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The sedative effect of acepromazine combined with 2 doses of tramadol [3 and 5 mg/kg body weight (BW)] was compared with the sedative effect of acepromazine alone in dogs and the effects of each sedative protocol on cardiorespiratory variables were examined. This was a prospective, randomized, blinded, crossover study. Each of 6 dogs received 3 treatments at 1-week intervals. During all anesthetic episodes, dogs received 0.05 mg/kg BW acepromazine. Approximately 25 min later, dogs were given physiological saline (control) or tramadol [3 mg/kg BW (TR3) or 5 mg/kg BW (TR5)]. All drugs were administered intravenously. Variables evaluated included heart rate (HR), respiratory rate (RR), systolic, mean, and diastolic blood pressures (SAP, MAP, and DAP), and sedation [by use of a simple descriptive scale (SDS, range: 0 to 3) and a numeric rating scale (NRS, range: 0 to 10)]. Variables were recorded 25 min after acepromazine and for 80 min after saline or tramadol. Acepromazine administration resulted in mild sedation in most dogs and decreased RR, SAP, MAP, and DAP in all treatments. Tramadol administration did not significantly increase SDS or NRS scores compared to acepromazine alone. The only exception to this rule was observed at 20 min after TR3, when NRS was higher in this group than in the control treatment. Administration of tramadol (TR3 and TR5) decreased HR. Under the conditions of this study, sedation induced by acepromazine with tramadol was similar to that of acepromazine alone. The main adverse effects of the combination were a decrease in blood pressure and HR, without clinical significance.
L’effet sédatif de l’acépromazine combiné à deux doses de tramadol [3 et 5 mg/kg de poids corporel (PC)] a été comparé à l’effet sédatif de l’acépromazine seul chez des chiens et les effets de chaque protocole de sédation sur des variables cardio-respiratoires ont été examinés. Il s’agissait d’une étude prospective croisée, randomisée, réalisée à l’aveugle. Chacun des six chiens a reçu trois traitements à des intervalles de 1 semaine. Durant tous les épisodes anesthétiques, les chiens ont reçu 0,05 mg/kg PC d’acépromazine. Environ 25 min plus tard, les chiens ont reçu de la saline physiologique (témoin) ou du tramadol [3 mg/kg PC (TR3) ou 5 mg/kg PC (TR5)]. Toutes les drogues étaient administrées par voie intraveineuse. Les variables évaluées incluaient le rythme cardiaque (RC), le rythme respiratoire (RR), les pressions sanguines systolique, moyenne, et diastolique (PSS, PSM, et PSD), et la sédation [en utilisant une échelle descriptive simple (EDS, écart : 0 à 3) et une échelle de gradation numérique (EGN, écart : 0 à 10)]. Les variables ont été enregistrées 25 min après l’acépromazine et pendant 80 min après l’administration de saline ou de tramadol. L’administration d’acépromazine a résulté en une légère sédation chez la plupart des chiens et on nota une diminution de RR, PSS, PSM, et PSD avec tous les traitements. L’administration de tramadol ne fit pas augmenter de manière significative les pointages EDS et EGN lorsque comparée à l’acépromazine seul. La seule exception à cette règle a été observée à 20 min après TR3, alors que l’EGN était plus élevée dans ce groupe comparativement au témoin. L’administration de tramadol (TR3 et TR5) entraîna une diminution du RC. Dans les conditions de la présente étude, la sédation induite par l’acépromazine avec du tramadol était similaire à celle de l’acépromazine seul. Les principaux effets adverses de la combinaison étaient une diminution de la pression sanguine et du RC, mais sans signification clinique.
(Traduit par Docteur Serge Messier)
Acepromazine is the phenothiazine derivative most commonly used for sedation in dogs. The drug has been found to possess antagonistic action at dopaminergic receptors within the brain; this may explain the mechanism by which acepromazine induces sedation (1). Based on extensive data on the use of acepromazine in dogs, the degree of sedation following acepromazine administration in canine patients is mild to moderate (2–4).
It has been reported that acepromazine is devoid of analgesic properties (1). Therefore, acepromazine is commonly administered in combination with opioid analgesics to provide sedation and analgesia, which facilitates handling of dogs for placement of venous catheters, preparation for surgery, and diagnostic procedures. In addition to providing analgesia, there is evidence that an opioid analgesic enhances the degree of sedation induced by acepromazine in dogs. In a previous study, methadone, morphine, butorphanol, and tramadol were compared and methadone appeared to be the most effective for this purpose (3).
