Effects of carprofen and meloxicam with or without butorphanol on the minimum alveolar concentration of sevoflurane in dogs

Comparison Between Medetomidine and a Medetomidine-Vatinoxan Combination on Cardiorespiratory Variables in Dogs Undergoing Ovariectomy Anesthetized with Butorphanol, Propofol and Sevoflurane or Desflurane

The aim of this study was to compare the effects of a medetomidine-vatinoxan combination versus medetomidine alone on heart rate (HR) and mean arterial pressure (MAP) in a short-term surgery in dogs. Four groups of 10 dogs were administered as follows: medetomidine and sevoflurane; medetomidine and desflurane; medetomidine-vatinoxan and sevoflurane; and medetomidine-vatinoxan and desflurane. After administration, the increase in MAP soon stopped at 102-104 mmHg in the two groups administered medetomidine-vatinoxan, compared with significantly higher values of 143-126 mmHg achieved in the two groups administered medetomidine alone. The lowest MAPs in the two medetomidine-vatinoxan groups were 46-50 mmHg, while in the medetomidine groups, they were 58-79 mmHg. From 3 min onwards after administration, in the medetomidine-vatinoxan treatments, HR remained at values very close to those of pre-administration, between 83 and 118 beats min-1, while in the medetomidine treatments, it dropped to 36-43 beats min-1 and then slowly rose to reach 71-90 beats min-1. These results encourage the use of vatinoxan in clinical settings, particularly in anesthetic protocols for dogs when bradycardia and an increase in systemic pressure should be avoided. Further clinical studies are needed to manage the short periods of hypotension, as well as the slight reduction in sedative and pain-relieving medetomidine effects found, particularly when vatinoxan is in combination with desflurane rather than sevoflurane.

Influence of intravenous fentanyl or dexmedetomidine infusions, combined with lidocaine and ketamine, on cardiovascular response, sevoflurane requirement and postoperative pain in dogs anesthetized for unilateral mastectomy

OBJECTIVE

To compare the effects of constant rate infusions (CRI) of fentanyl or dexmedetomidine, combined with lidocaine and ketamine, on cardiovascular response during surgery, sevoflurane requirement and postoperative pain in dogs undergoing mastectomy.

STUDY DESIGN

Prospective, randomized, blinded, clinical trial.

ANIMALS

A total of 29 female dogs with mammary tumors.

METHODS

Premedication consisted of intramuscular acepromazine and morphine. General anesthesia was induced with intravenous propofol and maintained with sevoflurane. Dogs were randomized to be administered intravenous DLK [dexmedetomidine 1 μg kg-1 loading dose (LD) and 1 μg kg-1 hour-1; lidocaine 2 mg kg-1 LD and 3 mg kg-1 hour-1; ketamine 1 mg kg-1 LD and 0.6 mg kg-1 hour-1; n = 14] or FLK (fentanyl 5 μg kg-1 LD and 9 μg kg-1 hour-1; same doses of lidocaine and ketamine; n = 15) during anesthesia. Cardiorespiratory variables and end-tidal sevoflurane (Fe'Sevo) were recorded during surgery. The number of dogs administered ephedrine to treat arterial hypotension [mean arterial pressure (MAP) < 60 mmHg] was recorded. Meloxicam was administered to both groups. Postoperative pain and rescue analgesia requirement were assessed for 24 hours using the short form of the Glasgow Composite Measure Pain Scale. Data were compared using a mixed effects model or a Mann-Whitney test.

RESULTS

More dogs required ephedrine in FLK than in DLK (67% versus 7%). Heart rate was not significantly different between groups, whereas lower values of MAP (p ≤ 0.01) and Fe'Sevo (p = 0.018) were observed in FLK than in DLK. Rescue analgesia was administered to 2/15 dogs in FLK and 0/14 dogs in DLK.

CONCLUSIONS AND CLINICAL RELEVANCE

Based on the cardiovascular response during surgery, intraoperative infusions of FLK and DLK provided adequate antinociception. Infusion of DLK provided greater stability of blood pressure. Both protocols resulted in minimal need for additional analgesia within 24 hours postoperatively.

