Abstract
OBJECTIVE To evaluate effects of the peripherally acting α2-adrenoceptor antagonist MK-467 on cardiopulmonary function in sheep sedated with medetomidine and ketamine.
ANIMALS 9 healthy adult female sheep.
PROCEDURES Each animal received an IM injection of a combination of medetomidine (30 μg/kg) and ketamine (1 mg/kg; Med-Ket) alone and Med-Ket and 3 doses of MK-467 (150, 300, and 600 μg/kg) in a randomized blinded 4-way crossover study. Atipamezole (150 μg/kg, IM) was administered 60 minutes later to reverse sedation. Cardiopulmonary variables and sedation scores were recorded, and drug concentrations in plasma were analyzed. Data were analyzed with a repeated-measures ANCOVA and 1-way ANOVA. Reference limits for the equivalence of sedation scores were set at 0.8 and 1.25.
RESULTS Heart rate, cardiac output, and Pao2 decreased and mean arterial blood pressure, central venous pressure, and systemic vascular resistance increased after Med-Ket alone. Administration of MK-467 significantly alleviated these effects, except for the decrease in cardiac output. After sedation was reversed with atipamezole, no significant differences were detected in cardiopulmonary variables among the treatments. Administration of MK-467 did not significantly alter plasma concentrations of medetomidine, ketamine, norketamine, or atipamezole. Sedation as determined on the basis of overall sedation scores was similar among treatments.
CONCLUSIONS AND CLINICAL RELEVANCE Concurrent administration of MK-467 alleviated cardiopulmonary effects in sheep sedated with Med-Ket without affecting sedation or reversal with atipamezole.
The α2-adrenoceptor agonist drugs are used for several clinical applications, including sedation, analgesia, and premedication before general anesthesia. Medetomidine, or more precisely its active enantiomer dexmedetomidine, is the most selective and potent α2-adrenoceptor agonist in clinical use, with a ratio for α2-adrenoceptor selectivity to α1-adrenoceptor selectivity of 1,620:1.1 Both medetomidine and dexmedetomidine are commonly used in combination with ketamine to reliably induce sedation in many animal species. The sedative and analgesic effects of medetomidine and dexmedetomidine are mediated via α2-adrenoceptors located in the brain and spinal cord.2 Activation of α2-adrenoceptors located in peripheral tissues, especially in vascular smooth muscle cells, results in adverse cardiopulmonary consequences. Specifically, the most notable adverse effects in sheep are hypoxemia3–6 and a decrease in HR and CO.7–10
It is known that MK-467 (also referred to as vatinoxan), a peripherally acting α2-adrenoceptor antagonist, poorly penetrates the CNS because of its low lipophilicity, as has been determined in rats and marmosets.11 Administration of MK-467 can attenuate adverse cardiovascular influences induced by various α2-adrenoceptor agonists in many animal species, including dogs,12–15 cats,16 horses,8,17,18 and sheep.8,10 Furthermore, MK-467 has no clinically relevant effect on the sedative efficacy of α2-adrenoceptor agonists,10,12,19 which suggests that its antagonistic actions are limited to peripheral receptors.
In a previous study of dogs,15 MK-467 enhanced the absorption of medetomidine on concurrent IM injection. Moreover, MK-467 substantially increased the apparent volume of distribution after simultaneous IV administration of α2-adrenoceptor agonists in dogs20 and horses,17,18 whereas this effect was only moderate in cats.21
Atipamezole is a potent α2-adrenoceptor antagonist that is commonly used in veterinary practice to reverse sedation induced by administration of medetomidine22,23 or medetomidine combined with ketamine.24,25 Relapse into sedation (ie, resedation) has been observed in many ruminant species, including dairy calves,26 cows,27 and reindeer,28 but no resedation was reported for sheep receiving atipamezole IV 60 minutes after IV administration of medetomidine.23
To the authors’ knowledge, there have been no reports on the concurrent administration of MK-467 and a sedative combination of medetomidine and ketamine in any animal species. The objective of the study reported here was to evaluate effects of MK-467 concurrently administered IM with medetomidine and ketamine on cardiopulmonary function and sedation in sheep. We hypothesized that MK-467 would attenuate adverse cardiopulmonary effects associated with medetomidine and ketamine. Furthermore, we assumed that if MK-467 affected sedation, it would be associated with differences in sedative drug concentrations in plasma.
Materials and Methods
Animals
Nine healthy adult nonpregnant female sheep were used in the study. Mean age of the animals was 1.7 years (range, 1 to 5 years), and mean body weight was 45 kg (range, 37 to 72 kg). Health of the sheep was confirmed on the basis of results for physical examination and comprehensive hematologic and biochemical analyses. Sheep were housed as a group and fed hay and concentrate; they had ad libitum access to drinking water. One month before the start of the study, the sheep were anesthetized, and the right carotid artery of each animal was surgically elevated into a subcutaneous position, as has been described for horses.29 The study was authorized by the National Animal Experiment Board of Finland (ESAVI/9394/04.10.07/2015).
Study design
A randomized blinded 4-period crossover study design was used. Each sheep received each of the 4 treatments; there was a washout period of at least 14 days between successive treatments.
Instrumentation
On the morning of an experiment, concentrates were withheld, but sheep had access to water and hay. Each animal was placed in a sling that supported it in a standing position. The ventral aspect of the neck of each sheep was aseptically prepared for catheter insertion. An aliquot (5 mg) of a 20-mg/mL solution of lidocaine hydrochloridea was infiltrated SC at the catheter sites. A single-lumen polyethene central venous catheterb was inserted into the left jugular vein and advanced to the central vein by use of pressure waveforms to monitor the CVP. A 20-gauge venous cannulac was then inserted in the right jugular vein for lithium chloride injections. Finally, a 20-gauge arterial cannulad was inserted into the elevated carotid artery. The central venous and arterial catheters were connected to pressure transducers,e which were calibrated with a mercury manometer. The transducers were connected to a monitorf and calibrated (zeroed) to atmospheric pressure at the level of the manubrium.
Treatments
All drugs for each treatment were mixed in the same syringe for IM administration. Treatments were medetomidine hydrochlorideg (30 μg/kg) and ketamine hydrochlorideh (1 mg/kg; Med-Ket) alone and Med-Ket and MK-467 hydrochloridei at 150 μg/kg (Med-Ket-MK150), 300 μg/kg (Med-Ket-MK300), and 600 μg/kg (Med-Ket-MK600). Drug mixtures were freshly prepared for each injection by dissolving MK-467 hydrochloride powder (15, 30, and 60 mg) in 3 mL of medetomidine and 1.5 mL of saline (0.9% NaCl) solution to create solutions with ratios for medetomidine hydrochloride concentration to MK-467 hydrochloride concentration of 1:5, 1:10, and 1:20. Complete dissolution was promoted by sonication in lukewarm tap water. Immediately before each treatment was administered, 2 mL of ketamine was added to the mixture. The injection volume was identical for all treatments and was adjusted in accordance with body weight (0.065 mL/kg). All injections were administered IM into the triceps brachii muscle, alternating between the right and left side (time 0; baseline). Sixty minutes after injection of each treatment, atipamezole hydrochloridej (150 μg/kg) was administered IM into the contralateral triceps brachii muscle of each sheep.
