Procedural sedation is often needed in small animal clinical practice to allow safe and effective animal handling without undue stress. Rapid and smooth postprocedural recovery from sedation is advantageous for the animal, its owner, and the veterinary staff. One of the main benefits of α2-adrenoceptor agonists, such as MED and its pharmacologically active enantiomer dexmedetomidine, is the reversibility of sedation by administration of the α2-adrenoceptor antagonist atipamezole.1–4 Disadvantages of α2-adrenoceptor agonists are their cardiovascular adverse effects, such as vasoconstriction, bradycardia, and decreased cardiac output,5,6 that limit their safe use especially in animals with systemic disease.
Atipamezole is a potent and specific antagonist of centrally and peripherally located α2-adrenoceptors.2 It reverses the sedative and some cardiovascular and respiratory effects induced by MED,5,7 thereby facilitating more rapid recovery from MED-induced sedation.4,8 However, transient decreases in blood pressure soon after atipamezole administration, which are thought to be caused by atipamezole's vasodilatory effect, have been reported.7,9
The peripheral α2-adrenoceptor antagonist vatinoxan (also known as MK-467 and L-659′066)10 alleviates the undesired cardiovascular depression associated with dexmedetomidine and MED11–13 without substantially altering the level of sedation.14–16 Vatinoxan crosses the blood-brain barrier poorly in mammals and has antagonistic action only at α2-adrenoceptors located in peripheral tissues,10 which explains the differences in clinical outcomes after administration of vatinoxan or atipamezole. Vatinoxan has distinct effects on the pharmacokinetics of MED and dexmedetomidine (eg, by increasing drug absorption after IM administration of either α2-adrenoceptor agonist and by promoting drug clearance),17–20 which result in more rapid onset and shorter duration of sedation.18,21 Concurrent administration of vatinoxan improves the tolerability and attenuates the adverse cardiovascular effects of MED and dexmedetomidine in dogs and cats.11,13,22 However, when sedation is reversed with atipamezole in animals treated with vatinoxan and MED or dexmedetomidine, vatinoxan might amplify the reduction in blood pressure attributed to atipamezole because both agents induce vasodilatation by blocking vascular α2-adrenoceptors.7,13
It has been reported that atipamezole reverses sedation in dogs sedated with dexmedetomidine and vatinoxan,14 but there are no reports on the possible cardiovascular consequences of concomitant administration of vatinoxan and atipamezole in dogs sedated with α2-adrenoceptor agonists, to our knowledge. The purpose of the study reported here was to investigate the cardiovascular and sedation reversal effects of IM administration of atipamezole in dogs treated with MED or MEDVAT. To this end, the influences of vatinoxan and atipamezole on cardiovascular functions, respiration, sedation-related factors, and plasma drug concentrations in dogs sedated with MED were evaluated. We hypothesized that treatment with atipamezole and vatinoxan would improve cardiovascular performance by decreasing SVRI, thereby increasing both heart rate and CI, without inducing clinically relevant hypotension. We also expected that vatinoxan administration would not have any significant effect on respiration. Atipamezole is known to increase the clearance and decrease the elimination half-life of MED.23 Consequently, we further hypothesized that by improving perfusion in peripheral tissues, vatinoxan would decrease the plasma exposure to MED and thus hasten recovery from MED-induced sedation in dogs.
Materials and Methods
Animals
Approval (ESAVI/7187/04.10.03/2012) from the National Animal Experimental board to use 8 purpose-bred Beagles for the study was granted. Dogs were approximately 2 years old and had a mean ± SD body weight of 14.7 ± 1.5 kg. They were considered healthy on the basis of history and results of a comprehensive general examination that included a CBC and routine serum biochemical analyses. Dogs were housed in groups in a kennel with daily maintenance and activities. They were fed commercial food and had free access to water. Food was withheld 12 hours prior to experiments. After each experimental day was completed, the dog was assessed for recovery (heart rate and CSS) from sedation and fed and meloxicam (0.2 mg/kg) was administered SC for analgesia before transfer back to the kennel. After completion of the study, all dogs were placed with families in homes.
Study design
In this prospective, randomized,a blinded, experimental crossover study, each dog received 2 IM treatments at an interval of ≥ 2 weeks. On each of the 2 experimental days, each dog was treated IM with MED hydrochlorideb (20 μg/kg) or MED (20 μg/kg) mixed with vatinoxan hydrochloridec (400 μg/kg) 30 minutes before administration of atipamezole hydrochlorided (5 mg/mL). One dog was evaluated on each experimental day, and the overall duration of the study was 2 months.
Medetomidine-vatinoxan solution was prepared for each dog undergoing the MEDVAT treatment on a given day just before administration by injecting 1 mL of MEDb solution and 1 mL of physiologic saline (0.9% NaCl) solution into an ampule containing 20 mg of vatinoxan powder and mixing the ampule contents until the solution was clear. The final drug concentration in the solution was 500 μg of MED/mL and 10 mg of vatinoxan/mL; hence, the injection volume was 0.04 mL/kg. The MED-to-vatinoxan dose ratio of 1:20 was selected on the basis of previously published experimental data.18 Atipamezole was administered according to information in the product characteristics summarye at the recommended dose of 5 times the preceding dose of MED.
Instrumentation
At the start of an experiment, each dog was placed on an examination table covered with an isolating foam mattress and heating pad. A cephalic vein was cannulated with a 22-gauge IV catheter. By use of a face mask, each dog was allowed to breathe 100% oxygen (5 L/min) before induction of anesthesia with propofol administered IV to effect (maximum dose, 8 mg/kg). The dog's trachea was intubated, and anesthesia was maintained by inhalation of isoflurane in oxygen (end-tidal isoflurane concentration, 1.5%) via a circle breathing system. Each dog was mechanically ventilated to maintain end-tidal Paco2 between 35 and 45 mm Hg. Monitoring (noninvasive blood pressure assessment, ECG, and pulse oximetry) and IV infusion of acetated Ringer solution (5 mL/kg/h) were started. A 20-gauge arterial catheterf was aseptically introduced into a femoral artery under local anesthesia and secured in place with 2 sutures and surgical tape. The arterial catheter was used for direct arterial blood pressure and cardiac output measurements and collection of blood samples for arterial blood gas analysis. Local anesthetic (0.25 mL of 2% lidocaine solution) was injected SC to mark the place for a small skin incision over a jugular vein. A 7F doublelumen central venous catheterg was inserted aseptically into the jugular vein and sutured in place; a light bandage was applied around the neck. To ensure the accurate location of the catheter's tip, the insertion site of the central venous catheter was premeasured and marked to indicate positioning of the tip of the catheter at the cranial border of the second rib's costochondral junction. Confirmation of a typical pressure waveform was performed after insertion. The central venous catheter was used for measurement of CVP and collection of venous blood samples for assessment of plasma drug concentrations. Isoflurane administration was discontinued after placement of both catheters. Full recovery from anesthesia (characterized by the dog's ability to walk and interact normally) was ensured by allowing the dog to recover for at least 60 minutes after extubation prior to obtaining baseline measurements.