Does maropitant provide more effective perioperative pain management than meloxicam in bitches undergoing ovariohysterectomy? The first report on the comparison of visceral algesia-analgesia for ovariohysterectomy

Ovariohysterectomy (OVH) is a widely used surgical procedure in small animal practice. In developing countries, injectable anesthetics such as ketamine and xylazine are commonly used in veterinary medicine. Pharmacological agents with analgesic activity, such as ketamine and meloxicam, are not sufficiently effective in reducing visceral pain. Therefore, this study aimed to investigate the visceral analgesia and anti-inflammatory effectiveness of maropitant compared with those of meloxicam during and after OVH in bitches. In this study, thirty-six bitches were randomly divided into the maropitant, meloxicam, and control groups. The heart rate (HR), peripheral oxygen saturation, and respiratory rate were monitored during the procedure. Pain scores were assessed using the University of Melbourne pain scale (UMPS). Rescue analgesia was not necessary for any bitch at any time point. Blood samples were collected before anesthesia induction and 24 h after the operation to determine C-reactive protein (CRP) levels. No significant difference was observed in HR between the control and meloxicam groups when the right ovary was removed, and the HR of the maropitant group was significantly (p < 0.05) lower than that of the control group. The pain scores of the maropitant group were significantly (p < 0.05) lower than those of the other groups. However, no significant differences were observed in CRP levels between the groups. In conclusion, compared to meloxicam, maropitant provided more effective visceral analgesia in bitches undergoing OVH, although no significant difference was found in its anti-inflammatory effect.

A randomized crossover study of the effect of butorphanol-lidocaine and tramadol-lidocaine on sevoflurane's minimum alveolar concentration in dogs

Inhalational anesthesia is routinely used in small animal surgery. Selecting a suitable drug combination is vital since it may negatively affect the patient's physiological condition. We conducted this study to examine the sparing effect of butorphanol-lidocaine (BUT-LID) and tramadol-lidocaine (TRM-LID) on sevoflurane's minimum alveolar concentration (MAC) in 10 healthy mongrel dogs aged 1-2 years and weighing 11.5 ± 0.8 kg (mean ± SD). Sevoflurane's MAC was measured on three separate occasions. The three dog treatment groups were control (CONT) anesthetized only with sevoflurane, TRM-LID (TRM, i.v. 1.5 mg kg^-1, then 1.3 mg kg^-1 h^-1 and LID, i.v. 2 mg kg^-1, then 3 mg kg^-1 h^-1) or BUT-LID treatment (BUT, i.v. 0.1 mg kg^-1 then 0.2 mg kg^-1 h^-1 and LID, i.v. 2 mg kg^-1, then 3 mg kg^-1 h^-1). We hypothesized that both TRM-LID and BUT-LID would result in a significant MAC sparing effect in healthy dogs. The TRM-LID treatment resulted in a non-significant MAC reduction. MAC was lowered significantly in the BUT-LID group (p = 0.009). The sevoflurane MAC-sparing effects of TRM-LID and BUT-LID treatments were 7.05 ± 22.20 and 19.90 ± 5.91%, respectively, a difference that was not statistically significant (p = 0.13). Bradycardia was observed in the TRM-LID (p < 0.001) treatment. The esophageal temperature was significantly higher for the TRM-LID treatment than the CONT (p < 0.001) treatment. No statistically significant changes were detected between the three groups in f _R, Pe'CO₂, and MABP. In conclusion, there was a significant sparing effect after adding BUT-LID co-infusion than the control group. No sparing effect was noticed when adding TRM-LID co-infusion. However, no difference in the MAC sparing percentages between the TRM-LID and BUT-LID treatments. The BUT-LID co-infusion resulted in a sevoflurane MAC reduction superior to TRM-LID in addition to minimal cardiorespiratory changes. Both BUT-LID and TRM-LID may be clinically beneficial to dogs during anesthesia. However, BUT-LID produced higher sparing effect and reduction of sevoflurane MAC value.

Sparing effect of tramadol, lidocaine, dexmedetomidine and their combination on the minimum alveolar concentration of sevoflurane in dogs

Background

Problems associated with using inhalational anaesthesia are numerous in veterinary anaesthesia practice. Decreasing the amount of used inhalational anaesthetic agents and minimising of cardiorespiratory disorders are the standard goals of anaesthetists.

Objective

This experimental study was carried out to investigate the sparing effect of intravenous tramadol, lidocaine, dexmedetomidine and their combinations on the minimum alveolar concentration (MAC) of sevoflurane in healthy Beagle dogs.