Measurements
The HR, CVP, SAP, DAP, and MAP were continuously monitored and recorded at baseline and every 5 minutes until 120 minutes after administration of the treatments. Respiratory rate was calculated by observing chest movements at 10-minute intervals. The CO was measured at baseline and 7, 15, 30, 45, 75, and 90 minutes after treatment by use of the lithium dilution method,k as described elsewhere.30 Standard values for hemoglobin (100 g/L) and sodium (140 mmol/L) were used initially, and these were later corrected with values obtained from simultaneously collected arterial blood samples.l,m Arterial blood samples were anaerobically collected via the arterial cannula into heparinized syringesn at baseline and 15, 30, 45, and 90 minutes after treatment, stored in ice water for ≤ 10 minutes, and analyzed with a blood gas analyzero to determine Pao2, Paco2, pH, base excess, and bicarbonate, lactate, and glucose concentrations. Measurements were corrected on the basis of rectal temperature. When appropriate, standard equations31,32 were used to calculate cardiopulmonary variables (Do2, stroke volume, left ventricle workload, left ventricle rate pressure product, SVR, and Cao2). A specific equation for sheep was used to calculate Sao2.33
Sedation was assessed visually on a scale from 0 to 10 (0 represented no sedation, and 10 represented deep sedation and unresponsiveness to manipulation and hand clapping) by an investigator (MA) who was unaware of the treatment administered to each sheep. Scores were recorded before administration of each treatment and at regularly scheduled intervals up to 120 minutes after treatment. At the end of each experiment, all catheters were removed, and meloxicamp (0.5 mg/kg, IV) was administered to alleviate pain.
Blood samples (6 mL) for analysis of drug concentrations in plasma were collected from the arterial cannula into EDTA-containing tubes at 3, 6, 10, 15, 20, 25, 30, 40, 50, and 90 minutes after treatment; stored in ice water; and centrifuged at 3,000 × g for 15 minutes to separate plasma. Plasma was harvested, and samples were stored at −20°C or colder until analyzed.
Concentrations of dexmedetomidine and levomedetomidine were determined by use of racemic d3-medetomidineq as the internal standard. Samples were subjected to solid-phase extractionr; concentrations were then measured with high-performance liquid chromatography-tandem mass spectrometry. Chiral separations was performed with 10mM ammonium acetate (pH, 4.5) and acetonitrile containing 0.1% formic acid as solvents. Quantitative detection was performed with a triple-quadrupole mass spectrometert in multireaction monitoring mode. For dexmedetomidine and levomedetomidine, the respective precursor ions (m/z) were 201.2 → 95.1, whereas the m/z for d3-medetomidine was 204.2 → 98.05. Chromatograms were processed with standard software.u The linear concentration range spanned from 0.10 to 10.0 ng/mL. Interassay accuracy of quality control samples (concentrations, 0.225, 1.0, 8.0, and 50 ng/mL) ranged from 94.4% to 998% for dexmedetomidine and from 92.5% to 99.2% for levomedetomidine.
Plasma samples were prepared for analysis of ketamine, norketamine, MK-467, and atipamezole. Plasma aliquots (50 μL) were placed in wells of a protein-precipitation platev; 250 μL of internal standard solution in acetonitrile (containing 100 ng of propranolol/mL and 20 ng of chlorpromazine/mL) was added. Contents of wells were mixed for 5 minutes and then were centrifuged at 2,952 × g for 20 minutes. Next, 50-μL aliquots of sample supernatants were transferred to 96-well ultraperformance liquid chromatography plates, diluted with 450 μL of 20% acetonitrile in water, and analyzed. Reference samples were prepared in drug-free sheep plasma (analyte concentrations, 0.02 to 20,000 ng/mL). Quality control samples were analyzed (concentrations, 0.2, 2, 20, 200, and 2,000 ng/mL). Analyses were performed with liquid chromatography-tandem mass spectrometryw,x by use of a reverse-phase C18 column.y Column temperature was 40°C, autosampler temperature was 10°C, and injection volume was 4 μL. The aqueous eluent (solution A) was 0.5% formic acid in water, and acetonitrile was the organic eluent (solution B). Eluent flow rate was 0.5 mL/min. Gradient elution involved changes in solution B as follows: 2% at 0 minutes, 2% at 0.5 minutes, 50% at 2 minutes, 90% at 2.5 minutes, 90% at 3 minutes, and 2% at 4 minutes. Nitrogen was used as a solvent (flow rate, 900 L/h) and cone gas (flow rate, 150 L/h). Solvent temperature was 650°C, and source temperature was 150°C. Capillary voltage was 1,000 V, and polarity was set as positive ionization.
The multireaction monitoring mode transition settings for ketamine were as follows: m/z, 238 → 125; cone voltage, 25 V; collision energy, 12 V; and retention time, 1.54 minutes. Settings for norketamine were as follows: m/z, 224 → 207; cone voltage, 25 V; collision energy, 10 V; and retention time, 1.51 minutes. Settings for MK-467 were as follows: m/z, 419 → 200; cone voltage, 25 V; collision energy, 20 V; and retention time, 1.55 minutes. Settings for atipamezole were as follows: m/z, 213 → 145; cone voltage, 40 V; collision energy, 20 V; and retention time, 1.86 minutes. Settings for the internal standard propranolol were as follows: m/z, 260 → 116; cone voltage, 28 V; collision energy, 18 V; and retention time, 1.9 minutes. Settings for the internal standard chlorpromazine were as follows: m/z, 319 → 58; cone voltage, 15 V; collision energy, 22 V; and retention time, 2.26 minutes. Quantitation range for ketamine, MK-467, and atipamezole was 1 to 10,000 ng/mL, whereas the quantitation range for norketamine was 0.5 to 2,000 ng/mL.
Pharmacological calculations
Values for AUC0–50 were calculated with the trapezoidal method. Values for Cmax and Tmax were derived from plasma concentration data.
Statistical analysis
Statistical power was estimated on the basis of data from studies of sheep10 and dogs12,14 conducted by our research group. Results indicated that a crossover design with 8 animals would allow the detection (paired 2-tailed test, α = 0.05, and power = 80%) of clinically meaningful differences among treatments (peak effects) in primary outcome variables as follows: HR (mean ± SD), 20 ± 10 beats/min; MAP, 17 ± 15 mm Hg; CO, 1.4 ± 1.2 L/min; and Pao2, 15 ± 10 mm Hg.
Cardiopulmonary variables and sedation scores were analyzed with a statistical program.z Differences among treatments for changes from baseline values were evaluated with a mixed model repeated-measures ANCOVA. The model included treatment, time, period, the treatment-by-time interaction, the period-by-time interaction, and a baseline covariate as fixed effects; animal, the animal-by-time interaction, and the animal-by-period interaction were random effects. Treatment comparisons for changes from baseline values by time and changes within treatment were computed from the same model with contrasts. Because treatment period could be clearly divided into 2 phases (before and after atipamezole administration), each phase was analyzed separately and compared with baseline values. In addition, the objective of the analysis of the sedation score was to determine that the level of sedation was equal for all treatments. The trapezoidal method was used to calculate the area under the sedation score–time curve from baseline to 70 minutes for each sheep. Logarithmic transformation (loge) of the values for the area under the sedation score–time curves was analyzed with a repeated-measures ANOVA. The model included treatment and period as fixed effects and animal and the period-by-animal interaction as random effects. Treatment differences and 95% confidence intervals were transformed back to the original scale and referred to as the geometric mean ratio. Similar to procedures used in bioequivalence studies, reference limits for equivalence34 were set at 0.8 and 1.25. Equivalence among treatments was to be accepted when the 95% confidence interval of the geometric mean ratio could be within reference limits.
