Medetomidine, an α2-adrenoceptor agonist, is commonly used as a premedication because of its consistent sedative and analgesic properties. However, similar to other α2-adrenoceptor agonists, medetomidine is associated with marked adverse effects of the cardiovascular system, which limit its administration to healthy animals only.1 Medetomidine causes biphasic changes in ABP.2 Activation of peripheral α2-adrenoceptors in vascular smooth muscle leads to vasoconstriction and increases SVR, which causes arterial hypertension and reflex bradycardia.3,4 Activation of α2-adrenoceptors in the brain causes sedation and analgesia as well as a decrease in sympathetic outflow in conjunction with augmentation of parasympathetic tone that results in centrally mediated bradycardia followed by a decrease in ABP.5,6
In small animal practice, anesthesia is commonly maintained with an inhalant anesthetic such as isoflurane or sevoflurane. Isoflurane affects hemo-dynamic homeostasis by decreasing SVR and depressing myocardial function.7,8 Thus, hypotension is a common problem associated with administration of inhalant anesthetics. Medetomidine decreases vasodilation and the subsequent reduction in ABP in isoflurane-anesthetized dogs; however, substantial increases in SVR after medetomidine administration can lead to marked decreases in HR and CO.9 In dogs anesthetized with isoflurane, anticholinergic agents, such as glycopyrrolate, are used to prevent or treat bradycardia induced by α2-adrenoceptor agonists and are associated with an increase in HR, CO, and ABP.3,10,11 However, transient arrhythmias such as second-degree atrioventricular block, VPCs, pulsus alternans, and sinus tachycardia have been reported in dogs concomitantly administered anticholinergics and α2-adrenoceptor agonists.12–15
MK-467 is a peripherally acting α2-adrenoceptor antagonist that does not cross the blood-brain barrier.16 In dogs, MK-467 prevents peripheral cardiovascular effects induced by dexmedetomidine or medetomidine.17–23 In a study21 in which MK-467 and medetomidine were concurrently administered to dogs prior to isoflurane anesthesia, the SVRI and, subsequently, ABP decreased during anesthesia. This raised concern about the ideal dose of MK-467 for dogs. Thus, we wanted to determine whether a lower dose of MK-467 than that used in the previous study21 would improve cardiovascular function in dogs anesthetized with isoflurane after premedication with medetomidine.
The purpose of the study reported here was to compare cardiovascular effects of premedication with medetomidine alone or in combination with 1 of 3 doses of MK-467 or after glycopyrrolate in dogs in which anesthesia was induced with ketamine and midazolam and maintained with isoflurane. Our hypotheses were that MK-467 attenuates medetomidine-induced cardiovascular changes in a dose-dependent manner before and during isoflurane anesthesia and that glycopyrrolate does not improve cardiovascular function.
Materials and Methods
Animals
All study procedures were approved by the National Animal Experiment Board of Finland (ESAVI-2010-07734/Ym-23). Eight healthy 5-year-old purpose-bred Beagles (6 neutered males and 2 neutered females) with a mean ± SD body mass of 13.1 ± 1.7 kg were used in the study. Dogs were considered healthy on the basis of results of a thorough physical examination, CBC, and serum biochemical analysis.
Study design
The study had a randomized crossover design. All dogs received each of 5 premedication protocols (medetomidinea [10 μg/kg, IV] alone [MED] and combined with MK-467b at each of 3 doses [50 {MKK50}, 100 {MMK100}, and 150 {MMK150} μg/kg, IV] and 15 minutes after glycopyrrolatec [10 μg/kg, SC; MGP]) before being anesthetized. There were at least 14 days between treatments. Each dog received the treatments in a random order, which was determined by the blind selection of slips of paper, each of which had a treatment name written on it, from an envelope.
Instrumentation
For each dog, food but not water was withheld for 12 hours before each anesthetic session. Each dog was anesthetized to facilitate instrumentation. The anesthesia was induced with propofold (maximum dose, 6 mg/kg, IV); the loss of palpebral reflex, rotation of the eye globe, and muscle relaxation were used as signs that sufficient propofol had been administered to allow endotracheal intubation. Following endotracheal intubation, anesthesia was maintained with isoflurane.e Instrumentation consisted of the aseptic placement of a 20-gauge cannulaf in a cepahlic vein, 22-gauge cannulaf in a dorsal pedal artery, and 7F double-lumen central venous catheterg in a jugular vein. The insertion site of the central venous catheter was determined by measurement from the cranial border of the second rib at the costochondral junction cranially along the jugular groove, and correct placement of the catheter was confirmed by the observation of typical CVP waveforms on a patient monitor. Acetated Ringer solutionh (10 mL/kg/h, IV) was infused through the cephalic catheter as soon as it was placed and for the duration of the instrumentation period. While anesthetized, dogs were positioned on a mattress fitted with a heating pad and covered with a blanket to maintain body temperature. After completion of instrumentation, dogs were allowed to recover from anesthesia for at least 60 minutes to ensure return of consciousness and normal locomotor activity prior to baseline measurement.
