Comparative study on the sedative effects of morphine, methadone, butorphanol or tramadol, in combination with acepromazine, in dogs

Abstract

Objective  To compare the effects of morphine (MOR), methadone (MET), butorphanol (BUT) and tramadol (TRA), in combination with acepromazine, on sedation, cardiorespiratory variables, body temperature and incidence of emesis in dogs.

Study design  Prospective randomized, blinded, experimental trial.

Animals  Six adult mixed-breed male dogs weighing 12.0 ± 4.3 kg.

Methods  Dogs received intravenous administration (IV) of acepromazine (0.05 mg kg⁻¹) and 15 minutes later, one of four opioids was randomly administered IV in a cross-over design, with at least 1-week intervals. Dogs then received MOR 0.5 mg kg⁻¹; MET 0.5 mg kg⁻¹; BUT 0.15 mg kg⁻¹; or TRA 2.0 mg kg⁻¹. Indirect systolic arterial pressure (SAP), heart rate (HR), respiratory rate (f_R), rectal temperature, pedal withdrawal reflex and sedation were evaluated at regular intervals for 90 minutes.

Results  Acepromazine administration decreased SAP, HR and temperature and produced mild sedation. All opioids further decreased temperature and MOR, BUT and TRA were associated with further decreases in HR. Tramadol decreased SAP whereas BUT decreased f_R compared with values before opioid administration. Retching was observed in five of six dogs and vomiting occurred in one dog in MOR, but not in any dog in the remaining treatments. Sedation scores were greater in MET followed by MOR and BUT. Tramadol was associated with minor changes in sedation produced by acepromazine alone.

Conclusions and clinical relevance  When used with acepromazine, MET appears to provide better sedation than MOR, BUT and TRA. If vomiting is to be avoided, MET, BUT and TRA may be better options than MOR.

Introduction

Neuroleptanalgesic combinations are commonly used in veterinary medicine to facilitate handling of small animals, as premedication and to provide analgesia for minor surgical procedures. Phenothiazine agents, α₂-adrenoceptor agonists and opioid analgesics are the most commonly used drugs to produce neuroleptanalgesia. When these drugs are administered together, synergism seems to occur; sedation and analgesia being greater than that achieved with either drug given alone (Hall et al. 2001a).

Phenothiazine derivatives, such as acepromazine, are often used to produce mild to moderate tranquilization/sedation in dogs. This class of drugs, also named as neuroleptics, interferes with dopamine transmission within the central nervous system (Baldessarini & Tarazi 2001). Other advantages related to the use of acepromazine in dogs include the reduced requirement of both injectable (Thurmon et al. 1996) and inhaled anaesthetics (Heard et al. 1986; Webb & O’Brien 1988), antiemetic (Valverde et al. 2004) and anti-arrhythmogenic effects (Dyson & Pettifer 1997). However, phenothiazine agents do not produce analgesia in dogs (Hall et al. 2001a).

Opioid analgesics are used primarily to produce analgesia without resulting in loss of consciousness. The analgesia produced by this class of drugs is mediated by stimulation of opioid receptors (μ, κ and δ) located mainly in the brain and in the dorsal horn of the spinal cord (Wagner 2002).

Morphine is the prototype opioid to which all others are compared. Morphine is assigned an analgesic potency of 1 and although other opioids are known to have greater analgesic potency, none has been shown to be more effective in relieving pain (Wagner 2002). Morphine possesses high affinity for μ receptors where it acts as an agonist, resulting in analgesia (Hall et al. 2001a). Adverse effects following MOR administration in dogs include vomiting (Blancquaert et al. 1986; Valverde et al. 2004) and release of histamine when administered intravenously (Robinson et al. 1988; Guedes et al. 2006).

Methadone is a synthetic opioid with high affinity for μ receptors and similar analgesic potency to MOR (Hall et al. 2001a). It was also reported that MET might act as an antagonist on N-methyl-d-aspartate (NMDA) receptors and this property might contribute to its analgesic effect as well as prevent development of tolerance (Wagner 2002). Methadone does not induce emesis (Blancquaert et al. 1986) and, in humans, it has a low potential for the release of histamine (Bowdle et al. 2004). In dogs, methadone alone induced mild sedation wheras the combination of methadone and acepromazine produced mild to intense sedation with minimal cardiorespiratory effects (Monteiro et al. 2008).

