings of heart rate and MAP were made for a 2 minute period prior to withdrawal of additional blood samples at 5, 10, 15, 30, 45, 60, 75, 90 and 120 minutes during 15
Ketamine/medetomidine anaesthesia in the rabbit P Hedenqvist et al.
anaesthesia for blood gas analysis (using a Stat Pro¢le 3, Nova Biomedical,Waltham, MA, USA). Monitoring was continued until the animal regained its righting re£ex. Total sleep time was measured as the latency from drug administration until return of the righting re£ex. At the end of the study, the rabbits were re-homed. Data processing and statistical analysis The e¡ects of each treatment according to the dose combination given, and the route of administration used was compared using repeated measures analysis of variance (ANOVA), using the computer program SPSS (SPSS Inc., Chicago, IL, USA). Overall group di¡erences were determined from the results of averaged univariate statistics, whereas further univariate comparisons determined speci¢c di¡erences between routes of administration. The results of these analyses (repeated measures one-way ANOVA) are presented in the form [F(df1,df2) ¼ X, p ¼ sig.], where df1 and df2 are the degrees of freedom of the numerator and the denominator, respectively.
Results
re£exes was not signi¢cantly di¡erent (p > 0.1) among the groups which received lower dose K/M either IM and SC (Table 1). The duration of loss of the ear pinch re£ex was signi¢cantly greater following high-dose ketamine and medetomidine (15/0.5 mg kg1) (76 22 minutes) than the lower dose (15/0.25 mg kg1) given either SC [41 23 minutes; F(1,5) ¼ 13.8, p ¼ 0.014] or IM [44 19 minutes; F(1,5) ¼ 13.9, p ¼ 0.014]. A similarly prolonged re£ex loss occurred when animals were given butorphanol together with lower dose K/M (78 30 minutes), this being signi¢cantly longer than obtain- ed with the lower SC dose of K/M alone [F(1,5) ¼ 11.4, p ¼ 0.02]. Total sleep time also tended to increase with the higher dose combination and in animals given butorphanol and K/M (107^109 minutes). This, however, was not signi¢cantly greater than the sleep time obtained with lower dose K/M administered by either route (79^86 minutes) (Table 1). The PWR was not lost uniformly in any of the groups. Three animals in each of the groups given only ketamine and medetomidine showed loss of this re£ex, whereas four animals receiving ketamine and medetomidine with butorphanol lost the PWR.These di¡erences were not signi¢cant (p > 0.1).
Depth of anaesthesia All groups lost the righting re£ex and ear pinch response. The time of loss of the righting and ear pinch re£exes did not di¡er signi¢cantly among the groups (p > 0.1) (Table 1). The duration of loss of these
Physiological variables Baseline blood gas analyses and arterial pressure measurements indicated that animals were physiologically normal prior to induction of anaesthesia
Table 1 The e¡ects upon autonomic re£ex responsiveness of either subcutaneously (SC) or intramuscularly (IM) administered ketamine/medetomidine (K/M), or SC administration (K/M) with butorphanol (B) in six New Zealand White rabbits. Sleep time was the duration of loss of righting re£ex.Values are mean 1 SD. Because not all animals lost their pedal withdrawal response, data from individual animals are presented
Drugs, doses (mg kg1)
Route Onset of loss of righting reflex (minutes) Duration of loss of ear pinch (minutes) Duration of loss of pedal withdrawal response (minutes) Sleep time (minutes)
K/M 15/0.25
K/M 15/0.25
K/M/B 15/0.25/0.4
K/M 15/0.5
IM 31
SC 3 0.4
SC 53
SC 32
44 19
41 23
78 30
76 22
yp ¼ 0.014 30, 30, 30
yp ¼ 0.014 10, 35, 50
§p ¼ 0.02 45, 50, 70, 90
35, 45, 60
86 26
79 29
107 11
109 19
Significant differences between treatments are defined by: y significantly different from K/M (15/0.5); § significantly different from K/M SC (15/0.25).
16
Veterinary Anaesthesia and Analgesia, 2002, 29, 14^19
Ketamine/medetomidine anaesthesia in the rabbit P Hedenqvist et al.