Normality assumptions were assessed with the Kolmogorov-Smirnov test. Logarithmic and inverse transformations were computed to normalize distributions when normality assumptions were not met. For all transformed variables, the model was fitted for changes in the transformed response; results for these variables were also in the transformed values. Similar to the main analysis, estimates of treatment differences were calculated over time from the fitted models. In addition, differences were calculated for selected time points, and within-treatment changes also were calculated. The 95% confidence intervals were calculated for treatment differences and within-treatment changes. Pharmacokinetic parameters were analyzed by use of a 1-way ANOVA followed by the Tukey post hoc test.aa Values of P < 0.05 were considered significant.
Results
Data for treatments of 2 sheep (1 sheep for the Med-Ket treatment and 1 sheep for the Med-Ket-MK150 treatment) were excluded because of an apparent partial IV administration of drug. This was deduced from the abnormal shape of the plasma drug concentration-time curves.
Cardiopulmonary data were summarized (Figure 1; Tables 1–3). Significant (P < 0.001) effects of treatment, time, and the treatment-by-time interaction were detected for HR, SAP, DAP, and MAP. For the Med-Ket treatment, HR remained significantly (P < 0.001) below the baseline value for the entire period until reversal with atipamezole; however, CVP increased significantly (P < 0.001) over time. Administration of MK-467 prevented initial increases in SAP, DAP, MAP, and CVP, and subsequent blood pressures were also significantly below baseline values after all doses of MK-467. Only 1 sheep had an MAP < 60 mm Hg after the Med-Ket-MK600 treatment.
Mean ± SD values for HR (A), MAP (B), CVP (C), and SVR (D) of 9 sheep receiving an IM injection (time 0; baseline) of a combination of medetomidine (30 μg/kg) and ketamine (1 mg/kg; Med-Ket [white squares]) or concurrent administration (within the same syringe) of Med-Ket and MK-467 at 150 μg/kg (Med-Ket-MK150 [black squares]), 300 μg/kg (Med-Ket-MK300 [triangles]), and 600 μg/kg (Med-Ket-MK600 [circles]) and for which sedation was reversed by administration of atipamezole (150 μg/kg, IM) at 60 minutes (arrow). There was a washout period of ≥ 14 days between treatments. *†‡§Within a treatment (*Med-Ket, †Med-Ket-MK150, ‡Med-Ket-MK300, and §Med-Ket-MK600), the value differs significantly (P < 0.05) from the baseline value. ‖¶#Within a time point, value for a treatment that included MK-467 (‖Med-Ket-MK300, ¶Med-Ket-MK600, and #Med-Ket-MK150) differs significantly (P < 0.05) from the value for the Med-Ket treatment.
Citation: American Journal of Veterinary Research 79, 9; 10.2460/ajvr.79.9.921
Mean ± SD values for cardiovascular variables of 9 sheep sedated with medetomidine and ketamine and administered atipamezole 60 minutes later to reverse sedation.
| Time after administration (min) | ||||||||
|---|---|---|---|---|---|---|---|---|
| Variable | Treatment | Baseline | 5 | 15 | 30 | 45 | 75 | 90 |
| CO (L/min)* | Med-Ket | 4.2 ± 0.54 | 3.4 ± 1.05† | 3.4 ± 1.26‡ | 3.6 ± 0.54 | 3.8 ± 0.96 | 4.6 ± 0.76 | 4.5 ± 0.81 |
| Med-Ket-MK150 | 4.7 ± 0.67 | 3.6 ± 0.72† | 3.9 ± 0.70§ | 4.6 ± 0.87 | 4.6 ± 1.13 | 5.4 ± 2.46 | 5.2 ± 1.77 | |
| Med-Ket-MK300 | 4.9 ± 0.90 | 4.1 ± 1.24§ | 4.6 ± 1.59 | 4.2 ± 0.56 | 4.1 ± 0.57 | 5.5 ± 2.21 | 4.9 ± 1.28 | |
| Med-Ket-MK600 | 4.9 ± 1.51 | 3.9 ± 0.66 | 4.1 ± 1.09§ | 4.0 ± 1.30† | 3.7 ± 0.93§ | 6.3 ± 1.69§ | 5.8 ± 2.51 | |
| SAP (mm Hg) | Med-Ket | 120 ± 8 | 127 ± 16 | 136 ± 10† | 132 ± 8§ | 124 ± 19 | 125 ± 13 | 130 ± 17 |
| Med-Ket-MK150 | 125 ± 15 | 122 ± 13 | 127 ± 13‖ | 114 ± 11‖ | 99 ± 7†‖ | 131 ± 14§ | 129 ± 12 | |
| Med-Ket-MK300 | 123 ± 11 | 117 ± 8 | 122 ± 9‖ | 102 ± 15‡ | 91 ± 14‡‖ | 135 ± 10†‖ | 133 ± 8§ | |
| Med-Ket-MK600 | 123 ± 14 | 119 ± 13 | 109 ± 17†‖ | 89 ± 12‡‖ | 84 ± 9‡‖ | 135 ± 14†‖ | 131 ± 7 | |
| DAP (mm Hg) | Med-Ket | 91 ± 4 | 94 ± 12 | 103 ± 10† | 102 ± 12§ | 97 ± 16 | 93 ± 9 | 95 ± 7 |
| Med-Ket-MK150 | 95 ± 11 | 91 ± 10 | 98 ± 7 | 88 ± 9‖ | 77 ± 8‡‖ | 97 ± 10 | 94 ± 13 | |
| Med-Ket-MK300 | 93 ± 10 | 88 ± 7 | 94 ± 6‖ | 78 ± 9‡‖ | 68 ± 13‡‖ | 98 ± 7 | 98 ± 9 | |