Tramadol is a synthetic opioid with low affinity for μ receptors and an analgesic potency of one-tenth of that of MOR (Duthie 1998). However, it has been reported in the literature that the analgesic properties of TRA result from both opioid and nonopioid mechanisms (Miranda & Pinardi 1998). This opioid was shown to inhibit the reuptake of norepinephrine and serotonin, achieving spinal modulation of pain and preventing impulses reaching the brain (Duthie 1998). In dogs, TRA produced similar analgesia to MOR in the early postoperative period following ovariohysterectomy (Mastrocinque & Fantoni 2003). The incidence of nausea and vomiting was lower than with other opioids in humans (Duthie 1998) and it does not release histamine (Barth et al. 1987). To the author’s knowledge, there are no data available about the use of TRA alone or in combination with tranquilizing/sedative drugs for premedication in dogs.

Butorphanol is a synthetic opioid with agonist–antagonist properties. Its analgesic effect results from activation of κ receptors. Conversely, BUT was also shown to have affinity for μ receptors where it acts as an antagonist. The analgesic potency of BUT is 3–5 times that of MOR. However, a ceiling effect was reported with no further increase in analgesia after 0.8–1.0 mg kg⁻¹ (Wagner 2002). In dogs, BUT alone or in combination with acepromazine was associated with minimal cardiopulmonary depression (Trim 1983; Cornick & Hartsfield 1992) and it was also shown to have an antiemetic effect (Moore et al. 1994).

The purpose of the present study was to compare the sedative effects of MOR, MET, BUT and TRA, in combination with acepromazine, in dogs. It was also aimed to evaluate the cardiorespiratory effects and adverse effects of each of the combinations.

We hypothesized that the degree of sedation provided by administration of acepromazine/opiod combinations to dogs would vary depending on the opioid chosen.

Materials and methods

Dogs

Six adult mixed-breed male dogs were used in the study. Mean ± SD weight of the dogs was 12.0 ± 4.3 kg. Health status was assessed by means of physical examination, a complete blood count and serum biochemical analyses; all findings were within reference ranges. The study was approved by the Animal Care and Use Committee of the University of Londrina (protocol 43/06).

Study design and experimental protocol

Food but not water was withheld for 12 hours prior to the experiment. Each dog was allowed to acclimate to a small quiet room with temperature at 25 °C for at least 30 minutes before each experiment was started. Subsequently, the dogs were instrumented with a Doppler (Doppler model 841-A; Parks Medical Electronics, Aloha, OR, USA) ultrasonic flow probe placed on the palmar digital artery with a sphygmomanometer and cuff (cuff width was 40–50% of limb circumference) placed above the carpus, to allow determination of indirect systolic arterial pressure (SAP). The accuracy of the sphygmomanometer was checked prior to the beginning of the study by means of a mercury manometer. Pulse rate (PR) was counted from the amplified doppler flow probe, respiratory rate (f_R) was measured by observing thoracic excursions and rectal temperature was determined by means of a digital thermometer. Pedal withdrawal reflex was evaluated by a toe pinch in the thoracic limb with a 20-cm haemostat with protective rubber tubing on each jaw that was clamped to the third ratchet for 15 seconds, or until a positive response was observed (crying, withdrawal of the limb or looking at the stimulated limb). The degree of sedation was assessed using a numeric descriptive scale (NDS) and a visual analogue scale (VAS). The NDS (Valverde et al. 2004) consisted of a scale ranging from 0 to 3, with 0: no sedation; 1: mild sedation (less alert but still active); 2: moderate sedation (drowsy, recumbent but can walk); and 3: intense sedation (very drowsy, unable to walk). The VAS consisted of a 10-cm line representing no sedation at the left end and the most sedation possible at the right end. An observer was responsible for placing a mark on the line that corresponded to the degree of sedation for the animal. The distance between the left end of the scale and the mark was considered the VAS score. To evaluate the degree of sedation, the dog was observed initially undisturbed and unrestrained on a stainless steel table with a rubber mat. Subsequently, with the dog in lateral recumbency, the remaining variables were measured. Finally, the assessor observed if the dog was able to walk when placed on the floor. A single blinded assessor, who was familiar with the dog’s normal behaviour, was responsible for assessing objective and subjective data throughout the study. Following determination of baseline data, each animal had a 20-SWG cephalic catheter placed for drug administration. Subsequently, the dogs received IV acepromazine at 0.05 mg kg⁻¹ (Acepran 0.2%; Univet, São Paulo, SP, Brazil). The final volume of acepromazine was corrected to a standardized volume of 1 mL with physiologic saline (NaCl 0.9%) and administered over 1 minute. Fifteen minutes after acepromazine administration (Time ACP), SAP, PR, f_R, temperature, NDS, VAS and pedal withdrawal reflex were recorded. Thereafter, dogs were assigned to receive IV administration of each of four treatments randomly in a cross-over design as follows: MOR 0.5 mg kg⁻¹ (Dimorf; Cristália, Itapira, SP, Brazil); MET 0.5 mg kg⁻¹ (Metadon; Cristália, Itapira, SP, Brazil); BUT 0.15 mg kg⁻¹ (Torbugesic; Fort Dodge, IA, USA); or TRA 2.0 mg kg⁻¹ (Tramadon; Cristália, Itapira, SP, Brazil). The final volume of each opioid was corrected to a standardized volume of 5 mL with physiologic saline and administered over 5 minutes. A minimum washout period of 7 days was allowed between treatments. Cardiorespiratory variables, NDS, VAS and pedal withdrawal reflex were evaluated again at 15-minute intervals for 90 minutes (Times 15, 30, 45, 60, 75 and 90). Temperature was measured at Times 60 and 90.