Treatment protocol
Immediately prior to use, MK-467 powder was dissolved in saline (0.9% NaCl) solution to achieve a concentration of 2 mg/mL. For the medetomidine and MK-467 treatments, the 2 drugs were mixed together in a syringe before administration. Twenty minutes before medetomidine administration, dogs were connected to patient monitors and baseline measurements were recorded (T–20). Intravenous administration of acetated Ringer solutionh (10 mg/kg/h) was resumed through the catheter in the cephalic vein (cephalic catheter), which was continued for the duration of the anesthetic session (experiment). In all treatments, the IV premedication was administered over 30 seconds through a 3-way stopcock and the cephalic catheter, which was flushed with saline solution (0.5 mL/kg) immediately thereafter. For the MGP treatment, glycopyrrolate was administered SC 15 minutes before the medetomidine (T–15). Medetomidine administration was designated as T0 for all treatments.
Twenty minutes after T0 (T20), anesthesia was induced with increments of ketaminei (0.5 mg/kg, IV) and midazolamj (0.1 mg/kg, IV). Ketamine was injected over 15 seconds, and then midazolam was also injected over 15 seconds. Then, there was a 30-second break. If the dog could not be intubated, additional doses of ketamine and midazolam were administered as previously described until the dog was successfully intubated, and the total amounts of ketamine and midazolam administered were recorded. Following intubation, anesthesia was maintained with isofluranee administered in oxygen (flow rate, 1 L/min) with a circle systemk to achieve an ETISO of 1.2%. Dogs were allowed to breath spontaneously throughout each experiment.
Beginning at T–20 and for the duration of each experiment, dogs were connected to a multichannel monitorl that recorded the continuous lead II ECG, SAP, diastolic arterial pressure, MAP, and CVP. In addition, ETISO and the inspired fraction of oxygen were recorded after the induction of anesthesia. The gas analyzer was calibrated before study initiation with calibration gasm supplied by manufacturer. The pressure transducersn were reset to atmospheric pressure prior to each experiment for each dog; the dog was positioned in lateral recumbency, and the level of the manubrium was used as zero reference. Cardiac output was measured with the lithium indicator dilution methodo as described24 by use of standard doses of lithium chloride (0.075 mmol). Standard values of 10 g of hemoglobin/L and 140 mmol of sodium/L were used and later corrected with actual values measured from simultaneously drawn arterial blood samples, which were anaerobically collected from the catheter in the dorsal pedal artery into preheparinized syringesp and stored in ice water for no longer than 15 minutes before being analyzed with a commercial blood gas analyzer.q Measurements recorded included Pao2, Paco2, Hba, and arterial lactate and sodium concentrations. Hemoglobin oxygen saturation was calculated as described.25 Subsequently, CI, stroke volume index, SVRI, Cao2, and Do2I were calculated by use of standard equations.26 Respiratory rate was measured by counting chest movements for 1 minute before anesthetic induction and by means of capnography during anesthesia. Rectal temperature was monitored throughout each experiment to maintain normothermia.
Cardiopulmonary variables and rectal temperature were recorded at baseline (T–20) and at 5 (T5), 15 (T15), 35 (T35), 45 (T45), and 60 (T60) minutes after medetomidine administration. Venous blood samples were collected from the central venous catheter into tubes containing potassium EDTA as an anticoagulant at T5, T15, T35, T45, T60, and T90. Those samples were centrifuged at 3,000 × g for 15 minutes. The plasma from each sample was decanted, divided into 2 tubes, and then frozen at −20°C until analyzed for dexmedetomidine, levomedetomidine, and MK-467 concentrations.
Isoflurane administration was ceased at 70 minutes after premedication (50 minutes after induction), and oxygen flow was increased to 5 L/min until the dog was extubated. Each dog received meloxicamr (0.2 mg/kg, SC) after each experiment.