Table 2 The e¡ects of each drug combination upon arterial pressure and heart rate before administration of anaesthetic (t ¼ 0) and peak changes in these variables during the period following either subcutaneous (SC) or intramuscular (IM) administration of ketamine/medetomidine (K/M), or SC administration of K/M with butorphanol (B) in six New Zealand White rabbits.Values are mean 1 SD Drugs, doses (mg kg1)
K/M 15/0.25
K/M 15/0.25
K/M/B 15/0.25/0.4
K/M 15/0.5
Route Mean arterial pressure t ¼ 0 (mm Hg) Mean arterial pressure maximum (mm Hg) Mean arterial pressure minimum (mm Hg) Heart rate t ¼ 0 (beats minute1) Heart rate Minimum (beats minute1)
IM 68 6 73 5 62 9 214 17 157 27
SC 74 8 74 9 57 14 229 44 166 24
SC 77 3 81 13 67 14 235 24 166 17
SC 75 13 87 14 75 11 209 36 152 38
on each occasion, although all animals showed tachycardia and tachypnoea which is a typical response of rabbits to handling and restraint. There were no signi¢cant di¡erences among the groups at this time point. The minimum arterial pressures recorded varied slightly among the groups (Table 2), but these di¡erences were not signi¢cant (p > 0.1). All animals developed a moderate bradycardia, but this was not signi¢cantly di¡erent among groups (p > 0.1) (Table 2).
Animals given butorphanol showed the greatest reduction in respiratory rate during anaesthesia, with the minimum rate of 31 13 breaths minute1 being signi¢cantly lower than that which occurred in animals given the lower dose of ketamine and medetomidine SC [49 8 breaths minute1; F(1,5) ¼ 8.03, p ¼ 0.036], IM [49 17 breaths minute1; F(1,5) ¼ 13.9, p ¼ 0.013] or those given the higher subcutaneous dose [46 17 breaths minute1; F(1,5) ¼ 64.5, p < 0.001]. However, this was not re£ected in
Table 3 The e¡ects of each drug combination upon respiratory rate and blood variables before administration of anaesthetic (t ¼ 0) and peak changes in these variables during the period following either subcutaneous (SC) or intramuscular (IM) administration of ketamine/medetomidine (K/M), or SC administration of K/M with butorphanol (B) in six New Zealand White rabbits.Values are mean 1 SD Drugs, doses (mg kg1)
K/M 15/0.25
K/M 15/0.25
K/M/B 15/0.25/0.4
K/M 15/0.5
Route Respiratory rate t ¼ 0 (breaths minute1) Respiratory rate minimum (breaths minute1)
IM 261 29 49 7 yp ¼ 0.013 4.12 0.3 (31.5 2.2) 6.25 0.53 (47.2 4.8) 7.48 0.03 7.37 0.03 12.5 1.86 (93.7 14.2) 5.72 0.66 (43.5 5.2)
SC >300 49 3 yp ¼ 0.036 3.86 0.4 (29.2 3) 5.98 0.66 (45 5.2) 7.48 0.02 7.38 0.02 11.97 0.4 (90.7 3) 5.85 0.66 (44.2 5.2)
SC >300 31 5
SC ffi290 46 7 yp > 0.001 3.72 0.4 (27.7 3) 6.78 0.8 (51.7 5.7) 7.49 0.03 7.37 0.03 11.84 0.93 (88.5 7.5) 5.3 0.8 (40.5 6)
PaCO2 t ¼ 0 kPa (mm Hg) PaCO2 maximum kPa (mm Hg) pH t ¼ 0 (units) pH minimum (units) PaO2 t ¼ 0 kPa (mm Hg) PaO2 minimum kPa (mm Hg)
3.72 0.4 (27.7 3) 6.78 0.53 (51.7 3.7) 7.47 0.03 7.36 0.02 12.23 0.53 (92.2 3.7) 5.19 0.05 (39 0.3)
Significant differences between treatments are defined by: ysignificant differences from K/M/B.
Veterinary Anaesthesia and Analgesia, 2002, 29, 14^19
17
Ketamine/medetomidine anaesthesia in the rabbit P Hedenqvist et al.
any signi¢cant di¡erences in arterial PCO2, pH or PO2 among the groups (Table 3).