Statistical analysis

Differences among treatments in SAP, PR, f_R and temperature were analyzed by a 2-way repeated measures anova with time and treatment as main factors. When an overall treatment effect was detected, the Bonferroni correction for multiple pairwise comparisons was performed to determine what treatments differed. The same approach was used to compare the effect of acepromazine (Time ACP) on parametric data (SAP, PR, f_R and temperature) with mean values at baseline. A one-way repeated measures anova followed by a Dunnet’s test was used to detect differences within each treatment between Time ACP and T15–T90. Variables not normally distributed (NDS and VAS) were analyzed by a Friedman test followed by a Dunn’s multiple comparison test to assess differences among treatments at all time points and differences over time. For all analyses, values of p < 0.05 were considered significant.

Results

Effects of acepromazine administration (Time ACP)

Mean values of SAP, PR, f_R and rectal temperature did not differ at baseline or at Time ACP among treatments (Tables 1 and 2). Following acepromazine administration, PR decreased in MOR, BUT and TRA. Although PR also decreased in MET, the difference was not statistically significant. Systolic arterial pressure decreased significantly in MOR, MET and BUT treatments after acepromazine administration. The change in f_R was not significant with acepromazine administration (Table 1). A decrease in rectal temperature was observed in all treatments after acepromazine. The mean reduction for all treatments was 0.4 °C (Table 2). Acepromazine resulted in mild sedation in most dogs. The median NDS was 1.0 in all treatments whereas the median VAS for each treatment were 3.4, 3.0, 3.5 and 3.1 in MOR, MET, BUT and TRA respectively (Table 3).

Table 1.   Mean ± SD pulse rate (PR), systolic arterial pressure (SAP) and respiratory rate (f_R) in six dogs at baseline (BL), at 15 minutes after intravenous (IV) administration of 0.05 mg kg⁻¹ of acepromazine (ACP) and at 15, 30, 45, 60, 75 and 90 minutes after IV administration of 0.5 mg kg⁻¹ of morphine (MOR), 0.5 mg kg⁻¹ of methadone (MET), 0.15 mg kg⁻¹ of butorphanol (BUT) or 2.0 mg kg⁻¹ of tramadol (TRA)