Analysis of plasma dexmedetomidine, levomedetomidine, and MK-467 concentrations
Plasma concentrations of dexmedetomidine, levomedetomidine, and MK-467 were determined by use of high-performance liquid chromatography–tandem mass spectrometry as described.23 The analytic methods were validated for range, precision, accuracy, carryover, interference of analytes, internal standards, and analyte stability. A validation for matrix effects was also performed to comply with regulatory guidance.27 The interassay accuracy of quality control samples at concentrations of 0.03, 0.45, and 4.0 ng/mL ranged from 91% to 96% for dexmedetomidine and 93% to 98% for levomedetomidine. The intra-assay accuracy of back-calculated calibration standards for MK-467 ranged from 90% to 109% and that of quality control samples at concentrations of 3, 45, and 450 ng/mL ranged from 98% to 112%. The AUClast of dexmedetomidine, levomedetomidine, and MK-467 were calculated by use of noncompartmental analysis with commercial software.s
Statistical analysis
All analyses involving cardiopulmonary data were performed with a commercially available statistical software program.t Descriptive statistics were used to summarize cardiovascular data within each treatment at the predetermined data collection times. For each variable evaluated, the change from baseline was used as the response for statistical modeling. Differences among treatments were evaluated by use of repeated-measures ANCOVA. Each model included a baseline covariate; fixed effects for treatment, data collection time (time), period (experiments 1 through 5), the interaction between treatment and time, and the interaction between period and time; and random effects for dog, the interaction between dog and time, and the interaction between dog and period. A carryover effect was not assessed because it was not expected to arise in the study. The fitted models were used to estimate treatment effects during the treatment period as well as at each time. For each variable, the mean and accompanying 95% confidence interval were calculated for each treatment overall and at each time as well as the mean difference and accompanying P values for all pairwise comparisons between treatments. Because of the exploratory nature of the comparisons conducted, adjustments for multiplicity were not used.
The plasma dexmedetomidine, levomedetomidine, and MK-467 concentration data were analyzed with a different statistical software programu than that used for the cardiovascular data. The Shapiro-Wilk test was used to assess the data distributions for normality. An ANOVA was used to compare the AUClast for dexmedetomidine and levomedetomidine among treatments, followed by pairwise comparisons between treatments, which were performed with a paired t test and Bonferroni correction. Results were reported as the mean ± SD for all variables. Values of P < 0.05 were considered significant for all analyses.
Results
Results over time for SVRI, MAP, HR (Figure 1), and other cardiovascular variables (Table 1) were summarized. In general, the mean HR was lowest for the MED treatment and greatest for the MGP treatment throughout the observation period. Meanwhile, the mean MAP and SVRI were highest for the MGP treatment and lowest for the MMK150 treatment. The mean CI was highest for the MMK150 treatment during the premedication period (at T5 and T15), whereas it was consistently lowest for the MGP treatment during the anesthetic period (T35 through T60). Arrhythmias, such as sinoatrial and second-degree atrioventricular blocks, were detected during the premedication period for all treatments. Occasional single VPCs were detected during the premedication period for all treatments except the MMK150 treatment, and arterial pulse deficits were occasionally detected after VPCs when dogs received the MGP treatment. Mean rectal temperature decreased slightly over time, but was always > 36.9°C and did not differ significantly among the treatments at any time.
Mean ± SD SVRI (A), MAP (B), and HR (C) at predetermined times for 8 healthy 5-year-old Beagles that received each of 5 premedication protocols (medetomidine [10 μg/kg, IV] alone [MED; solid line with black circles] and combined with MK-467 at each of 3 doses [50 {MKK50; dashed line with white squares}, 100 {MMK100; dashed line with white triangles}, and 150 {MMK150; dashed line with white circles} μg/kg, IV] and 15 minutes after glycopyrrolate [10 μg/kg, SC; MGP; solid line with black squares]) prior to anesthesia. There were at least 14 days between each experiment. Baseline (BL) was recorded before any treatments 20 minutes before medetomidine administration (T0) for all protocols. Data were acquired at baseline and at 5, 15, 35, 45, and 60 minutes after medetomidine administration. Twenty minutes after medetomidine administration, anesthesia was induced with increments of ketamine (0.5 mg/kg, IV) and midazolam (0.1 mg/kg, IV) and maintained with isoflurane (ETISO, 1.2%) in oxygen. Isoflurane administration was discontinued 70 minutes after medetomidine administration, and dogs were allowed to recover. *Significant (P < 0.05) difference between MED treatment and MMK50, MMK100, or MMK150 treatment. †Significant (P < 0.05) difference between MGP treatment and MMK50, MMK100, or MMK150 treatment. ‡Significant (P < 0.05) difference between MMK50 treatment and MMK100 or MMK150 treatment. §Significant (P < 0.05) difference between MMK100 and MMK150 treatments. ‖Significant (P < 0.05) difference between MED and MGP treatments.