Discussion This study employed a cross-over design to enable several comparisons to be made without using additional animals. A comparison of K/M administered at a dose calculated to produce a light to medium plane of anaesthesia (15/0.25 mg kg1) showed that there was no di¡erence in depth or duration of anaesthesia, sleep time, or depression of blood pressure or respiratory function when the agents were administered by the IM or SC routes. In addition, the rate of onset of e¡ect of the anaesthetic combination was una¡ected by route of administration. Because SC administration is both technically simpler to achieve and likely to cause less discomfort to the animal, it is suggested that this route be favoured. The addition of butorphanol to the anaesthetic technique increased the duration of anaesthesia, but had no signi¢cant e¡ect on depth of anaesthesia in this study. The addition of butorphanol resulted in anaesthetic e¡ects that appeared equivalent to those produced by the higher dose of ketamine/ medetomidine (15 and 0.5 mg kg1) although there was a slightly greater depression of respiratory rate. Based on previous studies (Hedenqvist et al. 2001), we predicted that the higher dose of ketamine/medetomidine would consistently produce loss of the PWR, however, this was not the case, and was most probably due to individual variation in response to the agents used. This highlights one of the problems of administering ¢xed dose rates of anaesthetic combinations as a single IM or SC injection.We consider loss of the PWR to indicate onset of a phase of surgical anaesthesia su⁄cient for procedures such as ovariohysterectomy. It was apparent that the dosages chosen, irrespective of route of administration or butorphanol inclusion, failed to achieve surgical anaesthesia in every case. If an inadequate depth of anaesthesia is produced for a particular procedure, then additional ketamine and, or medetomidine could be administered, or anaesthesia deepened by the use of low concentrations of an inhalational agent such as iso£urane. Clinical experience subsequent to completion of this study has shown that 0.5^1.0% iso£urane can safely be used in this way. A similar e¡ect may also be obtained using an intravenously administered ‘top-up’ agent such as propofol. It is possible that surgical anaesthesia might be achieved by the addition of other analgesic agents 18
such as buprenorphine. If administered preoperatively, this would have the advantage of improving intra- and postoperative analgesia. However, as this has been shown to deepen anaesthesia in rats (Roughan et al. 1999; Hedenqvist et al. 2000a) the dose of ketamine/medetomidine may have to be adjusted until the e¡ects of other such combinations have been established in rabbits. The dose of butorphanol was selected on the basis of previously published studies of its analgesic e¡ects in rabbits (Flecknell & Liles 1990). Butorphanol appears to be rapidly absorbed following SC injection, as judged by its rapid reversal of m-agonists such as fentanyl and sufentanil in rats (Hu et al. 1991; Hedenqvist et al. 2000b; 2001), and experience in our laboratory indicates that it has similarly rapid absorption in rabbits, as judged by its e¡ects in reversing the fentanyl component of ‘Hypnorm’ ( Janssen Pharmaceuticals). It is possible that use of a higher dose of butorphanol may have produced greater analgesic e¡ects, but would most likely have aggravated respiratory function. The results of this study suggest that the use of butorphanol in combination with ketamine and medetomidine will prolong the period of anaesthesia, and may have some minor e¡ects in deepening the plane of anaesthesia. This e¡ect does not seem as marked as in other species, e.g. cats and dogs, in which the addition of butorphanol allows some reduction in the dose of ketamine and medetomidine required for surgical anaesthesia (Reid & Nolan1999). An obvious advantage with the technique described is that the e¡ects of medetomidine can be reversed rapidly and completely by administration of atipamezole (Va“he-Vahe 1990; Vainio 1990; Hedenqvist et al. 2000b; 2001). If reversal occurs within a few minutes of drug administration, then the e¡ects of ketamine alone (catalepsy) may be undesirable. However, when reversal occurs after 20^30 minutes, for example after an ovariohysterectomy, residual e¡ects of ketamine appear minor and rabbits soon resume normal ambulatory activity. Even when atipamezole is not administered, the combination of medetomidine and ketamine in rabbits appears to provide safe and e¡ective anaesthesia no mortality was reported when ketamine (20 mg kg1) medetomidine (0.3 mg kg1) and diazepam (0.75^1.5 mg kg1) were used to anaesthetize a large group of rabbits undergoing orthopaedic surgery (n ¼ 340) (Mero et al.1989). Nevertheless, in this, and other previously published studies of ketamine and medetomidine in rabbits, moderate to marked Veterinary Anaesthesia and Analgesia, 2002, 29, 14^19
Ketamine/medetomidine anaesthesia in the rabbit P Hedenqvist et al.
hypoxia has been noted (Daunt et al. 1992; Hellebrekers et al.1997; Hedenqvist et al.2001) and it is recommended that oxygen supplementation be provided when these agents are used for anaesthesia. It should be noted that this depression of respiratory function is not unique to the use of ketamine/medetomidine, but also occurs with other injectable anaesthetic regimens in this species (Flecknell & Mitchell 1984; Borkowski et al. 1990).We therefore consider that, in addition to oxygen supplementation, endotracheal intubation of rabbits should be performed as this allows PPV when respiratory depression is severe. Our previous studies (Hedenqvist et al. 2000a,b; 2001) together with subsequent clinical experience have con¢rmed that endotracheal intubation can be carried out successfully after induction of anaesthesia with ketamine and medetomidine.
Acknowledgements John Roughan was supported by the Welcome Trust, Luis Antunes by the Portuguese Ministry of Science and Technology (JNICT) and Patricia Hedenqvist by the Swedish MRC.