| Med-Ket-MK600 | 92 ± 9 | 87 ± 11 | 85 ± 13‖ | 8 ± 13‡‖ | 64 ± 11‡‖ | 102 ± 14§ | 99 ± 7§ | |
| SV (mL) | Med-Ket | 57.2 ± 12.3 | 60.4 ± 16.3 | 64.1 ± 17.3 | 68.5 ± 8.8 | 71.4 ± 21.0 | 68.4 ± 12.2§ | 65.9 ± 23.6 |
| Med-Ket-MK150 | 58.5 ± 14.8 | 54.4 ± 8.0 | 67.9 ± 16.7 | 72.9 ± 23.2 | 61.4 ± 19.0 | 77.1 ± 23.5 | 74.2 ± 25.1 | |
| Med-Ket-MK300 | 67.0 ± 19.7 | 66.5 ± 15.8 | 72.1 ± 17.6 | 59.4 ± 23.0 | 57.5 ± 17.8 | 79.3 ± 21.0 | 70.1 ± 24.5 | |
| Med-Ket-MK600 | 56.8 ± 17.3 | 60.3 ± 14.4 | 54.7 ± 23.4 | 45.3 ± 16.7† | 45.4 ± 20.9§ | 74.7 ± 26.0§ | 73.5 ± 44.5 | |
| LVW (kg·m/min) | Med-Ket | 5.86 ± 0.84 | 4.95 ± 1.89§ | 5.43 ± 1.90 | 5.48 ± 0.70 | 5.34 ± 1.55 | 6.68 ± 1.67 | 6.66 ± 1.12 |
| Med-Ket-MK150 | 6.92 ± 1.50 | 5.08 ± 1.10† | 5.86 ± 1.31 | 6.26 ± 1.36 | 5.50 ± 1.67§ | 8.53 ± 4.82 | 7.97 ± 3.40 | |
| Med-Ket-MK300 | 7.08 ± 1.70 | 5.71 ± 2.04§ | 6.53 ± 2.08 | 5.12 ± 1.22† | 4.33 ± 1.06‡‖ | 8.71 ± 4.15 | 7.56 ± 1.99 | |
| Med-Ket-MK600 | 7.05 ± 2.70 | 5.37 ± 1.06§ | 5.33 ± 1.74† | 4.22 ± 1.90‡‖ | 3.62 ± 1.06‡‖ | 10.41 ± 2.68§ | 8.99 ± 4.49 | |
| LVRPP | Med-Ket | 7,740 ± 10,90 | 6,671 ± 1,449‡ | 6,280 ± 798‡ | 5,923 ± 222‡ | 5,822 ± 1,022‡ | 7,268 ± 1,850 | 8,077 ± 2,654 |
| (beats/min·mm Hg) | Med-Ket-MK150 | 8,980 ± 1,851 | 6,969 ± 1,370§ | 6,494 ± 945‡ | 6,633 ± 1,351‡ | 6,736 ± 963‡ | 8,046 ± 1,735 | 7,987 ± 1,997 |
| Med-Ket-MK300 | 8,073 ± 1,874 | 6,497 ± 1,263‡ | 6,670 ± 902‡ | 6,783 ± 1,313‡ | 5,710 ± 674‡ | 8,371 ± 2,202 | 8,595 ± 2,274 | |
| Med-Ket-MK600 | 9,313 ± 2,000 | 7,144 ± 1,525‡ | 8,075 ± 1,890‖ | 6,793 ± 1,169‡ | 6,533 ± 1,501‡ | 10,606 ± 3,921‖ | 10,097 ± 2,586 | |
Sheep received an IM injection (time 0; baseline) of a combination of medetomidine (30 μg/kg) and ketamine (1 mg/kg; Med-Ket) or concurrent administration (within the same syringe) of Med-Ket and MK-467 at 150 μg/kg (Med-Ket-MK150), 300 μg/kg (Med-Ket-MK300), and 600 μg/kg (Med-Ket-MK600); atipamezole (150 μg/kg, IM) was administered at 60 minutes. There was a washout period of ≥ 14 days between treatments.
The CO for 5 minutes was measured at 7 minutes.
Within a treatment, value differs significantly (P = 0.01) from the baseline value.
Within a treatment, value differs significantly (P < 0.001) from the baseline value.
Within a treatment, value differs significantly (P < 0.05) from the baseline value.
Within a time point within a variable, value differs significantly (P < 0.05) from the value for the Med-Ket treatment.
LVRPP = Left ventricle rate pressure product. LVW = Left ventricle workload. SV = Stroke volume.
Mean ± SD values for selected blood gas and metabolic variables for the same 9 sheep as in Table 1.
| Time after administration (min) | ||||||
|---|---|---|---|---|---|---|
| Variable | Treatment | Baseline | 15 | 30 | 45 | 90 |
| Pao2 (mm Hg) | Med-Ket | 98.1 ± 7.8 | 71.1 ± 12.4 | 68.6 ± 9.3‡ | 73.7 ± 10.0‡ | 99.5 ± 12.7 |
| Med-Ket-MK150 | 99.0 ± 7.7 | 76.3 ± 13.7‡ | 81.9 ± 11.3‡‖ | 86.2 ± 8.6† | 103.0 ± 6.3 | |
| Med-Ket-MK300 | 103.3 ± 6.6 | 79.2 ± 11.8‡‖ | 83.1 ± 7.9‡‖ | 92.9 ± 8.0† | 97.3 ± 9.4 | |
| Med-Ket-MK600 | 94.7 ± 5.7 | 79.7 ± 11.3‡‖ | 81.7 ± 8.2†‖ | 89.6 ± 10.9 | 92.8 ± 7.8 | |
| Paco2 (mm Hg) | Med-Ket | 39.5 ± 4.6 | 49.5 ± 8.6‡ | 51.5 ± 6.2‡ | 49.4 ± 6.9‡ | 41.0 ± 7.9 |
| Med-Ket-MK150 | 40.2 ± 2.3 | 52.2 ± 7.6‡ | 51.9 ± 4.9‡ | 50.0 ± 4.4‡ | 43.2 ± 3.7 | |
| Med-Ket-MK300 | 39.8 ± 3.3 | 48.9 ± 7.3‡ | 51.4 ± 7.3‡ | 46.8 ± 3.4‡ | 42.4 ± 2.2 | |
| Med-Ket-MK600 | 39.6 ± 2.8 | 50.8 ± 5.2‡ | 51.2 ± 4.0‡ | 45.7 ± 6.2‡ | 41.3 ± 6.4 | |
| Glucose (mmol/L) | Med-Ket | 4.6 ± 1.5 | 5.8 ± 1.5‡ | 6.7 ± 1.8‡ | 7.8 ± 1.9‡ | 8.6 ± 2.5‡ |
| Med-Ket-MK150 | 4.3 ± 0.5 | 4.5 ± 0.5‖ | 4.7 ± 0.9‖ | 4.7 ± 0.9‖ | 5.0 ± 0.8§‖ | |
| Med-Ket-MK300 | 4.9 ± 1.8 | 5.0 ± 1.9‖ | 4.9 ± 2.0‖ | 4.9 ± 1.9‖ | 5.1 ± 1.9‖ | |
| Med-Ket-MK600 | 4.3 ± 0.3 | 4.3 ± 0.6‖ | 4.2 ± 0.6‖ | 4.1 ± 0.5‖ | 4.1 ± 0.5‖ | |
| Lactate (mmol/L) | Med-Ket | 1.6 ± 1.1 | 1.5 ± 1.1 | 1.2 ± 0.6 | 1.1 ± 0.5 | 1.9 ± 1.4 |
| Med-Ket-MK150 | 1.9 ± 1.5 | 1.3 ± 0.6 | 1.0 ± 0.4§ | 0.8 ± 0.3† | 1.30 ± 0.7 | |
| Med-Ket-MK300 | 1.7 ± 1.8 | 1.8 ± 1.9 | 1.3 ± 1.2 | 1.0 ± 0.8§ | 1.6 ± 2.8 | |
| Med-Ket-MK600 | 1.3 ± 0.7 | 1.6 ± 1.2 | 1.2 ± 0.8 | 1.0 ± 0.6§ | 1.0 ± 0.7 | |
| Base excess (mmol/L) | Med-Ket | 7.1 ± 2.3 | 10.0 ± 3.1‡ | 11.9 ± 3.3‡ | 12.2 ± 4.2‡ | 8.4 ± 3.9§ |