Citation: American Journal of Veterinary Research 78, 11; 10.2460/ajvr.78.11.1245
Mean ± SD SVRI (A), MAP (B), and HR (C) at predetermined times for 8 healthy 5-year-old Beagles that received each of 5 premedication protocols (medetomidine [10 μg/kg, IV] alone [MED; solid line with black circles] and combined with MK-467 at each of 3 doses [50 {MKK50; dashed line with white squares}, 100 {MMK100; dashed line with white triangles}, and 150 {MMK150; dashed line with white circles} μg/kg, IV] and 15 minutes after glycopyrrolate [10 μg/kg, SC; MGP; solid line with black squares]) prior to anesthesia. There were at least 14 days between each experiment. Baseline (BL) was recorded before any treatments 20 minutes before medetomidine administration (T0) for all protocols. Data were acquired at baseline and at 5, 15, 35, 45, and 60 minutes after medetomidine administration. Twenty minutes after medetomidine administration, anesthesia was induced with increments of ketamine (0.5 mg/kg, IV) and midazolam (0.1 mg/kg, IV) and maintained with isoflurane (ETISO, 1.2%) in oxygen. Isoflurane administration was discontinued 70 minutes after medetomidine administration, and dogs were allowed to recover. *Significant (P < 0.05) difference between MED treatment and MMK50, MMK100, or MMK150 treatment. †Significant (P < 0.05) difference between MGP treatment and MMK50, MMK100, or MMK150 treatment. ‡Significant (P < 0.05) difference between MMK50 treatment and MMK100 or MMK150 treatment. §Significant (P < 0.05) difference between MMK100 and MMK150 treatments. ‖Significant (P < 0.05) difference between MED and MGP treatments.
Citation: American Journal of Veterinary Research 78, 11; 10.2460/ajvr.78.11.1245
Mean ± SD SVRI (A), MAP (B), and HR (C) at predetermined times for 8 healthy 5-year-old Beagles that received each of 5 premedication protocols (medetomidine [10 μg/kg, IV] alone [MED; solid line with black circles] and combined with MK-467 at each of 3 doses [50 {MKK50; dashed line with white squares}, 100 {MMK100; dashed line with white triangles}, and 150 {MMK150; dashed line with white circles} μg/kg, IV] and 15 minutes after glycopyrrolate [10 μg/kg, SC; MGP; solid line with black squares]) prior to anesthesia. There were at least 14 days between each experiment. Baseline (BL) was recorded before any treatments 20 minutes before medetomidine administration (T0) for all protocols. Data were acquired at baseline and at 5, 15, 35, 45, and 60 minutes after medetomidine administration. Twenty minutes after medetomidine administration, anesthesia was induced with increments of ketamine (0.5 mg/kg, IV) and midazolam (0.1 mg/kg, IV) and maintained with isoflurane (ETISO, 1.2%) in oxygen. Isoflurane administration was discontinued 70 minutes after medetomidine administration, and dogs were allowed to recover. *Significant (P < 0.05) difference between MED treatment and MMK50, MMK100, or MMK150 treatment. †Significant (P < 0.05) difference between MGP treatment and MMK50, MMK100, or MMK150 treatment. ‡Significant (P < 0.05) difference between MMK50 treatment and MMK100 or MMK150 treatment. §Significant (P < 0.05) difference between MMK100 and MMK150 treatments. ‖Significant (P < 0.05) difference between MED and MGP treatments.