Measurements
Prior to each experiment, blood pressure transducersh were calibrated with a mercury manometer and zeroed to atmospheric pressure at the level of the manubrium. Each dog was positioned on the examination table in lateral recumbency with minimal gentle restraint and connected to the blood pressure transducers for continuous measurement of central venous pressure, MAP, DAP, and SAP. For ECG monitoring,i adhesive ECG electrodes were placed on the forelimbs and left hind limb; the lead II tracing was monitored continuously. When the dog was lying calmly on the table, heart rate and blood pressure were allowed to stabilize before baseline measurements were obtained. After recording of all baseline values and collection of blood samples, MED or MEDVAT was injected into the right gluteal muscle; 30 minutes later, atipamezole was injected into the left gluteal muscle (designated as the 0-minute time point). Prior to any drug injection, negative pressure was applied to the syringe; if no blood was apparent, needle placement for extravascular drug administration was confirmed. For each dog, a heating pad and blankets were used to maintain normothermia.
At data collection time points when multiple variables were assessed, heart rate (auscultated heart beats counted during a 1-minute period), respiratory rate (thoracic movements counted during a 1-minute period), and blood pressures were always recorded first. These variables were assessed before (−30 [baseline] and −10 minutes) and after (5, 10, 15, 20, 30, 45, 60, and 90 minutes) administration of atipamezole. Heart rate was also determined at the end of the study (210 minutes after atipamezole administration). Cardiac output was measured as described previously24 at baseline and −10, 5, 15, 30, 45, 60, and 90 minutes by the lithium dilution methodj with a standard dose of lithium chloridek (0.075 mmol) injected via the central venous catheter. Initial standard values of 10 g/dL for hemoglobin concentration and 140 mmol/L for sodium concentration were later corrected with actual values obtained from arterial blood samples. Just prior to measurements of cardiac output, arterial blood samples were collected into heparinized syringes and analyzed immediately for blood gases.l Cardiac index, RPP, stroke volume index, and SVRI were calculated by use of standard equations.25
For each dog, a venous blood sample (6 mL) for plasma drug concentration analyses was collected from the central venous catheter into EDTA tubes before (−10 minutes) and after (5, 10, 15, 20, 30, 60, 90, and 210 minutes) administration of atipamezole. Venous blood samples were centrifuged, and plasma was separated into tubes that were kept frozen at ≤ −20°C until analyzed for dexmedetomidine, levomedetomidine, vatinoxan, and atipamezole concentrations.
Each dog was assigned a CSS (ranging from 0 [no sedation] to 20 [deep sedation])14 by an investigator (HT) who was unaware of the drug treatment or any values of cardiovascular variables before (−30 and −10 minutes) and after (5, 10, 15, 20, 30, 45, 60, and 90 minutes) atipamezole administration. Each dog was assigned a CSS by an unblinded investigator (KN) at 210 minutes after atipamezole administration to ensure full recovery from sedation. For each dog, a total CSS (Appendix) was calculated as the sum of scores for spontaneous posture (0 to 4), palpebral reflex (0 to 3), position of the eye (0 or 2), jaw and tongue relaxation (0 to 4), resistance to positioning in lateral recumbency (0 to 3), and general appearance including response to noise (0 to 4). The maximum achievable total score was 20.
Concentrations of dexmedetomidine, levome-detomidine, and vatinoxan in plasma samples were determined by liquid chromatography coupled with tandem mass spectrometry after solid-phase extraction.m Racemic deuterium–labeled MEDn was used as internal standard for dexmedetomidine and levomedetomidine. An internal standardo for vatinoxan was also used. Reversed-phase separationp and a gradient solvent system (0.1% formic acid in water and acetonitrile) were used before quantitative detection of vatinoxan. Chiral separationq of dexmedetomidine and levomedetomidine was achieved by use of 10mM ammonium acetate and acetonitrile as solvents. Quantitative detection of the analytes was performed in multireaction monitoring mode with a triple quadrupole mass spectrometer.r For dexmedetomidine and levomedetomidine and for deuterium–labeled MED, the m/z of the respective precursor ions were 201.2 and 204.2, respectively. The m/z for monitored fragment ions used for quantitation were 95.1 for dexmedetomidine and levomedetomidine and 98.05 for the internal standard. The linear concentration ranges for dexmedetomidine and levomedetomidine were 0.100 ng/mL to 10.0 ng/mL. The accuracy of the quality control samples (at 0.225, 1.0, and 8.0 ng/mL) ranged from 91.3% to 99.1% for dexmedetomidine and from 94.6% to 104.2% for levomedetomidine. The linear concentration range of the assay for vatinoxan was 25 ng/mL to 460 ng/mL. For vatinoxan, the interassay accuracy of the quality control samples (at 70, 250, 380, and 2500 ng/mL) ranged from 96.9% to 112%.
Concentrations of atipamezole in plasma samples were analyzed by liquid chromatography coupled with tandem mass spectrometrys after precipitation of a 100-μL volume of each sample with 200 μL of acetonitrile containing 100 ng of propranolol/mL as the internal standard on a 96-well precipitation plate. After mixing, the samples were kept in a refrigerator for 20 minutes at 8°C, and then centrifuged at for 20 minutes. The supernatants were transferred to 96-well plates, and 50-μL aliquots were transferred to another 96-well plate and diluted 1:20 with 20% acetonitrile in PBS solution (pH, 7.4) prior to analysis. Both diluted and undiluted samples were analyzed. Reference standards and quality control samples were prepared in blank dog plasma. The selected reaction monitoring was m/z of 213 > 117 for atipamezole and m/z of 260 > 116 for internal standard propanolol. Quantitation was based on the peak area ratios of the analyte and the internal standard. The calibration range was 0.1 to 5,000 ng/mL, and all quality control samples (at 0.2, 2, 20, 200, and 2,000 ng/mL) were within 91% to 115% of the nominal concentration.