	BL	ACP	Time after opioid (treatment) administration (minutes)
			15	30	45	60	75	90
*Significantly different from ACP; †significantly different from MET; ‡significantly different from MOR (p < 0.05).
PR (beats minute^−1)
MOR	99 ± 19*	76 ± 3	60 ± 3*	59 ± 5*†	58 ± 9*†	58 ± 6*†	58 ± 11*†	58 ± 10*†
MET	94 ± 17	83 ± 10	73 ± 29	91 ± 37	85 ± 28	91 ± 31	92 ± 30	89 ± 26
BUT	100 ± 23*	84 ± 24	64 ± 12*	63 ± 14*†	64 ± 18*	63 ± 14*†	64 ± 17*†	68 ± 16*
TRA	97 ± 14*	79 ± 9	68 ± 15	62 ± 10*†	63 ± 11*	65 ± 19†	62 ± 18*†	67 ± 22
SAP (mmHg)
MOR	130 ± 13*	101 ± 5	101 ± 9	94 ± 7	95 ± 7	95 ± 6	96 ± 8	98 ± 8
MET	131 ± 16*	113 ± 11	108 ± 8	106 ± 10	107 ± 8	104 ± 11	105 ± 11	107 ± 12
BUT	123 ± 9*	107 ± 6	101 ± 10	101 ± 8	102 ± 8	103 ± 9	106 ± 13	108 ± 12
TRA	126 ± 10	113 ± 6	102 ± 7*	98 ± 8*	98 ± 7*	100 ± 8*	100 ± 11*	99 ± 13*
fR (breaths minute^−1)
MOR	38 ± 17	27 ± 20	47 ± 52	40 ± 44	27 ± 16	24 ± 8	23 ± 7	23 ± 6
MET	39 ± 27	27 ± 12	40 ± 41	36 ± 37	34 ± 33	30 ± 23	28 ± 22	26 ± 13
BUT	45 ± 23	25 ± 10	17 ± 6*‡	17 ± 4*	16 ± 5*	16 ± 4*	17 ± 4*	18 ± 4*
TRA	44 ± 35	26 ± 15	23 ± 17	17 ± 4	20 ± 10	17 ± 6	17 ± 6	20 ± 7

Table 2.   Mean ± SD rectal temperature in six dogs at baseline (BL), at 15 minutes after IV administration of 0.05 mg kg⁻¹ of acepromazine (ACP) and at 60 and 90 minutes after intravenous (IV) administration of 0.5 mg kg⁻¹ of morphine (MOR), 0.5 mg kg⁻¹ of methadone (MET), 0.15 mg kg⁻¹ of butorphanol (BUT) or 2.0 mg kg⁻¹ of tramadol (TRA)

	BL	ACP	Time after opioid (treatment) administration (minutes)
			60	90
*Significantly different from ACP; †significantly different from MET; ‡significantly different from MOR (p < 0.05).
MOR	38.9 ± 0.3*	38.4 ± 0.3	37.1 ± 0.2*	36.8 ± 0.2*
MET	38.9 ± 0.3*	38.5 ± 0.4	36.8 ± 0.4*	36.4 ± 0.6*
BUT	38.9 ± 0.4*	38.4 ± 0.3	37.6 ± 0.4*†	37.6 ± 0.5*†‡
TRA	38.9 ± 0.1*	38.6 ± 0.4	37.9 ± 0.4*†‡	37.8 ± 0.4*†‡

Table 3.   Median (25th to 75th percentile range) numeric descriptive scale (NDS) and visual analog scale (VAS) sedation scores in six dogs at 15 minutes after IV administration of 0.05 mg kg⁻¹ of acepromazine (ACP) and at 15, 30, 45, 60, 75 and 90 minutes after intravenous (IV) administration of 0.5 mg kg⁻¹ of morphine (MOR), 0.5 mg kg⁻¹ of methadone (MET), 0.15 mg kg⁻¹ of butorphanol (BUT) or 2.0 mg kg⁻¹ of tramadol (TRA)