| Med-Ket-MK150 | 5.6 ± 3.2 | 9.2 ± 3.8† | 11.7 ± 4.1‡ | 13.6 ± 4.3‡ | 9.3 ± 3.8§ | |
| Med-Ket-MK300 | 6.4 ± 3.6 | 8.8 ± 4.2† | 10.9 ± 4.2‡ | 12.4 ± 3.6‡ | 8.1 ± 2.2 | |
| Med-Ket-MK600 | 4.9 ± 3.4 | 7.5 ± 3.9 | 10.5 ± 3.7‡ | 11.3 ± 4.7‡ | 12.1 ± 5.4‡‖ | |
| Bicarbonate (mmol/L) | Med-Ket | 29.6 ± 2.2 | 33.4 ± 3.3‡ | 35.0 ± 3.7‡ | 35.2 ± 4.3‡ | 30.9 ± 3.8 |
| Med-Ket-MK150 | 28.8 ± 2.8 | 33.1 ± 3.9‡ | 35.3 ± 3.7‡ | 36.6 ± 4.0‡ | 32.3 ± 3.4§ | |
| Med-Ket-MK300 | 29.3 ± 3.3 | 32.4 ± 4.3‡ | 34.5 ± 4.2‡ | 35.3 ± 3.4‡ | 31.2 ± 2.1 | |
| Med-Ket-MK600 | 28.1 ± 3.1 | 31.6 ± 3.6§ | 34.2 ± 3.2‡ | 34.3 ± 4.0‡‖ | 34.3 ± 5.2‡ | |
| Hemoglobin (g/dL) | Med-Ket | 9.4 ± 0.8 | 8.88 ± 0.86§ | 8.53 ± 0.59† | 8.58 ± 0.95‡ | 9.58 ± 1.12 |
| Med-Ket-MK150 | 9.3 ± 0.97 | 8.1 ± 0.78†‖ | 7.7 ± 0.60‡‖ | 7.5 ± 0.60‡‖ | 9.2 ± 0.93 | |
| Med-Ket-MK300 | 9.1 ± 0.61 | 8.2 ± 0.83§‖ | 7.7 ± 0.63‡‖ | 7.5 ± 0.64‡‖ | 9.2 ± 0.93 | |
| Med-Ket-MK600 | 9.4 ± 1.05 | 8.3 ± 0.77‖ | 7.7 ± 0.67‡‖ | 7.5 ± 0.63‡‖ | 8.9 ± 0.81 | |
See Table 1 for key.
Mean ± SD values of calculated cardiopulmonary parameters for the same 9 sheep as in Table 1.
| Time after administration (min) | ||||||
|---|---|---|---|---|---|---|
| Variable | Treatment | Baseline | 15 | 30 | 45 | 90 |
| Sao2 (%) | Med-Ket | 95.4 ± 0.9 | 87.4 ± 6.3‡ | 86.8 ± 5.1‡ | 89.1 ± 4.1‡ | 95.3 ± 1.8 |
| Med-Ket-MK150 | 95.5 ± 1.0 | 89.6 ± 5.6‡ | 91.8 ± 3.2†‖ | 93.2 ± 1.7‖ | 96.0 ± 0.6 | |
| Med-Ket-MK300 | 96.0 ± 0.7 | 90.9 ± 3.4‡ | 92.4 ± 2.2†‖ | 94.5 ± 1.2‖ | 95.0 ± 1.7 | |
| Med-Ket-MK600 | 94.9 ± 0.8 | 91.1 ± 3.6†‖ | 92.0 ± 2.2§‖ | 93.7 ± 2.2‖ | 94.5 ± 1.4 | |
| Do2 (mL/kg/min) | Med-Ket | 12.2 ± 2.1 | 8.9 ± 2.2‡ | 9.3 ± 1.4† | 8.7 ± 1.3† | 13.9 ± 3.4 |
| Med-Ket-MK150 | 12.9 ± 1.6 | 8.5 ± 1.0‡ | 10.2 ± 2.1† | 10.1 ± 2.4† | 14.0 ± 3.0 | |
| Med-Ket-MK300 | 14.2 ± 3.4 | 10.9 ± 3.6† | 9.9 ± 2.3‡ | 9.6 ± 1.8‡ | 14.0 ± 3.8 | |
| Med-Ket-MK600 | 14.4 ± 6.9 | 9.9 ± 2.9‡ | 9.3 ± 3.1‡ | 8.2 ± 2.0‡ | 15.0 ± 3.0 | |
| Cao2 (mL/L) | Med-Ket | 128.5 ± 10.6 | 114.3 ± 13.0‡ | 106.5 ± 7.1‡ | 111.1 ± 10.9‡ | 130.9 ± 16.1 |
| Med-Ket-MK150 | 127.7 ± 12.1 | 105.3 ± 9.9‡‖ | 102.2 ± 6.2‡‖ | 101.3 ± 7.3‡‖ | 127.3 ± 16.9 | |
| Med-Ket-MK300 | 126.2 ± 8.4 | 108.1 ± 12.3‡ | 103.2 ± 8.2‡ | 102.8 ± 8.7‡‖ | 123.5 ± 13.5 | |
| Med-Ket-MK600 | 128.0 ± 13.6 | 109.1 ± 9.6‡ | 101.5 ± 7.9‡‖ | 100.5 ± 8.2‡‖ | 117.5 ± 9.6 | |
See Table 1 for key.
After atipamezole was administered, there were no significant differences in HR, MAP, CVP, or SVR between the Med-Ket treatment and Med-Ket-MK-467 treatments. For the Med-Ket treatment, atipamezole administration increased HR, which no longer differed significantly from the baseline value. In addition, reversal with atipamezole resulted in an increase in CO and Do2 for all treatments, with values no longer significantly different from the baseline values.
At 15 minutes, Pao2 was significantly lower than the baseline values for all treatments. Values of Pao2 < 60 mm Hg were recorded for 3 sheep after treatment with Med-Ket and 1 sheep after treatment with Med-Ket-MK150. For Med-Ket-MK300 and Med-Ket-MK600, 2 sheep (the same 2 sheep for both treatments) had Pao2 values between 60 and 70 mm Hg.
Rectal temperature was significantly lower than the baseline value at ≥ 30 minutes for all treatments, including during the recovery period after reversal of sedation with atipamezole. However, the decreases were small (rectal temperature was never < 37.4°C). The plasma glucose concentration increased significantly (P < 0.001) after Med-Ket treatment and remained elevated after reversal with atipamezole, whereas no changes from the baseline value were detected after any dose of MK-467.
Comparisons of area under the plasma concentration-time curve for sedation scores revealed that all treatments resulted in an equal level of sedation. However, 2 sheep had a relatively slow recovery after the Med-Ket treatment, and another 2 sheep had a decrease in activity from 90 minutes until the end of the observation period. For the Med-Ket-MK150 treatment, 1 sheep appeared slightly lethargic at 110 minutes after reversal with atipamezole. Immediately after atipamezole administration, some sheep had tremor of the facial muscles and signs of agitation.