Statistical analysis
Sample size calculations were performed on the basis of data from our research group's previous studies.13,14,18 Eight dogs were used to detect differences (peak effects) between treatments (with a paired 2-tailed test, α of 0.05, and power of 80%) as follows: difference in heart rate of 11 beats/min (50 ± 10 beats/min vs 61 ± 10 beats/min), difference in cardiac output of 1.4 L/min (4.0 ± 1.0 L/min vs 5.4 ± 1.2 L/min), and difference in MAP of 17 mm Hg (110 ± 10 mm Hg vs 93 ± 15 mm Hg).
With regard to changes from baseline at the predetermined time points, differences between treatments were evaluated with repeated-measures ANCOVA models. The models included the main effects of treatment and time point, 2-way interaction of treatment and time point, and a baseline covariate as fixed effects and the main effect of dog, 2-way interaction of dog and time point, and 2-way interaction of dog and treatment as random effects. Composite sedation scores by time point were analyzed nonparametrically with a Wilcoxon signed rank sum test. Normality assumptions were checked with Kolmogorov-Smirnov tests and by assessment of the skewness and kurtosis measures. For DAP, MAP, SAP, RPP, heart rate, blood pH, stroke volume (analyzed and reported as SVI), and cardiac output (analyzed and reported as CI), logarithmic transformation of data was computed to normalize the distribution. With all transformed variables, the model was fitted for changes in the transformed response. However, the descriptive results reported for these variables are in linear scale.
The estimates of treatment effects were calculated over time and by time point from the fitted models with contrasts. Furthermore, within-group changes were calculated for each time point from the same models. For the group differences and within-group changes, 95% confidence intervals and P values were calculated. Differences in AUCs of drug concentrations and CSSs between the treatments were compared with paired 2-tailed t tests. Significance was set at a value of P < 0.05. Computer softwaret was used for all statistical analyses.
Results
Both experiments were completed for all 8 dogs. Cardiovascular data were summarized (Figure 1; Table 1). After atipamezole administration in the MED experiment, heart rate (range for all dogs across all time points, 32 to 68 beats/min) was significantly less (P < 0.001) than that at baseline (range, 60 to 114 beats/min). At all time points, CI (range, 1.1 to 3.3 L/min/m2) remained significantly less (P < 0.001) than baseline (range, 2.3 to 6.7 L/min/m2) and SVRI (range, 5,700 to 23,000 dynes•s/cm5/m2) was significantly greater (P < 0.001) than baseline (3,700 to 10,000 dynes•s/cm5/m2). After atipamezole administration in the MEDVAT experiment, cardiovascular function among the 8 dogs was better than that after MED-atipamezole administration; at 45 through 90 minutes after atipamezole administration, heart rate (range, 48 to 102 beats/min) returned to or was greater than the baseline value (55 to 86 beats/min). At all time points, CI (range, 1.9 to 6.1 L/min/m2) was slightly decreased, compared with baseline (2.7 to 6.8 L/min/m2). Between 30 and 90 minutes after atipamezole administration, SVRI (range, 4,100 to 10,000 dynes•s/cm5/m2) was not significantly different from baseline (range, 3,900 to 7,300 dynes•s/cm5/m2). Atipamezole administration decreased MAP transiently in both the MED and MEDVAT experiments. When dogs received the MEDVAT treatment, MAP during the first 15 minutes after atipamezole administration (64 to 115 mm Hg) was significantly lower than values during that same period when dogs received the MED treatment (83 to 156 mm Hg). However, hypotension (MAP < 60 mm Hg) was not detected in any dog at any time point.
Mean ± SD heart rate (A), CI (B), MAP (C), and SVRI (D) in 8 dogs that received an IM injection of MED (20 μg/kg [squares]) or MED mixed with 400 μg of vatinoxan/kg (MEDVAT [circles]) 30 minutes before IM administration of atipamezole (100 μg/kg [indicated at 0 minutes by the triangle]) at a ≥ 2-week interval in a randomized crossover study. Error bars indicate the 95% confidence intervals of the means. Variables of interest were measured at various time points before (at −30 minutes [baseline] and −10 minutes) and at various time points after atipamezole administration. Values of CO and body surface area were used to calculate CI, and values of MAP, CVP, and CI were used to calculate SVRI. *At a given time point, there is a significant (P < 0.05) difference between treatments † At a given time point for a given treatment, there is a significant (P < 0.05) difference from baseline.
Citation: American Journal of Veterinary Research 80, 10; 10.2460/ajvr.80.10.912
Cardiovascular variables in 8 dogs that received an injection of MED (20 μg/kg) or MED mixed with 400 μg of vatinoxan/kg (MEDVAT treatment) 30 minutes before IM administration of atipamezole (100 μg/kg [given at 0 minutes]) at a ≥ 2-week interval in a randomized crossover study.