	ACP	Time after opioid (treatment) administration (minutes)
		15	30	45	60	75	90
*Significantly different from ACP; †significantly different from MET (p < 0.05).
NDS
MOR	1.0 (1.0–1.5)	1.5 (1.0–2.5)	2.0 (1.0–2.5)	2.0 (1.0–2.5)	1.5 (1.0–2.0)	1.5 (1.0–2.0)	1.0 (1.0–1.5)
MET	1.0 (1.0–1.0)	3.0 (2.5–3.0)*	3.0 (3.0–3.0)*	3.0 (2.5–3.0)*	3.0 (2.0–3.0)*	2.0 (1.5–2.0)	2.0 (1.0–2.0)
BUT	1.0 (1.0–1.5)	1.5 (1.0–2.0)	1.5 (1.0–2.0)	1.0 (1.0–2.0)	1.0 (1.0–2.0)	1.0 (1.0–2.0)	1.0 (0.5–2.0)
TRA	1.0 (1.0–1.5)	1.0 (1.0–1.5)†	1.0 (1.0–1.0)†	1.0 (1.0–1.0)†	1.0 (0.5–1.0)†	1.0 (0.5–1.0)	0.5 (0.0–1.0)
VAS
MOR	3.4 (2.2–3.7)	4.9 (4.5–6.7)	6.0 (5.3–7.6)*	6.2 (5.6–7.9)*	6.0 (5.4–7.6)*	5.8 (5.0–7.0)*	4.9 (4.5–5.6)
MET	3.0 (2.5–3.8)	5.9 (4.6–7.0)	7.4 (6.2–8.6)*	7.6 (6.6–8.6)*	7.7 (6.3–8.2)*	6.9 (5.6–7.5)	6.1 (4.1–6.7)
BUT	3.5 (2.4–4.4)	6.0 (4.5–6.2)	6.7 (6.0–7.1)*	6.8 (6.2–7.3)*	6.3 (6.1–7.0)*	5.4 (4.8–6.4)	4.1 (3.0–5.4)
TRA	3.1 (2.1–4.1)	5.1 (3.8–6.2)	5.3 (4.6–7.0)*	5.7 (4.2–7.1)*	5.1 (4.1–6.3)†	4.1 (3.4–5.4)†	3.0 (2.7–4.2)

Effects of opioid administration (Times 15–90)

The six dogs which received MET had intense sedation (NDS = 3) at least at one time point. Sedation ranged from mild to intense (NDS = 1–3) in dogs receiving MOR. One dog in MOR was assigned an NDS score of 3. In the remaining five dogs, NDS was either 1 (2/6 dogs) or 2 (3/6 dogs). In dogs which received BUT, sedation was mild to moderate; NDS = 1, in three dogs and = 2, in the remaining three animals. In five of six dogs given TRA, sedation was not increased after opioid administration and NDS was 1. Only one of six dogs in TRA had moderate sedation (NDS = 2) at T15.

From 15 to 60 minutes after treatment administration, NDS was higher in MET than in TRA. There were no other significant differences between treatments in NDS. Compared with NDS at Time ACP, NDS was increased in MET from T15 to T60 (Table 3).

As in NDS, the VAS sedation scores were greater in MET (Table 3). MET had greater VAS scores than TRA at T60 and T75. There was an increase in VAS scores in all treatments compared with values at Time ACP. The differences were significant from T30 to T75 in MOR, from T30 to T60 in MET and BUT and at T30 and T45 in TRA (Table 3).

Pulse rate was higher in MET than in MOR from T30 to T90 and higher in MET than in BUT and TRA at T30, T60 and T75. A decrease in PR, compared with values at Time ACP, was observed from T15 to T90 in MOR and BUT and at T30, T45 and T75 in TRA (Table 1). Bradycardia (PR < 60 beats minute⁻¹) was observed at least at one time point in 3/6 dogs in MOR, MET and BUT and in 4/6 dogs in TRA.

Dogs which received MOR had the lowest values of SAP. However, significant differences in SAP were not detected among treatments throughout the study. Systolic arterial pressure decreased significantly in dogs receiving TRA from T15 to T90 compared with Time ACP. In the remaining treatments, SAP values after ACP did not differ significantly after opioid administration (Table 1). Hypotension (SAP < 90 mmHg) was observed at least at one time point in 2/6, 1/6 and 2/6 dogs given MOR, BUT and TRA respectively.

Respiratory rate was higher in MOR than in BUT at T15. Other differences in f_R among treatments were not observed. Respiratory rate decreased in BUT from T15 to T90 compared with values at Time ACP (Table 1). Panting was observed in one of six dogs in MOR and MET.

Rectal temperature decreased after administration of all treatments at T60 and T90 compared with values at Time ACP. At T60, temperature was lower in MET than in BUT and TRA and lower in MOR than in TRA. At T90, temperature was lower in MOR and MET than in BUT and TRA. Compared with values at Time ACP, rectal temperature at T90 decreased by 2.1, 1.6, 0.8 and 0.8 °C in MET, MOR, BUT and TRA respectively (Table 2).