Plasma concentrations of dexmedetomidine, levomedetomidine, ketamine, norketamine, and MK-467 were determined (Figure 2). Data for Cmax, Tmax, and AUC0–50 were summarized (Table 4). One sheep had extremely low plasma drug concentrations after administration of the Med-Ket treatment, which suggested poor absorption or an error in drug administration; data for that sheep were excluded from the calculations because it was considered an outlier. No significant differences in the concentrations of dexmedetomidine, levomedetomidine, ketamine, or norketamine were detected among treatments. Mean ± SD atipamezole concentrations at 90 minutes after the initial injection were 33.1 ± 5.1 ng/mL, 31.3 ± 5.0 ng/mL, 36.4 ± 5.4 ng/mL, and 31.7 ± 5.3 ng/mL for Med-Ket, Med-Ket-MK150, Med-Ket-MK300, and Med-Ket-MK600, respectively; there were no significant differences among treatments. The mean AUC0–50 of norketamine did not differ significantly among treatments (17,729 ± 3,265 min·ng/mL, 15,701 ± 2,828 min·ng/mL, 18,198 ± 5,206 min·ng/mL, and 20,122 ± 4,745 min·ng/mL for Med-Ket, Med-Ket-MK150, Med-Ket-MK300, and Med-Ket-MK600, respectively).
Mean ± SE plasma concentration of dexmedetomidine (A), levomedetomidine (B), ketamine (C), norketamine (D), and MK-467 (E) in 9 sedated sheep from 0 to 50 minutes after injection of each of 4 sedation treatments. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 79, 9; 10.2460/ajvr.79.9.921
Mean ± SD values of pharmacokinetic parameters for the same 9 sheep as in Table 1.
| Drug | Treatment | Cmax (ng/mL) | Tmax (min) | AUC0–50 (min·ng/mL) |
|---|---|---|---|---|
| Dexmedetomidine | Med-Ket | 5.7 ± 2.1 | 13.3 ± 9.1 | 196 ± 31 |
| Med-Ket-MK150 | 5.7 ± 2.7 | 13.8 ± 7.3 | 183 ± 25 | |
| Med-Ket-MK300 | 6.2 ± 1.8 | 12.0 ± 5.8 | 183 ± 26 | |
| Med-Ket-MK600 | 5.8 ± 2.0 | 10.9 ± 4.9 | 181 ± 32 | |
| Levomedetomidine | Med-Ket | 5.7 ± 2.0 | 12.0 ± 9.5 | 183 ± 29 |
| Med-Ket-MK150 | 5.5 ± 2.4 | 13.2 ± 6.3 | 175 ± 28 | |
| Med-Ket-MK300 | 5.9 ± 2.1 | 11.4 ± 5.7 | 176 ± 30 | |
| Med-Ket-MK600 | 5.7 ± 1.7 | 9.8 ± 3.5 | 175 ± 35 | |
| Ketamine | Med-Ket | 445 ± 176 | 19.7 ± 17.1 | 19,612 ± 2,676 |
| Med-Ket-MK150 | 408 ± 244 | 25.8 ± 27.9 | 19,201 ± 3,745 | |
| Med-Ket-MK300 | 469 ± 217 | 22.0 ± 27.8 | 21,489 ± 5,672 | |
| Med-Ket-MK600 | 496 ± 157 | 18.7 ± 27.0 | 23,160 ± 5,060 | |
| MK-467 | Med-Ket | NA | NA | NA |
| Med-Ket-MK150 | 93.2 ± 20 | 20.1 ± 8.6 | 6,799 ± 1,669 | |
| Med-Ket-MK300 | 209 ± 58 | 28.7 ± 11.9 | 15,579 ± 4,497 | |
| Med-Ket-MK600 | 362 ± 99 | 25.6 ± 12.6 | 27,479 ± 7,912 |
NA = Not applicable.
See Table 1 for remainder of key.
Discussion
Concomitant IM administration (mixed in the same syringe) of the peripherally acting α2-adrenoceptor antagonist MK-467 attenuated some of the adverse cardiopulmonary effects induced by sedation with the Med-Ket treatment in sheep. Concomitant administration of MK-467 did not completely prevent early vasoconstriction-related cardiopulmonary effects of medetomidine, but MK-467 accelerated (in a dose-dependent manner) the return of certain physiologic variables to baseline values.
In general, the cardiopulmonary findings of the present study were in accordance with results of previous studies8,10 in which investigators administered MK-467 and dexmedetomidine IV to sheep, although the onset of the effect of MK-467 was slower after IM administration in the present study than after IV administration in those earlier studies. However, ketamine might also have affected the results of the present study. Although the dose was clinically relevant for sheep,24 it was quite small when compared with the dose used in earlier experiments.35 In 1 study,8 MK-467 attenuated the reduction in HR and change in MAP when it was administered before medetomidine to sheep. In another study,10 MK-467 (250 μg/kg) was mixed with dexmedetomidine (5 μg/kg) and administered as a rapid IV bolus to sheep, and it prevented all dexmedetomidine-induced changes in hemodynamic variables.
Cardiovascular effects of medetomidine in dogs are characterized as an initial increase in blood pressures accompanied by reflex bradycardia and a reduction in CO,36,37 and corresponding effects in sheep have been reported.35,38,39 Similar initial effects were also evident in the present study, regardless of the administration of MK-467. Peripheral vasoconstriction was detected as an increase in systemic blood pressures and SVR. Consequently, HR, left ventricle workload, and CO decreased. Similar findings have been reported for conscious dogs after IM administration of medetomidine and MK-467 mixed in the same syringe.14,15 Nevertheless, the observed changes in CO for sheep of the present study were less obvious and of shorter duration than those reported for dogs.14,15 The delayed onset of the antagonistic effect of MK-467 could be explained by a more rapid absorption rate of medetomidine, as indicated by the earlier Tmax, compared with the Tmax for MK-467. Accordingly, the ratio for the plasma concentration of MK-467 to dexmedetomidine in the early drug absorption phase was probably not sufficient to prevent the effects of medetomidine. For example, in conscious dogs that received IM administration of medetomidine (20 μg/kg) and MK-467 (200 μg/kg), the ratio of MK-467 to dexmedetomidine in plasma was 40:1 at 5 minutes, which was not enough to reverse the peripheral effects of medetomidine.15 In the study reported here, the ratio of MK-467 to dexmedetomidine in plasma was approximately 35:1 at 6 minutes after injection of the Med-Ket-MK300 treatment. Furthermore, in isoflurane-anesthetized dogs receiving stepwise-increasing IV infusions of medetomidine and MK-467, a 50:1 ratio of MK-467 to dexmedetomidine in plasma attenuated medetomidine-induced effects on HR and MAP and returned them to baseline values.40 In the study reported here, a ratio of 50:1 was achieved at approximately 10, 20, and 40 minutes after injection of Med-Ket-MK600, Med-Ket-MK300, and Med-Ket-MK150, respectively. Therefore, it could be concluded that as the ratio of MK-467 to dexmedetomidine increased in plasma, MK-467 was better able to antagonize medetomidine-induced cardiovascular effects. However, in another study40 higher ratios for MK-467 to dexmedetomidine (ie, 125:1) in isoflurane-anesthetized dogs resulted in a decrease of MAP to < 60 mm Hg. In the present study, even for the highest dose of MK-467 (600 μg/kg) when the ratio of MK-467 to dexmedetomidine was 165:1 at 30 minutes after injection, MAP remained > 60 mm Hg and SVR did not differ significantly from the baseline value. This could have been attributable to a species difference from dogs or to effects of anesthesia with isoflurane in dogs40 because isoflurane alters the vasopressor effects of medetomidine in dogs by eliminating sympathetic tone.41 Furthermore, ketamine might have played a role in preventing hypotension because ketamine increases sympathetic tone, systemic42,43 and pulmonary blood pressures, HR, and cardiac index, although there is no effect on stroke index.43 In dogs, administration of ketamine (10 mg/kg, IV) after administration of xylazine (1 mg/kg, IV) returned HR to baseline values after a transient significant increase; it also significantly increased CO, which nevertheless remained below baseline values. Furthermore, left ventricle workload was increased, whereas SVR and left ventricle stroke index were significantly decreased after ketamine was administered, but SVR remained significantly above baseline values.44 In addition, dogs that received medetomidine (30 μg/kg, IM) combined with ketamine (3 mg/kg, IM) mixed in the same syringe had a higher HR, compared with the HR after IM administration of medetomidine alone.45 Similar findings were also reported for cats receiving ketamine (5 mg/kg, IM) and dexmedetomidine (10 μg/kg, IM).46 Thus, we believe that ketamine might have altered the hemodynamic effects of medetomidine and MK-467 in the sheep of the present study.