| Variable | ||||||
|---|---|---|---|---|---|---|
| Time (min) | Treatment | SAP (mm Hg) | DAP (mm Hg) | CVP (mm Hg) | SVI (mL/kg) | RPP (mm Hg/min) |
| −30 | MED | 168 ± 24 | 83 ± 11 | 1 ± 2 | 2.3 ± 0.4 | 9,750 ± 2,050 |
| MEDVAT | 169 ± 20 | 80 ± 15 | 2 ± 2 | 2.9 ± 0.9 | 7,500 ± 2,040 | |
| −10 | MED | 164 ± 20 | 92 ± 13* | 5 ± 2* | 1.8 ± 0.6* | 4,380 ± 1,360* |
| MEDVAT | 153 ± 25* | 76 ± 10† | 6 ± 2* | 2.3 ± 1.5* | 4,690 ± 1,370* | |
| 5 | MED | 143 ± 12* | 74 ± 10 | 4 ± 2* | 1.7 ± 0.4* | 5,140 ± 1,350* |
| MEDVAT | 132 ± 16* | 63 ± 13*† | 3 ± 2* | 1.9 ± 0.3* | 4,420 ± 806* | |
| 10 | MED | 148 ± 15* | 81 ± 9 | 4 ± 1* | — | 4,700 ± 1,270* |
| MEDVAT | 134 ± 16* | 69 ± 14*† | 3 ± 2* | — | 5,250 ± 1,510* | |
| 15 | MED | 146 ± 12* | 90 ± 26 | 4 ± 2* | 1.7 ± 0.4* | 5,180 ± 1,580* |
| MEDVAT | 138 ± 16* | 73 ± 13† | 3 ± 2* | 1.9 ± 0.3* | 5,600 ± 1,880* | |
| 20 | MED | 146 ± 19* | 89 ± 11 | 4 ± 2* | — | 5,470 ± 1,560* |
| MEDVAT | 147 ± 12* | 86 ± 10 | 3 ± 2 | — | 6,860 ± 2,020*† | |
| 30 | MED | 144 ± 14* | 82 ± 7 | 4 ± 2* | 1.7 ± 0.2* | 4,580 ± 652* |
| MEDVAT | 142 ± 13* | 75 ± 11 | 2 ± 2 | 2.2 ± 0.4 | 6,180 ± 1,500*† | |
| 45 | MED | 140 ± 10* | 81 ± 7 | 4 ± 2* | 1.9 ± 0.7* | 4,480 ± 832* |
| MEDVAT | 161 ± 11† | 86 ± 13 | 1 ± 2† | 2.0 ± 0.5* | 8,600 ± 2,560† | |
| 60 | MED | 143 ± 14* | 84 ± 7 | 4 ± 2* | 1.9 ± 0.4* | 4,320 ± 919* |
| MEDVAT | 155 ± 13* | 76 ± 14 | 1 ± 2† | 2.4 ± 0.5 | 6,670 ± 1,260*† | |
| 90 | MED | 142 ± 12* | 80 ± 9 | 4 ± 3* | 2.0 ± 0.4* | 4,010 ± 861* |
| MEDVAT | 162 ± 17† | 81 ± 11 | 1 ± 2† | 2.1 ± 0.5* | 8,220 ± 2,120† | |
Data are reported as mean ± SD.
For a given treatment, there is a significant (P < 0.05) difference from baseline at this time point
At a given time point, the MEDVAT treatment value is significantly (P < 0.05) different from that for the MED treatment.
SVI = Stroke volume index (calculated from data CI and heart rate).
Data for respiratory rate, selected arterial blood gas variables, and arterial blood pH and lactate concentration were summarized (Table 2). Compared with baseline values, respiratory rate was significantly decreased (P < 0.001) at all time points in both the MED and MEDVAT experiments. Partial pressures of oxygen and carbon dioxide remained stable, with only sporadic differences from baseline. During the 90-minute period after atipamezole administration, arterial blood lactate concentration was higher when dogs received the MED treatment, compared with findings after the dogs received the MEDVAT treatment.
Respiratory rates, selected arterial blood gas variables, and arterial blood pH and lactate concentration in the 8 dogs in Table 1.
| Variable | ||||||
|---|---|---|---|---|---|---|
| Time (min) | Treatment | RR (breaths/min) | Pao2 (mm Hg) | Paco2 (mm Hg) | Arterial blood pH | Arterial blood lactate concentration (mmol/L) |
| –30 | MED | 27 ± 6 | 95.6 ± 4.5 | 35.5 ± 2.3 | 7.37 ± 0.02 | 0.56 ± 0.24 |
| MEDVAT | 28 ± 16 | 96.8 ± 5.0 | 35.3 ± 1.9 | 7.35 ± 0.02 | 0.49 ± 0.17 | |
| –10 | MED | 16 ± 8* | 94.9 ± 5.1 | 34.3 ± 2.5 | 7.36 ± 0.02 | 0.93 ± 0.37* |
| MEDVAT | 9 ± 2*† | 94.0 ± 8.5 | 32.8 ± 6.3 | 7.35 ± 0.03 | 0.73 ± 0.34* | |
| 5 | MED | 15 ± 3* | 101.3 ± 8.1* | 34.6 ± 3.3 | 7.36 ± 0.02 | 1.40 ± 0.45* |
| MEDVAT | 10 ± 4*† | 96.0 ± 5.6† | 37.8 ± 2.9 | 7.33 ± 0.02*† | 0.90 ± 0.43*† | |
| 15 | MED | 17 ± 4* | 99.6 ± 5.2* | 35.3 ± 2.3 | 7.36 ± 0.02* | 1.33 ± 0.47* |
| MEDVAT | 18 ± 7* | 101.3 ± 7.4* | 37.0 ± 2.3 | 7.34 ± 0.03* | 0.95 ± 0.45*† | |
| 30 | MED | 17 ± 4* | 98.4 ± 5.5* | 34.9 ± 2.4 | 7.35 ± 0.02* | 1.43 ± 0.48* |
| MEDVAT | 20 ± 4* | 98.5 ± 6.4 | 37.3 ± 1.5 | 7.33 ± 0.02* | 0.79 ± 0.44*† | |
| 45 | MED | 15 ± 3* | 98.6 ± 5.0 | 35.0 ± 3.0 | 7.35 ± 0.02* | 1.41 ± 0.44* |
| MEDVAT | 20 ± 3*† | 98.9 ± 7.1 | 36.2 ± 1.6 | 7.34 ± 0.03* | 0.80 ± 0.49*† | |
| 60 | MED | 16 ± 5* | 100 ± 4.8* | 34.6 ± 2.5 | 7.35 ± 0.02* | 1.46 ± 0.4* |
| MEDVAT | 22 ± 5*† | 99.2 ± 6.1 | 36.1 ± 1.5 | 7.34 ± 0.03* | 0.73 ± 0.44*† | |
| 90 | MED | 15 ± 4* | 99.6 ± 4.5* | 34.4 ± 2.4 | 7.37 ± 0.02 | 1.40 ± 0.48* |
| MEDVAT | 22 ± 4*† | 98.1 ± 6.2 | 35.4 ± 2.1 | 7.35 ± 0.03 | 0.51 ± 0.29† | |
In both experiments, sedation and recovery from sedation were uneventful in all dogs. Composite sedation scores indicated that the dogs became markedly sedated after receiving the MED or MEDVAT treatment (Figure 2). The change in CSS from baseline did not differ significantly between the treatments prior to atipamezole administration. After atipamezole administration in the MEDVAT experiment, there was a steady decrease in CSS over time. In the MED experiment, an initial decrease in CSS was also detected; however, the CSS started to increase again at 30 minutes after atipamezole administration. The AUC for CSS from 5 to 210 minutes after atipamezole administration was significantly larger when dogs received the MED treatment (1,440 ± 520), compared with that AUC for the same period when dogs received the MEDVAT treatment (450 ± 210).