Methods

This study was conducted on six beagle dogs. Sevoflurane MAC was determined by the tail clamp method on five separate occasions. The dogs received no treatment (control; CONT), tramadol (TRM: 1.5 mg kg^-1 intravenously followed by 1.3 mg kg^-1 h^-1), lidocaine (LID: 2 mg kg^-1 intravenously followed by 3 mg kg^-1 h^-1), dexmedetomidine (DEX: 2 μg kg^-1 intravenously followed by 2 μg kg^-1 h^-1), and their combination (COMB), respectively. Cardiorespiratory variables were recorded every five minutes and immediately before the application of a noxious stimulus.

Results

The COMB treatment had the greatest sevoflurane MAC-sparing effect (67.4 ± 13.9%) compared with the other treatments (5.1 ± 25.3, 12.7 ± 14.3, and 40.3 ± 15.1% for TRM, LID, and DEX treatment, respectively). The cardiopulmonary variables remained within the clinically acceptable range following COMB treatment, although the mean arterial pressure was higher and accompanied by bradycardia.

Conclusions

Tramadol-lidocaine-dexmedetomidine co-infusion produced a remarkable sevoflurane MAC-sparing effect in clinically healthy beagle dogs and could result in the alleviation of cardiorespiratory depression caused by sevoflurane. Cardiorespiratory variables should be monitored carefully to avoid undesirable side effects induced by dexmedetomidine.

Minimum alveolar concentration: Key concepts and a review of its pharmacological reduction in dogs. Part 2

OBJECTIVE

To outline the major components of the minimum alveolar concentration (MAC) and review the literature regarding pharmacological manipulation of the MAC of halothane, isoflurane, sevoflurane, enflurane, and desflurane in dogs. The pharmacological agents included are alpha-2 agonists, benzodiazepines, propofol, opioids, lidocaine, acepromazine, non-steroidal anti-inflammatory agents (NSAIDs), maropitant, and NMDA antagonists. Part 2 of this review will focus on the effect of opioids, lidocaine, NSAIDs, maropitant, acepromazine, and NMDA antagonists on MAC.

DATABASES USED

PubMed, Google Scholar, CAB Abstracts. Search terms used: minimum alveolar concentration, MAC, dog, canine, inhaled anesthetic potency, isoflurane, sevoflurane, desflurane, enflurane, and halothane.

CONCLUSIONS

Opioids, lidocaine, NSAIDs, maropitant, acepromazine, and NMDA antagonists have been shown to reduce the MAC of inhaled anesthetics in dogs and allow for clinically important decreases in inhalant anesthetic use. Thus, the use of these agents potentially decrease the adverse cardiovascular and pulmonary effects associated with the use of high concentrations of inhaled anesthetics.

Plethysmography variability index for prediction of fluid responsiveness during graded haemorrhage and transfusion in sevoflurane-anaesthetized mechanically ventilated dogs

OBJECTIVE

To examine the accuracy of plethysmography variability index (PVI) as a noninvasive indicator of fluid responsiveness in hypovolaemic dogs.

STUDY DESIGN

Prospective experimental study.

ANIMALS

Six adult healthy sevoflurane-anaesthetized Beagle dogs.

METHODS

Dogs were anaesthetized with 1.3-fold their individual minimum alveolar concentration of sevoflurane. The lungs were mechanically ventilated after neuromuscular blockade with vecuronium bromide. Cardiopulmonary variables including mean arterial blood pressure (MAP), central venous pressure (CVP), transpulmonary thermodilution cardiac output (TPTDCO), stroke volume (SV), perfusion index (PI), pulse pressure variation (PPV), stroke volume variation (SVV) and PVI were determined during six stages of graded venous blood withdrawal (5 mL kg^-1 increments) and six stages of graded blood infusion (5 mL kg^-1 increments). The cardiopulmonary variables were analysed using paired t test or Wilcoxon signed rank test. Correlations between PPV and SVV or PVI were analysed by linear regression. The accuracy of PPV, SVV and PVI for predicting fluid responsiveness was examined by using receiver operating characteristic curve analysis. A value of p < 0.05 was considered statistically significant.

RESULTS

Blood withdrawal resulted in significant increases in PPV and PVI and decreases in MAP, CVP, TPTDCO, SV and PI. Blood infusion resulted in significant increases in MAP, CVP, TPTDCO, SV and PI and decreases in PPV and PVI. PPV and PVI showed a relevant correlation (p < 0.001, r² = 0.62) and threshold values of PPV ≥ 16% (sensitivity 71%, specificity 82%) and PVI ≥ 12% (sensitivity 78%, specificity 72%) for identifying fluid responsiveness. SVV did not change.