References Borkowski GL, Danneman PJ, Russell GB et al. CM (1990) An evaluation of three intravenous anesthetic regimens in New Zealand rabbits. Lab Anim Sci 40, 270^276. Daunt DA, Blum JR, HammTE et al. (1992) E¡ects of medetomidine plus ketamine with and without oxygen supplementation in rabbits.Vet Surg 21,159. Flecknell PA, Liles JH (1990) Assessment of the analgesic actions of opioid agonistantagonists in the rabbit. J AssocVet Anaesth17, 24^29. Flecknell PA, Mitchell M (1984) Midazolam and fentanyl£uanisone: assessment of anaesthetic e¡ects in laboratory rodents and rabbits. Lab Anim18,143^146. Flecknell PA, Liles JH,Williamson HA (1990) The use of lignocaineprilocaine local anaesthetic cream for pain-free
Veterinary Anaesthesia and Analgesia, 2002, 29, 14^19
venepuncture in laboratory animals. Lab Anim 24, 142^146. Hedenqvist P, Roughan JV, Flecknell PA (2000a) E¡ects of repeated anaesthesia with ketamine/medetomidine and of pre-anaesthetic administration of buprenorphine in rats. Lab Anim 34, 207^211. Hedenqvist P, Roughan JV, Flecknell PA (2000b) Sufentanil and medetomidine anaesthesia in the rat and its reversal with atipamezole and butorphanol. Lab Anim 34, 244^251. Hedenqvist P, Roughan JV, Orr H et al. (2001) Assessment of ketamine/medetomidine anaesthesia in the New Zealand White rabbit.Vet Anaesth Analg 28,18^25. Hellebrekers LJ, De Boer E-JW,Van Zuylen MA et al. (1997) A comparison between medetomidineketamine and medetomidinepropofol anaesthesia in rabbits. Lab Anim 31,58^69. Hu C, Flecknell PA, Liles JH (1991) Fentanyl and medetomidine anaesthesia in the rat and its reversal using atipamazole and either nalbuphine or butorphanol. Lab Anim 26,15^22. Mero M, Vainionpaa S, Vasenuis J et al. (1989) Medetomidineketaminediazepam anaesthesia in the rabbit. Acta Vet Scand 85,135^137. Reid J, Nolan AM (1999) Intravenous anaesthetics. In: BSAVA Manual of Small Animal Anaesthesia andAnalgesia. Seymour C, Gleed R (eds). British Small Animal Veterinary Association Publications, Cheltenham, UK. pp. 87^97. Roughan JV, Ojeda OB, Flecknell PA (1999) The in£uence of pre-anaesthetic administration of buprenorphine on the anaesthetic e¡ects of ketamine/medetomidine and pentobarbitone in rats and the consequences of repeated anaesthesia. Lab Anim 33, 234^242. Va“he-VaheA (1990) Clinical e¡ectiveness of atipamezole as a medetomidine antagonist in cats. J Small Anim Prac 31, 193^197. Vainio O (1990) Reversal of medetomidine-induced cardiovascular and respiratory changes with atipamezole in dogs.Vet Rec127, 447^450. Received 29 January 2001; accepted11 May 2001.
19
R E S E A R C H PA P E R
Anaesthesia with ketamine/medetomidine in the rabbit: influence of route of administration and the effect of combination with butorphanol P Hedenqvist DVM, HE Orry BVMS, Cert LAS, JV Roughany & PA Flecknelly M.A Vet MB, PhD, DLAS, Dip ECVA, MRCVS
BSc, PhD,
LM Antunesy
MSc DVM
Veterinary Resources, Karolinska Institute, Stockholm, Sweden
yComparative Biology Centre, Medical School, Newcastle upon Tyne, UK
Correspondence: JV Roughan, Comparative Biology Centre, Medical School, Framlington Place, Newcastle upon Tyne NE2 4HH, UK.
Abstract Objective To compare the characteristics of anaesthesia induced with ketamine/medetomidine administered by the subcutaneous and intramuscular routes and to assess the e¡ects of the addition of butorphanol to this combination. Study design Prospective randomised study. Animals Six female New Zealand White rabbits. Methods Rabbits were given one of four combinations of ketamine and medetomidine (K/M) either subcutaneously (SC) or intramuscularly (IM) on four successive occasions with a 7-day interval between treatments. The dose combinations were; 15/0.25 mg kg1 SC; 15/0.25 mg kg1 IM; 15/0.5 mg kg1 SC, and 15/0.25 mg kg1 together with 0.4 mg kg1 butorphanol (K/M/B) SC. The e¡ects of anaesthesia on arterial blood gas values and cardiovascular variables were recorded at predetermined time points. Toe and ear pinch re£exes were judged to determine the duration of surgical anaesthesia. Loss of the righting re£ex was used to measure the duration of sleep time. Analyses used repeated measures analysis of variance. Results All groups lost the righting re£ex and ear pinch response. Three animals in the groups that received K/M alone lost their toe pinch re£ex, 14
whereas four lost this re£ex when given K/M/B. Time of onset of loss of the righting, toe and ear pinch re£exes did not di¡er signi¢cantly among the groups. The higher dose combination of medetomidine with ketamine and the combination of K/M/B produced a greater duration of loss of the ear pinch response than the lower dose of K/M administered by either route. No signi¢cant di¡erences were found among the groups in the duration of loss of the toe pinch re£ex. All animals developed a moderate bradycardia (mean heart rate <166 beats minute1) and moderatehypoxaemia(meanPaO2 < 6.0 kPa). Animalsgiven butorphanol showed the greatest reduction in respiratory rate (31 13 breaths minute1, p < 0.05) but this was not re£ected in any signi¢cant differences in arterial PCO2, PO2 or pH among the groups. Conclusions Administration of K/M by the SC route produced equivalent e¡ects in comparison to intramuscular administration. The addition of butorphanol increased the duration of anaesthesia, but produced a slight increase in the degree of respiratory depression. All dose rates resulted in hypoxaemia so oxygen should be administered when these combinations are used in rabbits. Clinical relevance Subcutaneous administration is both technically simpler and may cause less discomfort to the animal than IM injection, and so is preferred. The combination of K/M with butorphanol
Ketamine/medetomidine anaesthesia in the rabbit P Hedenqvist et al.