Tramadol has been classified as an opioid analgesic although it was found to provide analgesia by opioid and non-opioid mechanisms (5). In humans, its opioid analgesic properties were associated with the production of the active metabolite O-desmethyltramadol (M1) (6), which was found to possess greater affinity for opiate receptors than the parent drug tramadol (7). Drowsiness has been reported as an adverse effect after tramadol administration in humans (8).
There is conflicting evidence about the sedative effect of tramadol in dogs. In one study, a dose-related sedative effect was observed in dogs administered 1, 2, or 4 mg/kg body weight (BW) tramadol by intravenous (IV) injection (9), whereas no sedation was evidenced after IV tramadol (approximately 4 mg/kg BW) to dogs in another study (10). When combined with acepromazine, tramadol appeared to be less effective in enhancing the degree of sedation induced by the phenothiazine than methadone, morphine, and butorphanol (3). However, the dose of tramadol (2 mg/kg BW) was not equipotent to the dose of other opioids and this may have accounted for the low efficacy of tramadol in this later study (3). The present study aimed to compare the sedative effect of acepromazine combined with 2 different doses of tramadol [3 mg/kg BW (TR3) or 5 mg/kg BW (TR5)] to the sedative effect of acepromazine alone in dogs. We hypothesized that tramadol would enhance sedation induced by acepromazine. A second objective of this study was to evaluate the effects of each sedative protocol on cardiorespiratory variables of dogs.
The present study was initiated after approval by the institutional Animal Research Ethical Committee (protocol 324-2014). Six healthy adult crossbreed dogs (4 male and 2 female) were used. Average weight of the dogs was 18.7 ± 3.1 kg (mean ± SD). Healthy status was confirmed by physical examination and laboratory evaluation [complete blood (cell) count (CBC) and biochemistry profile].
This was a prospective, randomized, blinded, crossover study. Each dog received 3 treatments on different occasions, with 1-week washout intervals. During all anesthetic episodes, dogs were administered acepromazine (Acepran 0.2%; Vetnil, Louveira, São Paulo, Brazil), 0.05 mg/kg BW, IV. Approximately 25 min after the administration of acepromazine, physiological saline (control) IV or tramadol IV (Cloridrato de tramadol 50 mg/mL; Hipolabor Farmacêutica, Belo Horizonte, Minas Gerais, Brazil) at TR3 or TR5, was given.
Dogs were fasted for 12 h prior to each experiment, but had free access to water. A 20-gauge catheter was introduced into a cephalic vein and the dogs were acclimated for 20 min to the laboratory environment. Baseline values for each variable were then recorded with the dogs gently restrained in lateral recumbency. Heart rate (HR) was measured with a stethoscope and respiratory rate (RR) was counted by observing chest wall movements. An oscillometric device (PetMap Classic; Ramsey Medical, Tampa, Florida, USA) was used for indirect measurement of systolic, mean, and diastolic blood pressures (SAP, MAP, and DAP, respectively). The blood pressure cuff was positioned proximal to the carpus in the nondependent limb and the cuff size was chosen according to the manufacturer’s directions. For every time point, 5 consecutive measurements of blood pressure were performed and averaged.
Sedation was scored by use of a simple descriptive scale (SDS) and a numeric rating scale (NRS). The SDS ranged from 0 to 3 as follows: 0 — no sedation; 1 — mild sedation, less alert but still active; 2 — moderate sedation, drowsy, recumbent but can walk; or 3 — intense sedation, very drowsy, unable to walk (11,12). The NRS ranged from 0 to 10, where 0 represented no sedation and 10 represented the most sedation possible (12). For the NRS, only whole numbers could be selected. A single observer, unaware of the treatment administered, was responsible for scoring SDS and NRS values. This person was familiar with both scoring systems. Dogs were initially observed without interaction with the observer. Thereafter, objective variables (HR, RR, SAP, MAP, and DAP) were recorded. Finally, the observer encouraged the dogs to stand and walk. Based on noninteractive and interactive behaviors, the observer recorded SDS and NRS scores.