Citation: American Journal of Veterinary Research 78, 11; 10.2460/ajvr.78.11.1245
Mean ± SD values for cardiovascular variables at predetermined times for 8 healthy 5-year-old Beagles that received each of 5 premedication protocols (medetomidine [10 μg/kg, IV] alone [MED] and combined with MK-467 at each of 3 doses [50 {MKK50}, 100 {MMK100}, and 150 {MMK150} μg/kg, IV] and 15 minutes after glycopyrrolate [10 μg/kg, SC; MGP]).
| Measurement acquisition time | |||||||
|---|---|---|---|---|---|---|---|
| Variable | Treatment | BL | T5 | T15 | T35 | T45 | T60 |
| SAP (mm Hg) | MED | 183 ± 20 | 203 ± 15 | 173 ± 11 | 151 ± 10 | 138 ± 14 | 128 ± 13 |
| MMK50 | 195 ± 19 | 194 ± 18† | 168 ± 11† | 134 ± 15*† | 117 ± 18*† | 115 ± 17*† | |
| MMK100 | 184 ± 19 | 188 ± 12*† | 166 ± 15† | 126 ± 14*† | 114 ± 14*† | 112 ± 14*† | |
| MMK150 | 188 ± 10 | 177 ± 15*†‡ | 157 ± 17† | 120 ± 13*†‡ | 108 ± 17*† | 104 ± 16*† | |
| MGP | 191 ± 17 | 262 ± 23* | 241 ± 29* | 158 ± 22 | 135 ± 25 | 130 ± 26 | |
| DAP (mm Hg) | MED | 87 ± 12 | 122 ± 8 | 101 ± 10 | 94 ± 10 | 77 ± 12 | 67 ± 13 |
| MMK50 | 91 ± 14 | 103 ± 9*† | 89 ± 11*† | 77 ± 18*† | 63 ± 17*† | 62 ± 15 | |
| MMK100 | 82 ± 7 | 98 ± 7*† | 80 ± 10*† | 69 ± 12*† | 60 ± 9*† | 57 ± 10† | |
| MMK150 | 88 ± 14 | 87 ± 9*†‡§ | 77 ± 9*† | 68 ± 11*† | 60 ± 13*† | 56 ± 12† | |
| MGP | 88 ± 13 | 170 ± 27* | 166 ± 29* | 100 ± 16 | 82 ± 16 | 74 ± 20 | |
| CVP (mm Hg) | MED | 5.6 ± 2.2 | 14.8 ± 2.6 | 12.0 ± 2.5 | 8.0 ± 1.7 | 6.3 ± 1.6 | 5.9 ± 1.6 |
| MMK50 | 6.0 ± 1.7 | 12.6 ± 1.1*† | 10.4 ± 1.3*† | 6.4 ± 1.4* | 5.1 ± 0.8* | 4.9 ± 0.6 | |
| MMK100 | 6.3 ± 0.7 | 11.5 ± 1.9*†‡ | 9.0 ± 0.8*†‡ | 5.4 ± 0.5*† | 4.5 ± 0.9*† | 4.6 ± 0.9* | |
| MMK150 | 4.8 ± 2.4 | 8.6 ± 2.4*†‡§ | 7.1 ± 2.6*†‡ | 4.6 ± 1.2*† | 4.1 ± 0.6*† | 3.9 ± 0.6* | |
| MGP | 6.0 ± 1.5 | 13.8 ± 1.2 | 11.4 ± 1.3 | 7.3 ± 0.7 | 5.6 ± 0.9 | 5.3 ± 1.3 | |
| CI (L/min/m2) | MED | 4.6 ± 0.9 | 1.4 ± 0.3 | 1.7 ± 0.4 | 4.2 ± 0.8 | 4.0 ± 0.6 | 4.1 ± 0.9 |
| MMK50 | 4.1 ± 1.6 | 1.5 ± 0.3 | 1.9 ± 0.2 | 4.2 ± 0.8† | 4.0 ± 0.5† | 4.1 ± 1.0† | |
| MMK100 | 4.4 ± 0.8 | 1.9 ± 0.4 | 2.4 ± 0.4*‡ | 4.8 ± 1.1*†‡ | 4.4 ± 0.8† | 4.2 ± 1.1† | |
| MMK150 | 4.0 ± 0.8 | 2.2 ± 0.4*† | 2.5 ± 0.5*‡ | 4.0 ± 0.6†§ | 4.3 ± 0.9† | 4.0 ± 1.1† | |
| MGP | 3.6 ± 1.1 | 1.6 ± 0.7 | 2.3 ± 0.9 | 3.8 ± 0.5* | 3.7 ± 0.3* | 3.6 ± 0.4* | |
| Stroke volume index (mL/beat/kg) | MED | 2.1 ± 0.4 | 1.7 ± 0.3 | 2.1 ± 0.3 | 2.0 ± 0.4 | 1.6 ± 0.2 | 1.7 ± 0.3 |
| MMK50 | 2.1 ± 0.7 | 1.7 ± 0.5† | 1.9 ± 0.3† | 1.5 ± 0.3* | 1.6 ± 0.3 | 1.6 ± 0.3 | |