Box-and-whisker plots of CSSs for the dogs in Figure 1 during the MED (A) and MEDVAT (B) experiments. Each dog was assigned a CSS (ranging from 0 [no sedation] to 20 [deep sedation])14 before (−30 and −10 minutes) and after (5, 10, 15, 20, 30, 45, 60, 90, and 210 minutes) atipamezole administration at 0 minutes (arrow). The CSS was calculated as the sum of scores for spontaneous posture (0 to 4), palpebral reflex (0 to 3), position of the eye (0 to 2), jaw and tongue relaxation (0 to 4), resistance to positioning in lateral recumbency (0 to 3), and general appearance including response to noise (0 to 4). For each box, the horizontal line represents the median value and the upper and lower boundaries represent the 75th and 25th percentiles, respectively. Whiskers represent the range. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 80, 10; 10.2460/ajvr.80.10.912
In the MEDVAT experiment, plasma dexmedetomidine concentrations peaked before atipamezole administration (Figure 3). In the MED experiment, the highest mean concentration was measured at 5 minutes after atipamezole administration. Vatinoxan reduced the exposure to the other drugs; in the MED and MEDVAT experiments, the AUCs between −10 and 210 minutes for dexmedetomidine were 208 ± 31 minutes•ng/mL and 163 ± 25 minutes•ng/mL, respectively, and those for levomedetomidine were 445 ± 97 minutes•ng/mL and 289 ± 59 minutes•ng/mL, respectively. Similarly, the AUC between 5 and 210 minutes for atipamezole was smaller when dogs received the MEDVAT treatment (2,665 ± 364 minutes•ng/mL), compared with the corresponding AUC when dogs received the MED treatment (3,966 ± 798 minutes•ng/mL).
Mean ± SD plasma dexmedetomidine concentrations in the dogs in Figure 1 during the MED (squares) and MEDVAT (circles) experiments. A venous blood sample was collected from each dog before (−10 minutes) and after (5, 10, 15, 20, 30, 60, 90, and 210 minutes) atipamezole administration at 0 minutes (arrow) for analysis. Error bars indicate the 95% confidence intervals of the means. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 80, 10; 10.2460/ajvr.80.10.912
Discussion
In the present study, the administration of atipamezole to the MED- or MEDVAT-treated dogs failed to reverse the adverse cardiovascular effects attributed to MED. In general, the reductions in heart rate and CI persisted following atipamezole administration and were more pronounced when dogs received the MED treatment than when they received the MEDVAT treatment. In the MED experiment, heart rate and CI remained 40% to 60% below baseline, even after atipamezole administration. The results of the present study were similar to those of a study26 by Vainio and Vähä-Vahe, which indicate that the initial restoration of heart rate is not sustained and is followed by a subsequent decrease when atipamezole is administered at a dose 2 to 4 times the preceding dose of MED. It has also been shown that atipamezole does not restore heart rate to presedation level in dogs treated with MED, although substantial, clinically relevant, and more sustained increases in heart rate after atipamezole administration have also been reported.1,4,8 Hence, the capacity of atipamezole to reverse the bradycardia attributed to α2-adrenoceptor agonists may be variable. In dogs, even very low doses of dexmedetomidine induce peripherally and centrally mediated bradycardia27 and also provide anxiolysis without sedative effect.28 This could explain why heart rate remains less than presedation values in MED-treated dogs even after reversal of sedation with atipamezole.
In the present study, atipamezole failed to restore SVRI or increase CI to baseline values when dogs received the MED treatment. Throughout the observational period after atipamezole administration, SVRI was doubled and CI remained depressed, compared with the respective baseline values. Thus, atipamezole was not able to effectively reverse the vasoconstrictive and bradycardic effects of MED. In a previous study5 of dogs wherein atipamezole (97.5 to 115 μg/kg) was administered IM 60 minutes after treatment with MED (39 to 46 μg/kg) that was combined with midazolam and butorphanol, buprenorphine, or saline solution, atipamezole (at a dose 2.5 times that of MED) was able to abolish the cardiovascular and respiratory effects of MED, although CI failed to return to baseline. In another study29 in which atipamezole (80 μg/kg) was administered IM 40 minutes after treatment with MED (20 μg/kg) that was combined with midazolam, atipamezole (at a dose 4 times that of MED) effectively and rapidly decreased systemic vascular resistance and increased MED-depressed heart rate and CI, both of which reached baseline values within 20 minutes. Similarly, atipamezole completely reversed all cardiovascular changes attributed to dexmedetomidine when atipamezole was administered IV at a high dose of 300 to 500 μg/kg.30,31 The results of previous studies are not fully comparable to those of the present study because of the differences in study designs. In a study by Pypendop et al,5 the dose of MED was double that used in the present study, and MED was combined with midazolam and an opioid; moreover, atipamezole was administered at a dose half that used in the present study and given 60 minutes after agonist administration. The protocol of Hayashi et al29 was more similar to that of the present study, although those researchers used MED that was combined with midazolam and the atipamezole dose was 20% lower and given 10 minutes later than the atipamezole dose used in the present study. All of the aforementioned data indicate that the dose of atipamezole and route and timing of its administration appear to influence the cardiovascular outcome of such treatment.
There are several possible reasons for the differences in the cardiovascular findings of the present study, compared with those of previous studies, namely differences in the drugs, doses, and routes and timings of administration used; differences in the methods of cardiovascular variable measurement (eg, ECG measurement of heart rate regardless of possible drug-induced changes in P waves and QRS complexes or measurement of cardiac output with a thermodilution vs lithium dilution method); and differences in the dogs' temperament and reactions to handling, monitoring, mensuration, and surroundings, which affect their activity and cardiovascular status during collection of data at baseline and during the recovery period. In the present study, dogs were gently restrained on an examination table until 90 minutes after atipamezole administration. Baseline values in the present study were comparable to those of previous studies,5,29 but lack of physical activity and residual sedation might have affected the dogs' cardiovascular function during recovery from sedation. Hence, further studies with larger numbers of dogs and that better represent a clinical setting are still required.