Pedal withdrawal reflex was absent in one dog in MET within 15 minutes after treatment administration and in another dog in the same treatment from T15 to T60. In the remaining treatments, pedal withdrawal reflex was present in all dogs throughout the study.

Retching was observed in five of six dogs and vomiting was observed in one dog in MOR. Retching and vomiting were not observed in any dog in MET, BUT and TRA. One of six dogs in MOR and two of six dogs in MET defecated during or immediately after opioid administration.

Discussion

A limitation of the present study was that not all the opioids used were employed in equipotent doses for analgesia. Morphine is the prototype opioid to which all others are compared with a relative potency of 1. Methadone was reported to have 1 to 1.5 the potency of MOR whereas BUT is 3–5 times more potent than MOR (Wagner 2002). Tramadol is said to have 1/10 the potency of MOR (Duthie 1998). Thus, the doses of MOR, MET and BUT employed in the present study (0.5, 0.5 and 0.15 mg kg⁻¹ respectively) may be considered to be equipotent whereas the dose of TRA (2.0 mg kg⁻¹) may not. The equipotent dose of TRA to 0.5 mg kg⁻¹ of MOR would be 5 mg kg⁻¹. However, this dose is higher than the dose commonly used in clinical practice (Gaynor 2002; Mastrocinque & Fantoni 2003). All the doses employed in the present study are within reported clinical ranges for dogs (Hall et al. 2001b; Gaynor 2002; Wagner 2002; Mastrocinque & Fantoni 2003).

Phenothiazine agents produce mild to moderate sedation. Peak sedation occurs within 20 minutes following IV administration of acepromazine. Although other drugs, such as alpha₂-adrenoceptor agonists, produce dose related sedation, increasing the dose of a phenothiazine does not result in enhancement of the degree of sedation and it may actually intensify the adverse effects (Hall et al. 2001a). When moderate to intense sedation is desired, phenothiazine derivatives should be administered in combination with other drugs with sedative properties such as opioid analgesics. The degree of sedation is greater with the combination than with either agent alone (Smith et al. 2001). In the present study, acepromazine alone produced mild sedation. However, when an opioid was administered 15 minutes after the phenothiazine, sedation was greater than that achieved with acepromazine alone as judged by our visual analog scale.

The sedative effect of opioids results from their interaction with μ and κ receptors (Muir 2002). However, other factors may influence sedation produced by opioid administration. In addition to the type of receptor activated, the dose, differences in pharmacokinetics, the number of observers assessing sedation and individual variation may affect the sedation score. In the present study, we aimed to minimize the influence of all these factors in the assessment of sedation by using equipotent doses, by administering the drugs intravenously, to minimize differences in the absorption times and bioavailability, by using a single blinded observer and by using a randomized cross-over design to minimize individual variation.

It has been reported in the literature that administration of μ-opioids alone results in mild to moderate sedation whereas κ-agonists produce mild sedation (Muir 2002). In a previous study, greater sedation was achieved when the μ-opioid oxymorphone was administered in combination with acepromazine than the combination of the κ-agonist BUT with acepromazine (Cornick & Hartsfield 1992). In the present study, a trend for better sedation was also observed when acepromazine was administered in combination with the μ opioids agonists (MOR and MET). The degree of sedation produced by the combination acepromazine/BUT varied with the scoring system used. Greater sedation was recorded with the VAS than with NDS. This finding is not unexpected as sedation is a subjective variable.

Tramadol is an opioid with low affinity for μ receptors, which also acts by inhibiting the reuptake of norepinephrine and serotonin (Gaynor 2002). This mixed mechanism of action might explain the analgesic effect produced by TRA administration in spite of its low affinity for opioid receptors. In addition to its ability to block these reuptake mechanisms, TRA is metabolized in the liver to O-desmethyltramadol (M1), which is a pharmacologically active metabolite. M1 was reported to have greater affinity for μ receptors than TRA itself (Poulsen et al. 1996) and it is thought that this metabolite contributes to the analgesic effect of TRA (Poulsen et al. 1996; Shipton 2000). M1 also appears to play a role in sedation produced by TRA as its IV administration resulted in sedation in dogs whereas IV (4 mg kg⁻¹) or oral (11 mg kg⁻¹) administration of the parent drug TRA did not (Kukanich & Papich 2004). In the present study, the degree of sedation produced by acepromazine/TRA was lower than that achieved with the other combinations. It might be hypothesized that the low affinity of TRA for μ receptors or the production of insufficiently high concentrations of M1 during the course of observation, or both, may be responsible for these results.