After recording baseline variables, the dogs were administered acepromazine through the cephalic catheter. Within 20 to 25 min after acepromazine administration, all variables were recorded (time point ACP). Thereafter, physiological saline (control), TR3, or TR5 was administered over 1 min through the cephalic catheter and all variables were reassessed at 20 min intervals for 80 min (time points 20, 40, 60, and 80).
Sample size calculation was performed using G*Power for Windows Version 3.1.6 (Heinrich Heine Universität Düsseldorf, Germany) and was based on sedation scores reported for dogs in previous studies (2–3). Results of the power analysis indicated 6 dogs would be necessary to detect 1.0 point differences between groups for SDS scores and 3.0 point differences between groups for NRS scores with a power of 80% at 5% level of significance.
Statistical analyses were performed by use of a computer software (Prism 5.0; GraphPad Software, La Jolla, California, USA). Data distribution was analyzed by the Kolmogorov-Smirnov test. For normally distributed variables (HR, RR, SAP, MAP, and DAP), comparisons between treatments were analyzed by a 2-way repeated measures analysis of variance (ANOVA) with time and treatment as factors. When a significant difference between treatments was identified, a Bonferroni correction for multiple comparisons was used to determine what treatments differed. A 1-way repeated measures ANOVA was performed to detect differences over time within each treatment. If a significant difference was detected, a Dunnett’s test for multiple comparisons was used to compare all time points with time point ACP. For comparisons between treatments and over time in sedation scores, a Friedman test was performed and post hoc analysis was conducted by use of the Dunn’s test for multiple comparisons. Differences were considered significant if P < 0.05.
All 6 dogs completed the study. Acepromazine administration resulted in mild sedation in most dogs (Table I, Figure 1). Heart rate did not change significantly after acepromazine but a decrease was observed in SAP, MAP, DAP, and RR in all treatments at time point ACP (Table II).
Administration of the experimental treatment (time points 20 to 80 min) did not significantly increase SDS or NRS scores compared to time point ACP. The only significant difference between treatments in sedation scores was observed at 20 min, when NRS was higher in the TR3 treatment compared to the control treatment. At 80 min, NRS scores decreased in all treatment groups compared to the values at time point ACP (Table I). The distribution of SDS scores in each treatment is summarized in Figure 1.
Blood pressure and RR did not differ among treatments throughout the study. In addition, SAP, MAP, DAP, and RR did not change significantly after administration of the experimental treatment. The only exception to this rule was observed at time point 40, when DAP was significantly higher in TR5 compared to the value at time point ACP. Administration of tramadol (TR3 and TR5) resulted in significant decreases in HR from 20 to 80 min and the value for the TR3 treatment was significantly lower compared to the control treatment at 40 min (Table II).
The present study revealed that combining TR3 or TR5 with 0.05 mg/kg BW acepromazine failed to enhance the degree of sedation induced by acepromazine. Mild to moderate sedation was observed after administration of acepromazine alone or the combination acepromazine-tramadol.
On the basis of SDS scores, the findings of this study are in agreement with previous studies that, in dogs, sedation following acepromazine alone ranges from mild to moderate (3,13). After intramuscular administration of acepromazine in dogs, peak sedative effect appears to occur within 15 to 30 min (2,4). In the present study, SDS and NRS scores assessed from 20 to 80 min in the control treatment were not significantly greater than at time point ACP (approximately 25 min after acepromazine administration). These results suggest that peak sedative effect after IV acepromazine occurs within 25 min after administration of the drug. If sedation was assessed more frequently in this study, it would be possible to determine more precisely the period of time between IV acepromazine administration and peak sedative effect.
When an opioid analgesic is administered in combination with acepromazine, sedation is expected to be greater than after acepromazine alone, but there is conflicting evidence. In one study, intramuscular acepromazine-methadone (0.05 mg/kg BW and 0.5 mg/kg BW, respectively) resulted in non-significantly increase in sedation score compared to 0.1 mg/kg BW acepromazine (2). In another study, sedation induced by acepromazine (0.05 mg/kg BW) was significantly improved when morphine (0.5 mg/kg BW), methadone (0.5 mg/kg BW) or butorphanol (0.2 mg/kg BW) were administered 15 min after acepromazine via IV injection (3). Conversely, the combination of acepromazine (0.05 mg/kg BW) with hydromorphone (0.1 mg/kg BW) failed to induce greater sedation than acepromazine alone (4). The discrepancies between studies are probably related to differences on the scoring systems used to subjectively assess sedation, routes of drug administration and pharmacological characteristics of opioid analgesics used.