has relatively minor e¡ects on the depth and duration of anaesthesia, so o¡ers little advantage to the use of K/M alone.
rabbits were fed a commercial pelleted diet (Rabma pellets, SDS), autoclaved hay and water ad libitum.
Keywords anaesthesia, medetomidine, rabbit.
Anaesthesia
butorphanol,
ketamine,
Introduction Developing methods for safe and e¡ective anaesthesia has often been challenging in rabbits in which unacceptable intra- and postoperative mortality has occurred with a variety of anaesthetic techniques. The use of fentanyl/£uanisone/midazolam, although safe in comparison with some techniques, produces a relatively long recovery (Flecknell & Mitchell 1984). We recently evaluated the use of various dose combinations of ketamine and medetomidine (K/M) and found that it produces smooth induction and recovery in rabbits (Hedenqvist et al. 2001). The dose rates needed to produce surgical anaesthesia (15 mg kg1 ketamine, 0.25 mg kg1 medetomidine) require relatively large volumes of anaesthetic to be injected intramuscularly (IM) (0.4 mL kg1), which can cause discomfort to the animal. In addition, the combination produces marked respiratory depression. In other species, the dose of medetomidine and ketamine required to produce surgical anaesthesia can be reduced by combining these agents with butorphanol, a mixed agonist/antagonist opioid. This study compared subcutaneous (SC) administration of ketamine/medetomidine with IM injection, and also investigated the e¡ects of the addition of butorphanol.
Materials and methods All procedures were conducted in accordance with the guidelines set out in the Animals (Scienti¢c Procedures) Act 1986 (1989), under project license number PPL 60/2362. Animals Six female young adult New Zealand White (NZW) rabbits were used for the study (mean body mass SD 2391 71 g). They were obtained from a colony free from respiratory pathogens (Charles River UK) and group housed in £oor pens on dust-free shavings (‘Gold Shavings’, SDS, Whitham, UK) in a room with a 07:0019:00 hours light cycle. Room temperature was maintained at 20 38C, relative humidity at 50%, with 18 air changes per hour. The Veterinary Anaesthesia and Analgesia, 2002, 29, 14^19
Rabbits each received four anaesthetic combinations with a 1-week interval between successive anaesthetics, with the order of treatments allocated using a Latin square design. The combinations administered were low dose K/M (15/0.25 mg kg1) given either SC or IM, the low dose of ketamine combined with a higher dose of medetomidine given SC (15/ 0.5 mg kg1) or the lower dose K/M combination (15/ 0.25 mg kg1) given together with butorphanol (0.4 mg kg1) SC. All drugs were mixed in a single syringe for injection. Local anaesthetic cream (EMLA, Astra Pharmaceutical Ltd, Herts, UK) was applied to the rabbits’ ears 45 minutes before induction of anaesthesia to prevent pain during catheter placement (Flecknell et al. 1990). An ‘over-the-needle’ catheter (20 SWG, ‘Ven£on’ Jelco, Critikon, London, UK) was placed in the central ear artery and connected to a pressure transducer (Kontron ‘Supermon’, St Albans, Herts, UK) to monitor arterial blood pressure. The Kontron module was connected to a data capture system (1401 plus, CED, Cambridge, UK) for continuous computer recording. A thermostatically controlled heat-pad equipped with a rectal probe (Harvard Apparatus, Edinburgh, UK), set to 388C (the normal body temperature of the conscious rabbit at rest), was used to maintain body temperature during anaesthesia. After placement of the catheters, baseline (t ¼ 0) recordings of respiratory rate, mean arterial blood pressure (MAP) and heart rate were obtained for a 2 minute period. An arterial blood sample (0.5^ 1.0 mL) was then obtained for blood gas analysis (t ¼ 0 minutes) and the anaesthetic combination was administered. Depth of anaesthesia was assessed by recording the presence or absence of a righting response (the animal showing ability to spontaneously right itself after being placed on its back), an ear pinch response (shaking the head or vocalization when the pinna was pinched using the investigator’s ¢ngernails) and a pedal withdrawal response (PWR, withdrawal of the hind limb in response to pinching one digit with the investigator’s ¢ngernails). Simple eye ointment was administered in both eyes to prevent drying of the corneal surface. Recordings of heart rate and MAP were made for a 2 minute period prior to withdrawal of additional blood samples at 5, 10, 15, 30, 45, 60, 75, 90 and 120 minutes during 15
Ketamine/medetomidine anaesthesia in the rabbit P Hedenqvist et al.