Results of the present investigation indicate that acepromazine in combination with MET, MOR or BUT results in good sedation; this effect being apparently greater with acepromazine/MET. Tramadol has little influence on sedation achieved with acepromazine alone. Thus, the combination acepromazine/TRA is not recommended when moderate to intense sedation is required. Peak sedative effect appears to occur within 30–45 minutes after administration of all combinations.

Pedal withdrawal reflex was used in the present study to assess acute somatic pain subjectively. However, the results of the present study should be interpreted carefully because drugs that influence vigilance, motor responses and autonomic reflexes, such as neuroleptanalgesic combinations, are thought to affect the subjective experience of pain making it difficult to differentiate sedative and analgesic effects of drugs (Ansah et al. 1998). Thus, one could not ascertain, based on the results of our study, that better analgesia was provided by the combination acepromazine/MET.

The effects of acepromazine on HR and arterial blood pressure of dogs have been previously reported. Acepromazine causes a decrease in arterial pressure (Popovic et al. 1972; Turner et al. 1974; Stepien et al. 1995), which is mediated through peripheral alpha-₁ adrenoceptor block and depression of the vasomotor centre within the hypothalamus, resulting in vasodilation (Thurmon et al. 1996). However, hypotension (SAP < 90 or MAP < 70 mmHg) was not reported following acepromazine administration in healthy animals. The effects of acepromazine on PR in dogs are variable. Tachycardia in response to a decrease in arterial blood pressure (Turner et al. 1974), a slight decrease (Popovic et al. 1972) or no change (Stepien et al. 1995) have been reported. In the study reported here, a 15% decrease in SAP and a 17% decrease in PR were observed after acepromazine administration. Hypotension was not observed in any of the dogs treated with acepromazine and only one dog developed bradycardia (PR = 56 beats minute⁻¹).

The cardiovascular effects of opioids are quite variable and may be influenced by the drug, its dose and species involved (Hall et al. 2001a). This class of drugs seems to affect myocardial contractility minimally or not at all. However, increased vagal tone may result in bradycardia (Hall et al. 2001a; Wagner 2002). A decrease in HR was reported in dogs treated with MET, oxymorphone and hydromorphone alone (Smith et al. 2001; Monteiro et al. 2008) and following administration of MET, buprenorphine, oxymorphone, hydromorphone or BUT in combination with acepromazine (Cornick & Hartsfield 1992; Stepien et al. 1995; Smith et al. 2001; Monteiro et al. 2008). The present study was in agreement with previous reports. A decrease in PR was observed after administration of acepromazine with MOR, BUT or TRA. However, PR did not change significantly after the combination acepromazine/MET and this finding is in contrast with previous studies. In a previous study performed in our laboratory, PR decreased after intramuscular administration of 0.5 mg kg⁻¹ of MET alone or in combination with acepromazine (0.05 mg kg⁻¹, IM) (Monteiro et al. 2008). The differences between the two studies may be due to the routes of administration employed (IV versus IM). In another study in conscious dogs, IV MET (1 mg kg⁻¹) also decreased HR (Hellebrekers et al. 1989). In the latter study, the authors suggested that bradycardia was mediated, at least partially, through vasopressin release after MET administration in doses of 1 mg kg⁻¹ or higher. Vasoconstriction and increased systemic vascular resistance occurs as a result of vasopressin release and induces a compensatory decrease in HR and cardiac output. These findings suggest that another mechanism might be involved in the decrease in PR after MET administration, in addition to the centrally mediated increase in vagal tone reported before. It is possible that, at the dose used in the present study (0.5 mg kg⁻¹, IV), MET did not induce significantly high concentrations of vasopressin release and consequently, systemic vascular resistance did not increase. Therefore, a compensatory bradycardia was not observed. Another hypothesis is that the alpha-adrenergic blocking properties of acepromazine overwhelmed the vasoconstriction induced by vasopressin.