No significant differences in cardiovascular variables were observed among treatments after reversal of sedation with atipamezole. However, SAP, DAP, and MAP were slightly but significantly increased from baseline values in sheep when they received MK-467 treatments. Similar effects on MAP were reported for sheep receiving atipamezole after administration of medetomidine alone9 or after administration of medetomidine combined with ketamine.38,39
In sheep, arterial hypoxemia is a common adverse effect induced by α2-adrenoceptor agonists of variable selectivity for α2- and α1-adrenoceptors,3 and the magnitude of arterial hypoxemia is dose dependent.9 Activation of peripheral α2-adrenoceptors is the main cause of hypoxemia in sheep.4 The peripheral α2-adrenoceptor agonist ST-91 (30 μg/kg, IV) induced histopathologic changes in the lungs of sheep similar to those caused by xylazine (150 μg/kg, IV).47 The authors of that study47 speculated that the pulmonary edema was attributable to altered permeability (permeability edema), whereby the activation of pulmonary intravascular macrophages resulted in production of some vasoactive mediators that can induce changes in permeability. However, no signs of early inflammation were observed in sheep after administration of dexmedetomidine (2 μg/kg, IV), despite the similarity of the histopathologic changes that were associated with an increase in pulmonary artery pressure and pulmonary arterial occlusion pressure.6 This suggests that the increased hydrostatic stress as a result of the stimulation of α2-adrenoreceptors is the underlying cause of pulmonary edema formation in sheep (hemodynamic edema) and that the permeability changes were a result of the stimulation rather than a cause.6 Consequently, when these peripheral pressor effects are antagonized by MK-467, it may prevent the formation of pulmonary edema and development of hypoxemia. In the present study, Pao2 was decreased from baseline values for all treatments. Nevertheless, MK-467 significantly increased Pao2, compared with effects for Med-Ket alone, which was reflected by maintaining Sao2 > 90% throughout the sedation phase. Although Paco2 was significantly increased and pH decreased for all treatments, both Paco2 and pH remained within their respective reference ranges.48,49 After atipamezole was administered, blood gas variables returned to baseline values, except for base excess and the bicarbonate concentration, which remained above baseline values. In the present study, pressure hypoxemia attributable to bloat was unlikely because we did not notice obvious ruminal distension. However, we did not monitor ruminal motility. On the other hand, relevant changes in cardiopulmonary hemodynamics have not been observed in nonanesthetized sheep after simple ruminal distension by insufflation of compressed air.50
In the study reported here, hemoglobin concentration was significantly decreased from the baseline value for all treatments, but it was significantly higher during the sedation phase for the Med-Ket treatment than for the MK-467 treatments. Similar results were reported for dogs receiving IV and IM administrations of medetomidine combined with MK-467.13 We speculated that MK-467 antagonized the peripheral effects of medetomidine on the splenic capsule because medetomidine alone reportedly causes an increase in hemoglobin concentrations from baseline values in dogs.13 In that same report,13 the cardiac index and Cao2 were significantly decreased from baseline values, with and without MK-467. This is comparable to results of the present study and findings for dogs receiving MK-467 with medetomidine combined with the opioid butorphanol.14 However, the reduction in Do2 and cardiac index may have had only a minor effect, if any, on oxygen supply because the oxygen extraction ratio and mixed-venous partial pressure of oxygen remained at baseline values in dogs after MK-467 treatments.13 Mixed-venous partial pressure of oxygen was not measured in the present study; therefore, we were unable to precisely evaluate the impact of reduced Do2 and CO on oxygen supply because the oxygen extraction ratio was not calculated. However, Do2 apparently was not compromised or did not reach a critical level in the present study because there was no indication of metabolic acidosis. The slight decrease in pH was probably attributable to an increase in Paco2. Moreover, the plasma lactate concentration was not increased by MK-467, which suggested that there was adequate tissue perfusion.
In dogs, MK-467 increases the absorption rate of medetomidine after concurrent IM administration in the same syringe.15 The authors of that study15 suggested that MK-467 antagonized the local vasoconstrictor effect of medetomidine, which increased perfusion and thus drug absorption. Nevertheless, MK-467 did not significantly affect the plasma concentration profiles of dexmedetomidine or levomedetomidine in the present study. We attributed this difference to ketamine, which may have direct vasodilatory effects on the peripheral vasculature.43 Furthermore, the low pH of the ketamine solution may have affected drug absorption. No reports are available concerning effects of ketamine on the absorption rate of α2-adrenoceptor agonists or other drugs after IM administration, but the absorption of MK-467 was more rapid for the sheep of the present study than for dogs receiving IM administration of medetomidine and MK-467,15 which suggested that ketamine may also enhance the absorption of MK-467.
In the present study, rectal temperature significantly decreased with all treatments. Even after reversal of sedation with atipamezole, rectal temperature remained below the baseline value. However, hypothermia was not detected. Therefore, we assume that the changes in rectal temperature did not markedly affect the disposition of the drugs.