CONCLUSIONS AND CLINICAL RELEVANCE

Noninvasive measurement of PVI predicted fluid responsiveness with moderate accuracy equal to PPV in sevoflurane-anaesthetized mechanically ventilated dogs. Provisional threshold values for identification of fluid responsiveness were PPV ≥ 16% and PVI ≥ 12%. Clinical trials are needed to confirm these threshold values in dogs.

ANESTHETIC MANAGEMENT OF AN INDO-PACIFIC BOTTLENOSE DOLPHIN (TURSIOPS ADUNCUS) REQUIRING SURGICAL DEBRIDEMENT OF A TAIL ABSCESS

This report describes the anesthetic management of a 14-yr-old, 160-kg, female Indo-Pacific bottlenose dolphin ( Tursiops aduncus ) that underwent surgical debridement for a refractory subcutaneous abscess twice within a 6-mo interval. The animal was otherwise in good physical condition at each anesthetic procedure. Following premedication with intramuscular midazolam and butorphanol, anesthesia was induced with propofol and maintained with sevoflurane by intubation. During surgery ventilation was controlled. Blood pressure was indirectly estimated using either oscillometric or pulse oximetry. Presumed hypotension was managed by adjusting the sevoflurane concentration and infusion of dopamine. During recovery, the dolphin regained adequate spontaneous respiration following intravenous administration of flumazenil and doxapram. The dolphin was extubated at 85 min and 53 min after the first and second surgeries, respectively. Successful weaning from the ventilator and initiation of spontaneous respiration was the most important complication encountered. Establishment of a reliable blood pressure measurement technique is critical to success for anesthesia in this species.

Interaction between maropitant and carprofen on sparing of the minimum alveolar concentration for blunting adrenergic response (MAC-BAR) of sevoflurane in dogs

Maropitant, a neurokinin-1 receptor antagonist, may provide analgesic effects by blocking pharmacological action of substance P. Carprofen is a non-steroidal anti-inflammatory drug commonly used for pain control in dogs. The purpose of this study was to evaluate the effect of a combination of maropitant and carprofen on the minimum alveolar concentration for blunting adrenergic response (MAC-BAR) of sevoflurane in dogs. Six healthy adult beagle dogs were anesthetized with sevoflurane four times with a minimum of 7-day washout period. On each occasion, maropitant (1 mg/kg) alone, carprofen (4 mg/kg) alone, a combination of maropitant (1 mg/kg) and carprofen (4 mg/kg), or saline (0.1 ml/kg) was subcutaneously administered at 1 hr prior to the first electrical stimulation for the sevoflurane MAC-BAR determination. The sevoflurane MAC-BAR was significantly reduced by maropitant alone (2.88 ± 0.73%, P=0.010), carprofen alone (2.96 ± 0.38%, P=0.016) and the combination (2.81 ± 0.51%, P=0.0003), compared with saline (3.37 ± 0.56%). There was no significant difference in the percentage of MAC-BAR reductions between maropitant alone, carprofen alone and the combination. The administration of maropitant alone and carprofen alone produced clinically significant sparing effects on the sevoflurane MAC-BAR in dogs. However, the combination of maropitant and carprofen did not produce any additive effect on the sevoflurane MAC-BAR reduction. Anesthetic premedication with a combination of maropitant and carprofen may not provide any further sparing effect on anesthetic requirement in dogs.

Comparison of cardiac output measurements using transpulmonary thermodilution and conventional thermodilution techniques in anaesthetized dogs with fluid overload

OBJECTIVE

To evaluate the agreement between cardiac output (CO) values obtained using a transpulmonary thermodilution technique (TPTDCO) and conventional thermodilution technique (TDCO) in anaesthetized dogs with fluid overload.

STUDY DESIGN

Prospective experimental study.

ANIMALS

Six healthy Beagle dogs aged 7-8 years.