Although some of the discordant findings between previous investigations and the present study may be difficult to reconcile, results of the present study indicated that cardiovascular function in dogs was improved when vatinoxan was coadministered with MED, both before and after reversal of sedation with atipamezole. With the MEDVAT treatment, heart rate, CI, RPP, and SVRI returned to or very close to reference values.25 Medetomidine induces cardiovascular changes both peripherally and centrally by causing vasoconstriction and baroreflex-mediated bradycardia30 and diminishing sympathetic tone.32 Vatinoxan affects only the peripheral organs, preserving cardiovascular functions in dexmedetomidine- and MED-sedated dogs,11–13 whereas atipamezole also reverses the central effects of α2-adrenoceptor agonists.2 In the dogs of the present study, atipamezole and vatinoxan appeared to interact in a favorable manner, although momentary decreases in arterial blood pressure attributable to α2-adrenoceptor blockade were noticed shortly after atipamezole administration. Similar initial decreases in arterial blood pressure after IM administration of atipamezole as a result of antagonism of the dexmedetomidine- and MED-mediated peripheral vasoconstriction have been found in other studies.5,7,31 In the present study, the magnitude of the momentary decrease in MAP was similar whether dogs did or did not receive vatinoxan. Because MAP was lower after MEDVAT administration than it was after MED administration at the −10-minute time point (ie, before atipamezole administration), the lowest MAP was detected when dogs received the MEDVAT treatment. Notably, no clinically relevant hypotension was detected in any dog at any time during the experiments in the present study.
A decrease in respiratory rate was evident in dogs following both treatments in the present study, and contrary to previous reports,7 atipamezole administration did not significantly increase respiratory rate. However, the gentle restraint of dogs on an examination table during recovery from sedation might have influenced their breathing. Nevertheless, Pao2, Paco2, and arterial blood lactate concentration and pH remained within reference ranges throughout the MED and MEDVAT experiments, although the comparatively higher arterial blood lactate concentration following MED administration suggested more pronounced compromise of tissue perfusion with that treatment. The limited published data available suggest that the decrease in cardiac output attributed to α2-adrenoceptor agonists does not cause detrimental hypoperfusion of vital organs in healthy dogs, but intestinal and skeletal blood flows are decreased.27,39 These findings might explain the higher arterial blood lactate concentration when the dogs received the MED treatment because vatinoxan improves perfusion in peripheral tissues.
The dogs' recoveries from sedation after atipamezole administration were calm and smooth with both treatments. However, dogs treated with MED without vatinoxan tended to relapse into sedation. Resedation was not noticed in dogs treated with MEDVAT. Similar to our findings, Vainio and Vähä-Vahe reported drowsiness after atipamezole administration in almost half of the medetomidine-sedated dogs.26 It was described that the dogs in that study26 could stand up and walk if forced to do so, but they preferred to lay down and were somnolent. Resedation or prolonged sedation after atipamezole administration has, nevertheless, been described as rare in dogs,4,33 but it is common in ruminants.34–36 Administration of vatinoxan appeared to ensure more complete recovery from sedation when atipamezole was used to reverse the effects of MED in sheep,37 which is in line with the finding for dogs of the present study. At the time of resedation, plasma dexmedetomidine concentrations when dogs received the MED treatment were significantly higher than those detected when dogs received the MEDVAT treatment. Because atipamezole is a competitive α2-adrenoceptor antagonist,38 high plasma agonist concentration would be expected to impair the desired antagonistic effect of atipamezole administration. Compared with the effects of the MED treatment in the dogs of the present study, coadministration of vatinoxan with MED maintained higher cardiac output (as evidenced by the higher CI values), thereby providing better hepatic perfusion39 and increasing the plasma clearance of dexmedetomidine; thus, more complete recoveries from sedation were facilitated.
Administration of vatinoxan with MED accelerates MED absorption after IM injection,18 as was apparent in the present study wherein peak plasma concentrations of dexmedetomidine in the dogs were observed earlier after MEDVAT treatment than they were after MED treatment. Vatinoxan is absorbed more slowly than MED when the drugs are concomitantly given IM.18 Therefore, early cardiovascular effects of MED were not prevented by vatinoxan, and an initial increase in SVRI coupled with decreases in heart rate and CI was also evident after MEDVAT administration, although to a lesser extent than those associated with the MED treatment. Compared with findings in the MED experiment, coadministration of vatinoxan with MED decreased dexmedetomidine concentrations faster in plasma after atipamezole administration, as presumed on the basis of results of a previous study,17 which indicated that administration of vatinoxan significantly increased the plasma clearance of dexmedetomidine overall. In the dogs of the present study, both α2-adrenoceptor antagonists (vatinoxan and atipamezole) probably hastened the elimination of MED23 as indicated by enhanced hemodynamic function and faster and more complete recovery from sedation after atipamezole administration, even though the plasma exposure to atipamezole was reduced by vatinoxan.
In the present study, administration of atipamezole alone failed to result in sustained decreases in SVRI or increases in heart rate and CI in MED-sedated dogs, and those dogs relapsed into sedation after initial arousal was observed. However, when dogs received the MEDVAT treatment, atipamezole was able to reverse the sedative effect of MED more efficiently and reduce the depression of cardiovascular function. When atipamezole was used for reversal of MEDVAT sedation in dogs, the 2 α2-adrenoceptor antagonists interacted favorably, without adverse effects. Hence, coadministration of the peripherally acting α2-adrenoceptor antagonist vatinoxan with MED might enhance the quality and safety of recovery from MED-induced sedation in dogs receiving atipamezole as reversal agent.
Acknowledgments
This manuscript represents a portion of a thesis submitted by Dr. Turunen to the University of Helsinki Department of Equine and Small Animal Medicine as partial fulfillment of the requirements for a PhD degree.
Supported by Vetcare Ltd, Mäntsälä, Finland for the study expenses, salaries (Turunen, Nevanperä), materials, drugs, drug concentration analysis, and statistical analysis. Vetcare Ltd did not have any involvement in the study design, data analysis and interpretation, or writing and publication of the manuscript.
Presented in abstract form at the 12th World Congress of Veterinary Anaesthesiology, Kyoto, Japan, September 2015 and at the autumn meeting of the Association of Veterinary Anesthetists, Berlin, Germany, November 2017.