| MMK100 | 2.4 ± 0.5 | 1.8 ± 0.5† | 2.3 ± 0.7†‡ | 1.6 ± 0.3* | 1.4 ± 0.3 | 1.6 ± 0.4 | |
| MMK150 | 2.1 ± 0.4 | 2.0 ± 0.5† | 2.0 ± 0.4† | 1.4 ± 0.3* | 1.4 ± 0.4 | 1.5 ± 0.5 | |
| MGP | 2.0 ± 0.3 | 1.4 ± 0.7* | 1.0 ± 0.4* | 1.4 ± 0.2* | 1.3 ± 0.2 | 1.3 ± 0.3 | |
| Respiratory rate (breaths/min) | MED | 21 ± 5 | 14 ± 6 | 10 ± 4 | 7 ± 2 | 10 ± 3 | 10 ± 5 |
| MMK50 | 19 ± 4 | 11 ± 6* | 10 ± 6 | 9 ± 4 | 9 ± 4 | 10 ± 4 | |
| MMK100 | 19 ± 4 | 11 ± 6* | 10 ± 3 | 7 ± 2 | 9 ± 3 | 11 ± 4 | |
| MMK150 | 19 ± 6 | 12 ± 8 | 10 ± 6 | 8 ± 4 | 8 ± 4 | 11 ± 5 | |
| MGP | 18 ± 3 | 10 ± 6* | 11 ± 7 | 6 ± 2 | 8 ± 3 | 9 ± 3 | |
| Paco2 (mm Hg) | MED | 34 ± 2 | 34 ± 4 | 31 ± 3 | 53 ± 7 | 46 ± 4 | 43 ± 4 |
| MMK50 | 33 ± 2 | 33 ± 3 | 32 ± 3 | 48 ± 6* | 46 ± 5 | 43 ± 4 | |
| MMK100 | 33 ± 1 | 33 ± 2 | 33 ± 3 | 45 ± 5* | 43 ± 3 | 41 ± 3 | |
| MMK150 | 33 ± 3 | 34 ± 4 | 33.4 ± 2 | 44 ± 6*†‡ | 43 ± 5*† | 41 ± 3 | |
| MGP | 34 ± 2 | 33 ± 23 | 32 ± 3 | 48 ± 8* | 46 ± 4 | 43 ± 4 | |
| Pao2 (mm Hg) | MED | 98 ± 6 | 86 ± 13 | 92 ± 9 | 538 ± 16 | 552 ± 31 | 566 ± 25 |
| MMK50 | 102 ± 5 | 80 ± 23 | 96 ± 6 | 551 ± 27 | 550 ± 25 | 546 ± 29* | |
| MMK100 | 100 ± 7 | 88 ± 9 | 91 ± 10 | 537 ± 42 | 551 ± 36 | 565 ± 44‡ | |
| MMK150 | 99 ± 6 | 92 ± 11 | 97 ± 6 | 547 ± 40 | 551 ± 22 | 565 ± 31 | |
| MGP | 101 ± 4 | 85 ± 8 | 94 ± 8 | 544 ± 29 | 553 ± 25 | 564 ± 22 | |
| Arterial lactate (mmol/mL) | MED | 0.6 ± 0.2 | 0.7 ± 0.1 | 0.9 ± 0.2 | 0.8 ± 0.2 | 0.9 ± 0.3 | 1.0 ± 0.4 |
| MMK50 | 0.7 ± 0.2 | 0.7 ± 0.2 | 0.7 ± 0.2 | 0.7 ± 0.1 | 0.7 ± 0.3 | 0.9 ± 0.3 | |
| MMK100 | 0.7 ± 0.5 | 0.7 ± 0.4 | 0.7 ± 0.4 | 0.7 ± 0.2 | 0.7 ± 0.2 | 0.9 ± 0.5 | |
| MMK150 | 0.5 ± 0.1 | 0.5 ± 0.1 | 0.6 ± 0.2 | 0.7 ± 0.3 | 0.7 ± 0.3 | 0.8 ± 0.3 | |
| MGP | 0.6 ± 0.2 | 0.7 ± 0.2 | 0.9 ± 0.3 | 0.7 ± 0.2 | 0.8 ± 0.2 | 1.1 ± 0.3 | |
| Cao2 (mL/dL) | MED | 17 ± 1 | 19 ± 1 | 19 ± 1 | 21 ± 1 | 20 ± 1 | 20 ± 1 |
| MMK50 | 18 ± 1 | 17 ± 4*† | 18 ± 1*† | 20 ± 1*† | 19 ± 1*† | 18 ± 1*† | |
| MMK100 | 17 ± 1 | 18 ± 1*† | 17 ± 1*†‡ | 19 ± 1*† | 18 ± 1*† | 17 ± 1*† | |
| MMK150 | 17 ± 1 | 17 ± 1*† | 17 ± 1*†‡ | 18 ± 1*†‡ | 18 ± 1*† | 17 ± 1*† | |
| MGP | 17 ± 1 | 19 ± 1 | 20 ± 1* | 21 ± 1 | 21 ± 1* | 20 ± 1* | |
| Do2I (mL/min/m2) | MED | 780 ± 170 | 272 ± 61 | 327 ± 60 | 865 ± 146 | 808 ± 97 | 790 ± 156 |
| MMK50 | 723 ± 323 | 264 ± 77 | 353 ± 37 | 838 ± 165 | 755 ± 91 | 756 ± 213 | |
| MMK100 | 735 ± 147 | 329 ± 72 | 420 ± 86 | 898 ± 201† | 787 ± 154 | 739 ± 192 | |
| MMK150 | 666 ± 137 | 383 ± 71 | 416 ± 76 | 730 ± 123§ | 779 ± 158 | 687 ± 179* | |
| MGP | 600 ± 195 | 311 ± 137 | 474 ± 182 | 791 ± 121* | 781 ± 80 | 731 ± 100* | |