anaesthesia for blood gas analysis (using a Stat Pro¢le 3, Nova Biomedical,Waltham, MA, USA). Monitoring was continued until the animal regained its righting re£ex. Total sleep time was measured as the latency from drug administration until return of the righting re£ex. At the end of the study, the rabbits were re-homed. Data processing and statistical analysis The e¡ects of each treatment according to the dose combination given, and the route of administration used was compared using repeated measures analysis of variance (ANOVA), using the computer program SPSS (SPSS Inc., Chicago, IL, USA). Overall group di¡erences were determined from the results of averaged univariate statistics, whereas further univariate comparisons determined speci¢c di¡erences between routes of administration. The results of these analyses (repeated measures one-way ANOVA) are presented in the form [F(df1,df2) ¼ X, p ¼ sig.], where df1 and df2 are the degrees of freedom of the numerator and the denominator, respectively.
Results
re£exes was not signi¢cantly di¡erent (p > 0.1) among the groups which received lower dose K/M either IM and SC (Table 1). The duration of loss of the ear pinch re£ex was signi¢cantly greater following high-dose ketamine and medetomidine (15/0.5 mg kg1) (76 22 minutes) than the lower dose (15/0.25 mg kg1) given either SC [41 23 minutes; F(1,5) ¼ 13.8, p ¼ 0.014] or IM [44 19 minutes; F(1,5) ¼ 13.9, p ¼ 0.014]. A similarly prolonged re£ex loss occurred when animals were given butorphanol together with lower dose K/M (78 30 minutes), this being signi¢cantly longer than obtain- ed with the lower SC dose of K/M alone [F(1,5) ¼ 11.4, p ¼ 0.02]. Total sleep time also tended to increase with the higher dose combination and in animals given butorphanol and K/M (107^109 minutes). This, however, was not signi¢cantly greater than the sleep time obtained with lower dose K/M administered by either route (79^86 minutes) (Table 1). The PWR was not lost uniformly in any of the groups. Three animals in each of the groups given only ketamine and medetomidine showed loss of this re£ex, whereas four animals receiving ketamine and medetomidine with butorphanol lost the PWR.These di¡erences were not signi¢cant (p > 0.1).
Depth of anaesthesia All groups lost the righting re£ex and ear pinch response. The time of loss of the righting and ear pinch re£exes did not di¡er signi¢cantly among the groups (p > 0.1) (Table 1). The duration of loss of these
Physiological variables Baseline blood gas analyses and arterial pressure measurements indicated that animals were physiologically normal prior to induction of anaesthesia
Table 1 The e¡ects upon autonomic re£ex responsiveness of either subcutaneously (SC) or intramuscularly (IM) administered ketamine/medetomidine (K/M), or SC administration (K/M) with butorphanol (B) in six New Zealand White rabbits. Sleep time was the duration of loss of righting re£ex.Values are mean 1 SD. Because not all animals lost their pedal withdrawal response, data from individual animals are presented
Drugs, doses (mg kg1)
Route Onset of loss of righting reflex (minutes) Duration of loss of ear pinch (minutes) Duration of loss of pedal withdrawal response (minutes) Sleep time (minutes)
K/M 15/0.25
K/M 15/0.25
K/M/B 15/0.25/0.4
K/M 15/0.5
IM 31
SC 3 0.4
SC 53
SC 32
44 19
41 23
78 30
76 22
yp ¼ 0.014 30, 30, 30
yp ¼ 0.014 10, 35, 50
§p ¼ 0.02 45, 50, 70, 90
35, 45, 60
86 26
79 29
107 11
109 19
Significant differences between treatments are defined by: y significantly different from K/M (15/0.5); § significantly different from K/M SC (15/0.25).
16
Veterinary Anaesthesia and Analgesia, 2002, 29, 14^19
Ketamine/medetomidine anaesthesia in the rabbit P Hedenqvist et al.