In a previous study in dogs, sedation assessed after combining acepromazine (0.05 mg/kg BW) with tramadol (2 mg/kg BW) was only mildly increased and for a short period (15 min), compared to sedation assessed after acepromazine alone (3). In the present study, it was hypothesized that higher doses of tramadol (TR3 or TR5), combined with acepromazine, would result in consistently greater sedation scores than acepromazine alone. No significant differences were observed in SDS scores between the control treatment and the TR3 and TR5 treatments. In addition, no significant difference in NRS scores were detected between the control and TR5 treatment and NRS scores in the TR3 treatment were significantly higher than in the control treatment at a single time point (20 min) only. These findings indicate there is no clinically important enhancement in sedation induced by acepromazine when doses of tramadol as high as TR5 are combined with phenothiazine.
The inefficacy of tramadol in enhancing the degree of sedation induced by acepromazine in dogs may be related to its pharmacokinetic and pharmacodynamic characteristics. Sedation following opioid administration is attributable to agonistic action at opioid receptors at brain sites (5). In cloned human opioid receptors, a greater affinity for opioid receptors was found for the M1 metabolite than for the parent drug tramadol (7). It has been reported that dogs can only produce low levels of M1 and that this metabolite has a fast elimination phase in this species (9). Therefore, sedation may be impaired in dogs because of its inability to produce significant amounts of M1. This statement is supported by results of a previous study where administration of tramadol orally (11 mg/kg BW) or by IV (4.4 mg/kg BW), did not result in detectable signs of sedation, whereas administration of M1 resulted in mild sedation in dogs (10).
Acepromazine administration did not change HR in the present study. Heart rate measurements from 20 to 80 min in the TR3 and TR5 groups were typical of opioid analgesics in dogs. A sustained decrease of 25% to 30% in HR was observed in both groups compared with time point ACP. The negative chronotropic effect of opioid analgesics is related to their actions within the medulla oblongata increasing vagal tone (14). Although the effect of opioids on HR is dose-related, there is a plateau after which further increases in dose does not result in more bradycardia (15). In agreement with this previous study, our findings revealed the dose of tramadol did not influence the magnitude of the decrease in HR.
A decrease in SAP, MAP, and DAP was observed after acepromazine administration in all treatment groups. The effect of acepromazine on blood pressure is thought to result from antagonistic action at alpha-adrenergic receptors within vascular beds resulting in vasodilation (16). Moreover, in one study in dogs, acepromazine administration resulted in a decrease in stroke volume, which could also reduce blood pressure via a decrease in cardiac output (17). Tramadol administration did not have any influence on blood pressure such that SAP, MAP, and DAP values from 20 to 80 min were not significantly lower than at time point ACP.
Both acepromazine and opioids can reduce RR in dogs (15,17). The respiratory effects of acepromazine are associated with its sedative effect and relieving of anxiety (17). Despite causing a reduction in RR, acepromazine did not change blood gases in dogs (17). Administration of opioid analgesics such as fentanyl, result in dose-related respiratory depression in dogs that are awake, as represented by decreased pH and PaO2 and increased PaCO2 (15). Different from other opioid analgesics with high affinity for μ receptors, tramadol was found to cause less respiratory depression than morphine in humans (18) and in dogs, there was no change in RR after IV injection of tramadol at doses as high as 4 mg/kg BW (9). Finding that RR decreased below baseline in all treatment groups at time point ACP and did not change any further after administration of tramadol suggests that acepromazine was the main drug responsible for the decrease in RR in this study.
An indirect rather than a direct method was used for measuring blood pressure, which is a limitation of this study. Oscillometric devices have been used in dogs (19–21), and 1 study reported an acceptable accuracy (21). However, agreement with invasive blood pressure was poor in 2 other studies (19,20). Despite this limitation, in the present study it was aimed to assess the behavior of blood pressure over time and not to report absolute values.
Under the conditions of this study, sedation induced by acepromazine and tramadol was similar to that of acepromazine alone. The main adverse effects of the combination were a decrease in blood pressure and HR, without clinical significance.