ABBREVIATIONS
| AUC | Area under the time-concentration curve |
| CI | Cardiac index |
| CSS | Composite sedation score |
| CVP | Central venous blood pressure |
| DAP | Diastolic arterial blood pressure |
| MAP | Mean arterial blood pressure |
| MED | Medetomidine |
| MEDVAT | Medetomidine and vatinoxan |
| RPP | Rate pressure products |
| SAP | Systolic arterial blood pressure |
| SVRI | Systemic vascular resistance index |
Footnotes
Random list generator. Available at: www.random.org/lists. Accessed May 20, 2014.
Dorbene (1 mg/mL), Laboratorios SYVA S.A.U., León, Spain.
Vetcare Ltd, Mäntsälä, Finland.
Alzane (5 mg/mL), Laboratorios SYVA S.A.U., León, Spain.
Health Products Regulatory Authority. Summary of product characteristics. Available at: www.hpra.ie/img/uploaded/swedocuments/LicenseSPC_10495-001-001_26062015123938.pdf. Accessed Aug 31, 2018.
Arteriofix V, B. Braun Melsungen AG, Berlin, Germany.
CV-12702, Arrow International, Reading, Pa.
Gabarith PMSET, Becton Dickinson, Sandy, Utah.
S/5 Anesthesia Monitor, GE Healthcare, Helsinki, Finland.
LiDCO Plus hemodynamic monitor, LiDCO Ltd, London, England.
Lithium chloride (0.15 mmol/mL) solution for injection, LiDCO Ltd, London, England.
ABL 855, Radiometer, Copenhagen, Denmark.
Sep-Pak C18, 96-well extraction plates, Waters Co, Milford, Mass.
Toronto Research Chemicals Inc, Toronto, ON, Canada.
RS-79948, Tocris Bioscience, Bristol, England.
SunFire C18 column (2.1 × 150 mm; pore size, 3.5 μm), Waters Co, Milford, Mass.
Chiralpak AGP column (4 × 150 mm; pore size, 5 μm), Chiral Technologies Europe, Illkirch-Graffenstaden, France.
4000QTrap, MDS Sciex, Concord, ON, Canada.
Waters Acquity UPLC and Waters TQ-S triple quadrupole mass spectrometry, Waters Co, Milford, Mass.
SAS for Windows, version 9.3, SAS Institute Inc, Cary, NC.
References
1. Clarke KW, England GCW. Medetomidine, a new sedative-analgesic for use in the dog and its reversal with atipamezole. J Small Anim Pract 1989;30:343–348.
2. Virtanen R. Pharmacological profiles of medetomidine and its antagonist, atipamezole. Acta Vet Scand Suppl 1989; 85:29–37.
3. Vähä-Vahe AT. Clinical effectiveness of atipamezole as a medetomidine antagonist in cats. J Small Anim Pract 1990;31:193–197.
4. Vähä-Vahe AT. The clinical effectiveness of atipamezole as a medetomidine antagonist in the dog. J Vet Pharmacol Ther 1990;13:198–205.
5. Pypendop B, Serteyn D, Verstegen J. Hemodynamic effects of medetomidine-midazolam-butorphanol and medetomidine-midazolam-buprenorphine combinations and reversibility by atipamezole in dogs. Am J Vet Res 1996;57:724–730.
6. Lamont LA, Bulmer BJ, Grimm KA, et al. Cardiopulmonary evaluation of the use of medetomidine hydrochloride in cats. Am J Vet Res 2001;62:1745–1749.
7. Vainio O. Reversal of medetomidine-induced cardiovascular and respiratory changes with atipamezole in dogs. Vet Rec 1990;127:447–450.
8. Granholm M, McKusick BC, Westerholm FC, et al. Evaluation of the clinical efficacy and safety of intramuscular and intravenous doses of dexmedetomidine and medetomidine in dogs and their reversal with atipamezole. Vet Rec 2007;160:891–897.
9. Martin-Flores M, Sakai DM, Honkavaara J, et al. Hemodynamic effects of low-dose atipamezole in isoflurane-anesthetized cats receiving an infusion of dexmedetomidine. J Feline Med Surg 2018;20:571–577.
10. Clineschmidt BV, Pettibone DJ, Lotti VJ, et al. A peripherally acting alpha-2 adrenoceptor antagonist: L-659,066. J Pharmacol Exp Ther 1988;245:32–40.
11. Pagel PS, Proctor LT, Devcic A, et al. A novel alpha 2-adrenoceptor antagonist attenuates the early, but preserves the late cardiovascular effects of intravenous dexmedetomidine in conscious dogs. J Cardiothorac Vasc Anesth 1998;12:429–434.
12. Enouri SS, Kerr CL, McDonell WN, et al. Effects of a peripheral α2 adrenergic-receptor antagonist on the hemodynamic changes induced by medetomidine administration in conscious dogs. Am J Vet Res 2008;69:728–595.
13. Honkavaara JM, Restitutti F, Raekallio MR, et al. The effects of increasing doses of MK-467, a peripheral alpha2-adrenergic receptor antagonist, on the cardiopulmonary effects of intravenous dexmedetomidine in conscious dogs. J Vet Pharmacol Ther 2011;34:332–337.
14. Honkavaara JM, Raekallio MR, Kuusela EK, et al. The effects of L-659,066, a peripheral α2-adrenoceptor antagonist, on dexmedetomidine-induced sedation and bradycardia in dogs. Vet Anaesth Analg 2008;35:409–413.
15. Restitutti F, Honkavaara JM, Raekallio MR, et al. Effects of different doses of L-659′066 on the bispectral index and clinical sedation in dogs treated with dexmedetomidine. Vet Anaesth Analg 2011;38:415–422.
16. Rolfe NG, Kerr CL, Mcdonell WN. Cardiopulmonary and sedative effects of the peripheral α2-adrenoceptor antagonist MK 0467 administered intravenously or intramuscularly concurrently with medetomidine in dogs. Am J Vet Res 2012;73:587–594.
17. Bennett RC, Salla KM, Raekallio MR, et al. Effects of MK-467 on the antinociceptive and sedative actions and pharmacokinetics of medetomidine in dogs. J Vet Pharmacol Ther 2016;39:336–343.
18. Restitutti F, Kaartinen MJ, Raekallio MR, et al. Plasma concentration and cardiovascular effects of intramuscular medetomidine combined with three doses of the peripheral α2-antagonist MK-467 in dogs. Vet Anaesth Analg 2017;44:417–426.