Table 2 The e¡ects of each drug combination upon arterial pressure and heart rate before administration of anaesthetic (t ¼ 0) and peak changes in these variables during the period following either subcutaneous (SC) or intramuscular (IM) administration of ketamine/medetomidine (K/M), or SC administration of K/M with butorphanol (B) in six New Zealand White rabbits.Values are mean 1 SD Drugs, doses (mg kg1)
K/M 15/0.25
K/M 15/0.25
K/M/B 15/0.25/0.4
K/M 15/0.5
Route Mean arterial pressure t ¼ 0 (mm Hg) Mean arterial pressure maximum (mm Hg) Mean arterial pressure minimum (mm Hg) Heart rate t ¼ 0 (beats minute1) Heart rate Minimum (beats minute1)
IM 68 6 73 5 62 9 214 17 157 27
SC 74 8 74 9 57 14 229 44 166 24
SC 77 3 81 13 67 14 235 24 166 17
SC 75 13 87 14 75 11 209 36 152 38
on each occasion, although all animals showed tachycardia and tachypnoea which is a typical response of rabbits to handling and restraint. There were no signi¢cant di¡erences among the groups at this time point. The minimum arterial pressures recorded varied slightly among the groups (Table 2), but these di¡erences were not signi¢cant (p > 0.1). All animals developed a moderate bradycardia, but this was not signi¢cantly di¡erent among groups (p > 0.1) (Table 2).
Animals given butorphanol showed the greatest reduction in respiratory rate during anaesthesia, with the minimum rate of 31 13 breaths minute1 being signi¢cantly lower than that which occurred in animals given the lower dose of ketamine and medetomidine SC [49 8 breaths minute1; F(1,5) ¼ 8.03, p ¼ 0.036], IM [49 17 breaths minute1; F(1,5) ¼ 13.9, p ¼ 0.013] or those given the higher subcutaneous dose [46 17 breaths minute1; F(1,5) ¼ 64.5, p < 0.001]. However, this was not re£ected in
Table 3 The e¡ects of each drug combination upon respiratory rate and blood variables before administration of anaesthetic (t ¼ 0) and peak changes in these variables during the period following either subcutaneous (SC) or intramuscular (IM) administration of ketamine/medetomidine (K/M), or SC administration of K/M with butorphanol (B) in six New Zealand White rabbits.Values are mean 1 SD Drugs, doses (mg kg1)
K/M 15/0.25
K/M 15/0.25
K/M/B 15/0.25/0.4
K/M 15/0.5
Route Respiratory rate t ¼ 0 (breaths minute1) Respiratory rate minimum (breaths minute1)
IM 261 29 49 7 yp ¼ 0.013 4.12 0.3 (31.5 2.2) 6.25 0.53 (47.2 4.8) 7.48 0.03 7.37 0.03 12.5 1.86 (93.7 14.2) 5.72 0.66 (43.5 5.2)
SC >300 49 3 yp ¼ 0.036 3.86 0.4 (29.2 3) 5.98 0.66 (45 5.2) 7.48 0.02 7.38 0.02 11.97 0.4 (90.7 3) 5.85 0.66 (44.2 5.2)
SC >300 31 5
SC ffi290 46 7 yp > 0.001 3.72 0.4 (27.7 3) 6.78 0.8 (51.7 5.7) 7.49 0.03 7.37 0.03 11.84 0.93 (88.5 7.5) 5.3 0.8 (40.5 6)
PaCO2 t ¼ 0 kPa (mm Hg) PaCO2 maximum kPa (mm Hg) pH t ¼ 0 (units) pH minimum (units) PaO2 t ¼ 0 kPa (mm Hg) PaO2 minimum kPa (mm Hg)
3.72 0.4 (27.7 3) 6.78 0.53 (51.7 3.7) 7.47 0.03 7.36 0.02 12.23 0.53 (92.2 3.7) 5.19 0.05 (39 0.3)
Significant differences between treatments are defined by: ysignificant differences from K/M/B.
Veterinary Anaesthesia and Analgesia, 2002, 29, 14^19
17
Ketamine/medetomidine anaesthesia in the rabbit P Hedenqvist et al.
any signi¢cant di¡erences in arterial PCO2, pH or PO2 among the groups (Table 3).