METHODS

Dogs were anaesthetized with sevoflurane in oxygen, and catheters were inserted for TPTDCO and TDCO measurement. After instrumentation, baseline CO was measured using each technique at a central venous pressure (CVP) of 3-7 mmHg. Dogs were subsequently administered lactated Ringer's solution and 6% hydroxyethyl starch to induce fluid overload. CO measurements were obtained using each technique at CVP values of 8-12 mmHg, 13-17 mmHg, 18-22 mmHg and 23-27 mmHg. Agreements between CO measurements obtained with the respective techniques were analysed using Dunnett's test, Pearson's correlation coefficient and Bland-Altman analysis.

RESULTS

Thirty pairs of CO values were obtained, ranging from 1.45 L minute(-1) to 4.69 L minute(-1) for TPTDCO and from 1.30 L minute(-1) to 4.61 L minute(-1) for TDCO. TPTDCO and TDCO values correlated strongly (r(2)  = 0.915, p < 0.001). The bias and mean relative bias between TPTDCO and TDCO were 0.26 ± 0.30 L minute(-1) (limits of agreement - 0.29 to 0.81 L minute(-1) ) and 9.7%, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

TPTDCO and TDCO measurements obtained in anaesthetized dogs during fluid overload exhibited good agreement. Accordingly, transpulmonary thermodilution provides an accurate measurement of CO in dogs with fluid overload.

Effect of Acetaminophen Alone and in Combination with Morphine and Tramadol on the Minimum Alveolar Concentration of Isoflurane in Rats

Background

It has been observed that acetaminophen potentiates the analgesic effect of morphine and tramadol in postoperative pain management. Its capacity as an analgesic drug or in combinations thereof to reduce the minimum alveolar concentration (MAC) of inhalational anesthetics represents an objective measure of this effect during general anesthesia. In this study, the effect of acetaminophen with and without morphine or tramadol was evaluated on the isoflurane MAC.

Methods

Forty-eight male Wistar rats were anesthetized with isoflurane in oxygen. MAC_ISO was determined from alveolar gas samples at the time of tail clamping without the drug, after administering acetaminophen (300 mg/kg), morphine (3 mg/kg), tramadol (10 mg/kg), acetaminophen (300 mg/kg) + morphine (3 mg/kg), and acetaminophen (300 mg/kg) + tramadol (10 mg/kg).

Results

The control and acetaminophen groups did not present statistically significant differences (p = 0.98). The values determined for MAC_ISO after treatment with acetaminophen + morphine, acetaminophen + tramadol, morphine, and tramadol were 0.98% ± 0.04%, 0.99% ± 0.009%, 0.97% ± 0.02%, and 0.99% ± 0.01%, respectively.

Conclusions

The administration of acetaminophen did not reduce the MAC of isoflurane and did not potentiate the reduction in MAC_ISO by morphine and tramadol in rats, and therefore does not present a sparing effect of morphine or tramadol in rats anesthetized with isoflurane.

Postoperative comparison of four perioperative analgesia protocols in dogs undergoing stifle joint surgery

OBJECTIVE

To compare 4 analgesic protocols in dogs undergoing stifle joint surgery.

DESIGN

Randomized, blinded, prospective clinical trial. Animals-48 client-owned dogs that underwent stifle joint surgery.

PROCEDURES

Dogs undergoing tibial plateau leveling osteotomy were randomly assigned to receive a constant rate infusion of a combination of morphine, lidocaine, and ketamine; a lumbosacral epidural with morphine and ropivacaine; both treatments (ie, constant rate infusion and lumbosacral epidural); or only IM premedication with morphine. Indices of cardiorespiratory function and isoflurane requirement were recorded at 5-minute intervals during anesthesia. A validated sedation scoring system and the modified Glasgow composite measure pain score were used to assess comfort and sedation after surgery and anesthesia once the swallowing reflex returned and a body temperature of ≥ 36.7°C (98.1°F) was attained. Pain and sedation scores were acquired at 60-minute intervals for 4 hours, then at 4-hour intervals for 24 hours. Dogs with a postoperative pain score > 5 of 24 were given morphine as rescue analgesia.

RESULTS

No differences in heart rate, respiratory rate, systolic arterial blood pressure, end-tidal Pco2, end-tidal isoflurane concentration, and vaporizer setting were detected among groups. No differences in pain score, sedation score, rescue analgesia requirement, or time to first rescue analgesia after surgery were detected.

CONCLUSIONS AND CLINICAL RELEVANCE

Pain scores were similar among groups, and all 4 groups had similar rescue analgesia requirements and similar times to first administration of rescue analgesia. All 4 analgesic protocols provided acceptable analgesia for 24 hours after stifle joint surgery.