The hyperglycemic effect associated with α2-adrenoceptor agonists is a documented phenomenon for various species.51–53 It appears to be mainly attributable to the reduction in insulin release from beta cells of the pancreas as a result of acting as a peripheral α2A-adrenoceptor subclass agonist.54,55 In the present study, plasma glucose concentrations significantly increased after Med-Ket alone, and the addition of MK-467 prevented this increase. Similar findings were reported for dogs that received IV administration of dexmedetomidine combined with MK-467.56 Conversely, reversal with atipamezole failed to return the plasma glucose concentration to the baseline value for the Med-Ket treatment. In another study,51 medetomidine induced 2 phases of marked hyperglycemia in sheep. The glucose concentration initially increased immediately after administration of medetomidine (40 μg/kg, IV) and then stabilized before a secondary increase, which peaked at 240 minutes. Injection of atipamezole (200 μg/kg, IV) 60 minutes after administration of medetomidine did not reverse the first phase but prevented the second peak.51 Our results suggested that MK-467 was able to also prevent the initial glucose surge. Ketamine probably did not markedly influence the plasma glucose concentration. In dogs, administration of ketamine alone increased glucose concentrations only slightly,57 and no effect was detected in dogs treated with a combination of ketamine and xylazine.58
No significant differences in sedation scores were detected among treatments. In contrast to other ruminants, sheep have no or only extremely minor receptor hysteresis, and the depth of drug-evoked sedation is linearly associated with medetomidine concentrations in plasma.5,23 This is consistent with the findings for the present study that the level of sedation and plasma concentrations of dexmedetomidine and ketamine did not differ significantly among the treatments. Furthermore, 1 sheep had a low sedation score after administration of the Med-Ket treatment, which was associated with low dexmedetomidine concentrations in plasma. This can be attributed to inadvertent injection between muscles or into fatty tissue because this animal was relatively obese (body weight, 72 kg). Adipose tissue is relatively poorly perfused, which impairs drug absorption. In the present study, sheep were placed in a sling that kept them in a standing position with a limited ability to move, which may have impaired accurate evaluation of sedation.
Administration of atipamezole resulted in a smooth recovery from sedation, with no differences among treatments. Tremor of the facial muscles and signs of agitation observed in the present study were also reported in sheep administered atipamezole to reverse sedation after receiving medetomidine alone9,23 or medetomidine in combination with ketamine.38 This may have been caused by the relatively high dose of atipamezole administered at 60 minutes after the treatments, when the dexmedetomidine concentration in plasma was already quite low. Signs of agitation were not detected in lambs receiving atipamezole IV for reversal of sedation 15 minutes after IV administration of medetomidine (30 μg/kg).22 Furthermore, the relatively high concentrations of ketamine and norketamine when atipamezole was administered might have caused some rigidity of the muscles. Norketamine is an active metabolite of ketamine that retains some properties of the parent drug; in rats, it was found that norketamine is one-fifth to one-third as potent for anesthesia, compared with the potency for ketamine.43 In addition, norketamine has some analgesic effects.59 After receiving the highest dose of MK-467 (600 μg/kg), 2 sheep passed abnormally soft feces. An increased frequency of defecation with watery feces was also reported for horses that received MK-467 alone (200 μg/kg, IV).18
In the present study, CO was measured with the lithium dilution method. Medetomidine and ketamine can react with the lithium sensor and increase its voltage, which results in negative in vitro biases in lithium detection (≥ 10%).60 However, the magnitude of this effect is concentration dependent. In the study reported here, concentrations of the drugs in plasma were much lower than those that reportedly cause appreciable bias; only the ketamine concentration was > 300 ng/mL, which can induce a bias of 1.9%.60 Thus, we assumed that these drugs had only minimal effects on the accuracy of CO measurements. Finally, we also believe that exclusion of data sets for 2 sheep did not affect the results. Indeed, the statistical analyses were performed twice (ie, with and without the excluded data), and no relevant differences were observed in the results. Moreover, power calculations suggested that data for 8 animals would have been sufficient to detect differences.
In the present study, cardiopulmonary adverse effects induced by IM administration of Med-Ket were attenuated in a dose-dependent manner by concurrent administration of MK-467 (mixed in the same syringe), without significant effects on the level of sedation. Moreover, administration of atipamezole resulted in a smooth recovery from sedation without deleterious effects, regardless of administration of MK-467. The lowest dose of MK-467 (150 μg/kg) did not completely reverse the adverse effects of Med-Ket. On the other hand, the highest dose of MK-467 (600 μg/kg) provided no additional benefits. Thus, the middle dose of MK-467 (300 μg/kg) would appear to be appropriate for concurrent IM administration with medetomidine (30 μg/kg) and ketamine (1 mg/kg) in sheep.
Acknowledgments
Supported by Vetcare Ltd.
Presented in part in abstract form at the Autumn Meeting of the Association of Veterinary Anaesthetists, Prague, September 2016.
The authors thank Lauri Vuorilehto for assistance with analysis of plasma samples.
ABBREVIATIONS
| AUC0–50 | Area under the plasma concentration–time curve from time 0 to 50 minutes |
| Cao2 | Arterial oxygen content |
| Cmax | Maximum plasma drug concentration |
| CO | Cardiac output |
| CVP | Central venous pressure |
| DAP | Diastolic arterial blood pressure |
| Do2 | Oxygen delivery |
| HR | Heart rate |
| MAP | Mean arterial blood pressure |
| Sao2 | Arterial hemoglobin oxygen saturation |
| SAP | Systolic arterial blood pressure |
| SVR | Systemic vascular resistance |
| Tmax | Time to achieve maximum plasma concentration |
Footnotes
Orion Corp, Espoo, Finland.
Cavafix Certo, B. Braun, Melsungen, Germany.
Terumo Europe, Leuven, Belgium.
Becton Dickinson, Sandy, Utah.
Gabarith PMSET, Becton Dickinson, Sandy, Utah.
S/5 compact critical care monitor, Datex-Ohmeda, GE Healthcare, Helsinki, Finland.
Dorbene, 1 mg/mL, Syva Laboratories SA, León, Spain.
Ketador vet, 100 mg/mL, Richter Pharma AG, Wels, Austria.
Provided by Vetcare Ltd, Mäntsälä, Finland.
Alzane, 5 mg/mL, Syva Laboratories SA, León, Spain.
LidCO Plus hemodynamic monitor, LidCO Ltd, Cambridge, England.
ADVIA 2120i hematology system with autoslide, Siemens Healthcare GmbH, Erlangen, Germany.
Konelab PRIME 60i, Thermo Scientific, Wilmington, Del.
Pico50, Radiometer, Copenhagen, Denmark.
epoc, Epocal Inc, Ottawa, ON, Canada.
Metacam, 20 mg/mL, Vetmedica, Boeringer Ingelheim, Ingelheim am Rhein, Germany.
Reference standard: racemic medetomidine, TRC, Toronto, ON, Canada.
Sep-Pak tC18 96-well extraction plates, Waters Corp, Milford, Mass.
Chiralpak AGP column, 4 × 150 mm, 5 μm, Chiral Technologies Europe, Illkirch, France.
Triple-quadrupole mass spectrometer, 4000QTrap, MDS Sciex, Concord, ON, Canada.
Analyst software, version 1.6.1, Applied Biostystems Inc/MDS, Concord, ON, Canada.
Sirocco plate, Waters Corp, Milford, Mass.
Waters Acquity UPLC, Waters Corp, Milford, Mass.
Waters TQ-S triple-quadrupole MS, Waters Corp, Milford, Mass.
Waters Acquity BEH C18, 2.1 × 50 mm, 1.7 μm, Waters Corp, Milford, Mass.
SAS for Windows, version 9.3, SAS Institute Inc, Cary, NC.
SPSS statistics for Windows, version 24.0, IBM Corp, Armonk, NY.
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