Intravenous administration of MOR and meperidine results in histamine release; which causes vasodilation and decreases arterial blood pressure, this effect being prevented when the drugs are administered intramuscularly (IM) (Hall et al. 2001a). In addition to the route, the rate of administration also influences the release of histamine and this effect appears to be attenuated by slow IV administration (Moss & Rosow 1983). In the present study, we minimized the release of histamine by administering the opioids slowly over 5 minutes. Hence, it is not likely that arterial blood pressure was influenced by histamine release after administration of MOR or any other treatments.

A decrease in arterial blood pressure has been reported after administration of acepromazine in combination with buprenorphine, hydromorphone, oxymorphone or BUT in dogs, although severe hypotension was not reported following any of the combinations (Cornick & Hartsfield 1992; Stepien et al. 1995; Smith et al. 2001). The reduction in blood pressure seems to be more pronounced when higher doses of the phenothiazine (0.22 mg kg⁻¹, IV) were used (Cornick & Hartsfield 1992) and less likely to be influenced by increasing doses of opioids (Stepien et al. 1995). These findings are supported by a previous study which reported that the alpha-adrenergic block induced by acepromazine is dose related (Ludders et al. 1983). In the present study, a further decrease in SAP compared with mean values at time ACP was observed only after TRA administration, but TRA has not yet been reported to reduce cardiac output or to decrease systemic vascular resistance in dogs. Additionally, histamine release, which could result in decreased systemic vascular resistance, was not observed following TRA administration in people (Barth et al. 1987). Further research is required to evaluate the haemodynamic effects of TRA in dogs.

Acepromazine has little effect on respiratory function. Although f_R may decrease, blood-gases are not significantly changed (Popovic et al. 1972; Turner et al. 1974). Conversely, opioid administration may result in respiratory depression by decreasing ventilatory response to hypercapnia (Wagner 2002); this effect being more pronounced with the μ agonists than with the κ agonists (Hall et al. 2001a). When opioids are used alone and within clinical dose ranges in dogs, respiratory depression is unlikely to occur. This effect becomes more important when opioids are administered in combination with other respiratory depressant drugs such as injectable or inhaled general anaesthetics (Wagner 2002). However, when opioids were used in conjunction with acepromazine in healthy dogs, respiratory depression did not occur. Minimal changes were observed in pH, bicarbonate concentration, PaCO₂ and PaO₂ in dogs treated with acepromazine and buprenorphine, oxymorphone or BUT (Cornick & Hartsfield 1992; Stepien et al. 1995). In the present study, no dog had apnoea or obvious cyanosis. However, as blood-gases were not measured, it cannot be ascertained that respiratory depression did not occur.

The decrease in temperature after administration of all treatments in the present study was due to the effects of acepromazine and opioids on thermoregulatory mechanisms (Thurmon et al. 1996; Hall et al. 2001a; Wagner 2002). Rectal temperature decreased more in MOR and MET than in other treatments. It has been reported that opioids that cause panting may increase heat loss through airways (Wagner 2002). However, in the present study, only one of six dogs in MOR and MET were panting during the study and panting in these dogs was not related to the lowest temperatures observed.

Vomition is an adverse effect that may occur after administration of low lipid soluble opioids such as MOR, hydromorphone and oxymorphone (Valverde et al. 2004). This effect is thought to result from stimulation of δ receptors in the chemoreceptor trigger zone (CTZ) (Blancquaert et al. 1986). The incidence of vomition after IM administration of MOR (0.5 mg kg⁻¹) was 75% (Valverde et al. 2004). In the present study, premedication with acepromazine masked the true emetic effect of the opioids employed. One of six dogs (17%) vomited after MOR administration whereas none of the dogs vomited after MET, BUT and TRA, demonstrating a higher potential for MOR to induce vomiting in nonpainful, healthy dogs than for the other opioids tested, probably because of its lower lipid solubility. A previous study, using the same doses as the present one, reported a similar incidence of vomiting (25%) after acepromazine followed by MOR (Valverde et al. 2004).

Results of the present study suggest that, in the doses used, all combinations used are well-tolerated by healthy dogs. The major side effect observed was bradycardia. Good sedation was achieved with acepromazine in combination with MET, MOR or BUT, but not TRA. If vomiting is to be avoided, MET, BUT and TRA may be better options.

Acknowledgement

Partial funding for this research was provided by PROBIC-CESUMAR (Programa de Bolsas de Iniciação Científica – Centro Universitário de Maringá).