Discussion This study employed a cross-over design to enable several comparisons to be made without using additional animals. A comparison of K/M administered at a dose calculated to produce a light to medium plane of anaesthesia (15/0.25 mg kg1) showed that there was no di¡erence in depth or duration of anaesthesia, sleep time, or depression of blood pressure or respiratory function when the agents were administered by the IM or SC routes. In addition, the rate of onset of e¡ect of the anaesthetic combination was una¡ected by route of administration. Because SC administration is both technically simpler to achieve and likely to cause less discomfort to the animal, it is suggested that this route be favoured. The addition of butorphanol to the anaesthetic technique increased the duration of anaesthesia, but had no signi¢cant e¡ect on depth of anaesthesia in this study. The addition of butorphanol resulted in anaesthetic e¡ects that appeared equivalent to those produced by the higher dose of ketamine/ medetomidine (15 and 0.5 mg kg1) although there was a slightly greater depression of respiratory rate. Based on previous studies (Hedenqvist et al. 2001), we predicted that the higher dose of ketamine/medetomidine would consistently produce loss of the PWR, however, this was not the case, and was most probably due to individual variation in response to the agents used. This highlights one of the problems of administering ¢xed dose rates of anaesthetic combinations as a single IM or SC injection.We consider loss of the PWR to indicate onset of a phase of surgical anaesthesia su⁄cient for procedures such as ovariohysterectomy. It was apparent that the dosages chosen, irrespective of route of administration or butorphanol inclusion, failed to achieve surgical anaesthesia in every case. If an inadequate depth of anaesthesia is produced for a particular procedure, then additional ketamine and, or medetomidine could be administered, or anaesthesia deepened by the use of low concentrations of an inhalational agent such as iso£urane. Clinical experience subsequent to completion of this study has shown that 0.5^1.0% iso£urane can safely be used in this way. A similar e¡ect may also be obtained using an intravenously administered ‘top-up’ agent such as propofol. It is possible that surgical anaesthesia might be achieved by the addition of other analgesic agents 18
such as buprenorphine. If administered preoperatively, this would have the advantage of improving intra- and postoperative analgesia. However, as this has been shown to deepen anaesthesia in rats (Roughan et al. 1999; Hedenqvist et al. 2000a) the dose of ketamine/medetomidine may have to be adjusted until the e¡ects of other such combinations have been established in rabbits. The dose of butorphanol was selected on the basis of previously published studies of its analgesic e¡ects in rabbits (Flecknell & Liles 1990). Butorphanol appears to be rapidly absorbed following SC injection, as judged by its rapid reversal of m-agonists such as fentanyl and sufentanil in rats (Hu et al. 1991; Hedenqvist et al. 2000b; 2001), and experience in our laboratory indicates that it has similarly rapid absorption in rabbits, as judged by its e¡ects in reversing the fentanyl component of ‘Hypnorm’ ( Janssen Pharmaceuticals). It is possible that use of a higher dose of butorphanol may have produced greater analgesic e¡ects, but would most likely have aggravated respiratory function. The results of this study suggest that the use of butorphanol in combination with ketamine and medetomidine will prolong the period of anaesthesia, and may have some minor e¡ects in deepening the plane of anaesthesia. This e¡ect does not seem as marked as in other species, e.g. cats and dogs, in which the addition of butorphanol allows some reduction in the dose of ketamine and medetomidine required for surgical anaesthesia (Reid & Nolan1999). An obvious advantage with the technique described is that the e¡ects of medetomidine can be reversed rapidly and completely by administration of atipamezole (Va“he-Vahe 1990; Vainio 1990; Hedenqvist et al. 2000b; 2001). If reversal occurs within a few minutes of drug administration, then the e¡ects of ketamine alone (catalepsy) may be undesirable. However, when reversal occurs after 20^30 minutes, for example after an ovariohysterectomy, residual e¡ects of ketamine appear minor and rabbits soon resume normal ambulatory activity. Even when atipamezole is not administered, the combination of medetomidine and ketamine in rabbits appears to provide safe and e¡ective anaesthesia no mortality was reported when ketamine (20 mg kg1) medetomidine (0.3 mg kg1) and diazepam (0.75^1.5 mg kg1) were used to anaesthetize a large group of rabbits undergoing orthopaedic surgery (n ¼ 340) (Mero et al.1989). Nevertheless, in this, and other previously published studies of ketamine and medetomidine in rabbits, moderate to marked Veterinary Anaesthesia and Analgesia, 2002, 29, 14^19
Ketamine/medetomidine anaesthesia in the rabbit P Hedenqvist et al.
hypoxia has been noted (Daunt et al. 1992; Hellebrekers et al.1997; Hedenqvist et al.2001) and it is recommended that oxygen supplementation be provided when these agents are used for anaesthesia. It should be noted that this depression of respiratory function is not unique to the use of ketamine/medetomidine, but also occurs with other injectable anaesthetic regimens in this species (Flecknell & Mitchell 1984; Borkowski et al. 1990).We therefore consider that, in addition to oxygen supplementation, endotracheal intubation of rabbits should be performed as this allows PPV when respiratory depression is severe. Our previous studies (Hedenqvist et al. 2000a,b; 2001) together with subsequent clinical experience have con¢rmed that endotracheal intubation can be carried out successfully after induction of anaesthesia with ketamine and medetomidine.
Acknowledgements John Roughan was supported by the Welcome Trust, Luis Antunes by the Portuguese Ministry of Science and Technology (JNICT) and Patricia Hedenqvist by the Swedish MRC.
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