Effects of lidocaine, dexmedetomidine or their combination on the minimum alveolar concentration of sevoflurane in dogs

The aim of this study was to determine the effects of lidocaine (LIDO) and dexmedetomidine (DEX) or their combination (LIDO-DEX), administered by constant-rate infusion (CRI), on the minimum alveolar concentration (MAC) of sevoflurane in dogs. Seven healthy mongrel dogs were used with a 2-week washout interval between treatments in this study. Anesthesia was induced with propofol and maintained with sevoflurane in oxygen, and MAC of sevoflurane was determined after 90 min equilibration period in the dogs (SEV-MACBASAL). Then, sevoflurane MAC was determined again in the dogs after 45 min equilibration period of one of the following treatments: an intravenous loading dose of lidocaine 2 mg/kg followed by 6 mg/kg/hr CRI (SEV-MACLIDO); an intravenous loading dose of dexmedetomidine 2 µg/kg followed by 2 µg/kg/hr CRI (SEV-MACDEX); or their combination (SEV-MACLIDO-DEX). These SEV-MACs were determined in duplicate. Data were analyzed using ANOVA and post hoc Tuckey test when appropriate. The SEV-MACBASAL was 1.82 ± 0.06%, SEV-MACLIDO was 1.38 ± 0.08%, SEV-MACDEX was 1.22 ± 0.10%, and SEV-MACLIDO-DEX was 0.78 ± 0.06%. The CRI administration of lidocaine, dexmedetomidine and their combination produced a significant reduction in the MAC of sevoflurane by 26.1 ± 9.0% (P<0.0001), 43.7 ± 11.8% (P<0.0002) and 54.4 ± 9.8% (P<0.0001), respectively. The MAC reduction was significantly greater after the CRI combination of lidocaine and dexmedetomidine when compared with lidocaine CRI (P<0.0001) or dexmedetomidine CRI treatments (P<0.025).

Sparing effect of robenacoxib on the minimum alveolar concentration for blunting adrenergic response (MAC-BAR) of sevoflurane in dogs

Robenacoxib is a newer nonsteroidal anti-inflammatory drug approved for dogs and cats. This study was designed to evaluate the effect of robenacoxib on the minimum alveolar concentration for blunting adrenergic response (MAC-BAR) of sevoflurane in dogs. Sevoflurane MAC-BAR was determined by judging dogs’ response to a noxious electrical stimulus (50 V, 50 Hz and 10 msec) for 10 sec in 6 beagle dogs on two occasions at least a 7-day interval. In each occasion, saline (0.1 ml/kg) or robenacoxib (2 mg/kg) was administered subcutaneously at 1 hr prior to the MAC-BAR determination. Robenacoxib significantly decreased the sevoflurane MAC-BAR (3.44 ± 0.53% for saline vs. 2.84 ± 0.38% for robenacoxib, P=0.039). These results suggest that subcutaneous robenacoxib provides a clinically relevant sparing effect on anesthetic requirement.

Reduction of the sevoflurane minimum alveolar concentration induced by methadone, tramadol, butorphanol and morphine in rats

This study aimed to estimate the reduction in the minimum alveolar concentration (MAC) of sevoflurane induced by low and high doses of methadone (5 and 10 mg/kg), tramadol (25 and 50 mg/kg), butorphanol (5 and 10 mg/kg) or morphine (5 and 10 mg/kg) in the rat. A control group received normal saline. Sixty-three adult male Sprague-Dawley rats were anaesthetized with sevoflurane ( n = 7 per group). Sevoflurane MAC was then determined before and after intraperitoneal administration of the opioids or saline. The duration of the sevoflurane MAC reduction and basic cardiovascular and respiratory measurements were also recorded. The baseline MAC was 2.5 (0.3) vol%. Methadone, tramadol and morphine reduced the sevoflurane MAC (low dose: 31 ± 10, 38 ± 15 and 30 ± 13% respectively; high dose: 100 ± 0, 83 ± 17 and 77 ± 25%, respectively) in a dose-dependent manner. The low and high doses of butorphanol reduced the sevoflurane MAC to a similar extent (33 ± 7 and 31 ± 4%, low and high doses, respectively). Two rats developed apnoea following administration of high-dose butorphanol and methadone. These anaesthetic-sparing effects are clinically relevant and may reduce the adverse effects associated with higher doses of inhalational anaesthetics.