19. Pypendop BH, Honkavaara J, Ilkiw JE. Pharmacokinetics of dexmedetomidine, MK-467 and their combination following intramuscular administration in male cats. Vet Anaesth Analg 2017;44:823–831.
20. Honkavaara J, Restitutti F, Raekallio M, et al. Influence of MK-467, a peripherally acting α2-adrenoceptor antagonist on the disposition of intravenous dexmedetomidine in dogs. Drug Metab Dispos 2012;40:445–449.
21. Honkavaara J, Pypendop B, Ilkiw J. The impact of MK-467 on sedation, heart rate and arterial blood pressure after intramuscular coadministration with dexmedetomidine in conscious cats. Vet Anaesth Analg 2017;44:811–822.
22. Pypendop BH, Honkavaara J, Ilkiw JE, et al. Cardiovascular effects of dexmedetomidine, with or without MK-467, following intravenous administration in cats. Vet Anaesth Analg 2017;44:52–62.
23. Salonen S, Vuorilehto L, Vainio O, et al. Atipamezole increases medetomidine clearance in the dog: an agonist-antagonist interaction. J Vet Pharmacol Ther 1995;18:328–332.
24. Mason DJ, O'Grady M, Woods PJ, et al. Assessment of lithium dilution cardiac output as a technique for measurement of cardiac output in dogs. Am J Vet Res 2001;62:1255–1261.
25. Haskins S, Pascoe PJ, Ilkiw JE, et al. Reference cardiopulmonary values in normal dogs. Comp Med 2005;55:156–161.
26. Vainio O, Vähä-Vahe T. Reversal of medetomidine sedation by atipamezole in dogs. J Vet Pharmacol Ther 1990;13:15–22.
27. Murrell JC, Hellebrekers LJ. Medetomidine and dexmedetomidine: a review of cardiovascular effects and antinociceptive properties in the dog. Vet Anaesth Analg 2005;32:117–127.
28. Korpivaara M, Laapas K, Huhtinen M, et al. Dexmedetomidine oromucosal gel for noise-associated acute anxiety and fear in dogs—a randomised, double-blind, placebo-controlled clinical study. Vet Rec 2017;180:356.
29. Hayashi K, Nishimura R, Yamaki A, et al. Antagonistic effects of atipamezole on medetomidine-midazolam induced sedation in dogs. J Vet Med Sci 1994;56:1009–1011.
30. Bloor BC, Frankland M, Alper G, et al. Hemodynamic and sedative effects of dexmedetomidine in dog. J Pharmacol Exp Ther 1992;263:690–697.
31. Flacke WE, Flacke JW, Bloor BC, et al. Effects of dexmedetomidine on systemic and coronary hemodynamics in the anesthetized dog. J Cardiothorac Vasc Anesth 1993;7:41–49.
32. Schmeling WT, Kampine JP, Roerig DL, et al. The effects of the stereoisomers of the α2-adrenergic agonist medetomidine on systemic and coronary hemodynamics in conscious dogs. Anesthesiology 1991;75:499–511.
33. Kaartinen MJ, Cuvelliez S, Brouillard L, et al. Article survey of utilization of medetomidine and atipamezole in private veterinary practice in Quebec in 2002. Can Vet J 2007;48:725–730.
34. Ranheim B, Horsberg TE, Nymoen U, et al. Reversal of medetomidine-induced sedation in reindeer (Rangifer tarandus tarandus) with atipamezole increases the medetomidine concentration in plasma. J Vet Pharmacol Ther 1997;20:350–354.
35. Ranheim B, S⊘li NE, Ryeng KA, et al. Pharmacokinetics of medetomidine and atipamezole in dairy calves: an agonist-antagonist interaction. J Vet Pharmacol Ther 1998;21:428–432.
36. Ranheim B, Arnemo JM, Ryeng KA, et al. A pharmacokinetic study including some relevant clinical effects of medetomidine and atipamezole in lactating dairy cows. J Vet Pharmacol Ther 1999;22:368–373.
37. Adam M, Raekallio MR, Keskitalo T, et al. The impact of MK-467 on plasma drug concentrations, sedation and cardiopulmonary changes in sheep treated with intramuscular medetomidine and atipamezole for reversal. J Vet Pharmacol Ther 2018;41:447–456.
38. Virtanen R, Savola JM, Saano V. Highly selective and specific antagonism of central and peripheral alpha 2-adrenoceptors by atipamezole. Arch Int Pharmacodyn Ther 1989;297:190–204.
39. Restitutti F, Laitinen MR, Raekallio MR, et al. Effect of MK-467 on organ blood flow parameters detected by contrast-enhanced ultrasound in dogs treated with dexmedetomidine. Vet Anaesth Analg 2013;40:e48–e56.
Appendix
Modified composite sedation scoring system (0 to 20 points) used to assess sedation in dogs.
| Assessment (possible scores) | ||
|---|---|---|
| Position (0–4) | Dog is standing | 0 |
| Dog is standing but staggers | 1 | |
| Dog is in sternal recumbency with its head up | 2 | |
| Dog is in sternal recumbency with its head down | 3 | |
| Dog is in lateral recumbency with its head down | 4 | |
| Palpebral reflex (0–3) | Normal | 0 |
| Slightly reduced | 1 | |
| Weak | 2 | |
| Absent | 3 | |
| Eye position (0 or 2) | Central | 0 |
| Turned down | 2 | |
| Jaw and tongue relaxation (0–4) | Normal resistance on opening the dog's mouth and manipulation of the tongue | 0 |
| Dog closes jaws together | 1 | |
| Dog opens the jaws but there is strong resistance when the tongue is pulled | 2 | |
| Dog opens the jaws but there is slight resistance when the tongue is pulled | 3 | |
| Dog shows no resistance to jaw opening or tongue pulling | 4 | |
| Resistance to positioning in lateral recumbency (0–3) | Dog struggles normally or resists this positioning | 0 |
| Dog returns to sternal recumbency | 1 | |
| Some resistance but the dog remains in lateral recumbency | 2 | |
| No resistance or the dog is already in lateral recumbency | 3 | |
| General appearance (0–4) | Dog is awake with a normal reaction to surroundings | 0 |
| Dog appears slightly tired and its head is drooping | 1 | |
| Signs of mild sedation; dog clearly reacts to surroundings | 2 | |
| Signs of moderate sedation; dog reacts slightly to surroundings | 3 | |
| Signs of deep sedation; dog has no reaction to surroundings | 4 |