Baseline (BL) was recorded before any treatments 20 minutes prior to medetomidine administration (T0) for all protocols. Data were acquired at baseline (BL) and at 5 (T5), 15 (T15), 35 (T35), 45 (T45), and 60 (T60) minutes after medetomidine administration. Twenty minutes after medetomidine administration, anesthesia was induced with increments of ketamine (0.5 mg/kg, IV) and midazolam (0.1 mg/kg, IV) and maintained with isoflurane (ETISO, 1.2%) in oxygen. Isoflurane administration was discontinued 70 minutes after medetomidine administration, and dogs were allowed to recover.
Value differs significantly (P < 0.05) from the corresponding value for the MED treatment.
Value differs significantly (P < 0.05) from the corresponding value for the MGP treatment.
Value differs significantly (P < 0.05) from the corresponding value for the MMK50 treatment.
Value differs significantly (P < 0.05) from the corresponding value for the MMK100 treatment.
The total doses of ketamine and midazolam necessary for anesthetic induction and intubation were 1 and 0.2 mg/kg, respectively, for all dogs during all treatments, except for 3 random instances in which the total dose of ketamine necessary ranged from 1.5 to 2 mg/kg and the total dose of midazolam ranged from 0.2 to 0.4 mg/kg.
Plasma concentrations of dexmedetomidine and MK-467 were plotted over time (Figure 2). The mean ± SD AUClast for dexmedetomidine, levomedetomidine, and MK-467 were summarized (Table 2).
Mean ± SD plasma dexmedetomidine (A) and MK-467 (B) concentrations over time for 8 healthy 5-year-old Beagles that received each of 5 premedication protocols (medetomidine [10 μg/kg, IV] alone [MED; solid line with black circles] and combined with MK-467 at each of 3 doses [50 {MKK50; dashed line with white squares}, 100 {MMK100; dashed line with white triangles}, and 150 {MMK150; dashed line with white circles} μg/kg, IV] and 15 minutes after glycopyrrolate [10 μg/kg, SC; MGP; solid line with black squares]) prior to anesthesia. There were at least 14 days between each experiment. Data were acquired at 5, 15, 35, 45, 60, and 90 minutes after administration of medetomidine and MK-467. Twenty minutes after medetomidine administration, anesthesia was induced with increments of ketamine (0.5 mg/kg, IV) and midazolam (0.1 mg/kg, IV) and maintained with isoflurane (ETISO, 1.2%) in oxygen. Isoflurane administration was discontinued 70 minutes after medetomidine administration, and dogs were allowed to recover. Plasma MK-467 concentrations were determined only for the MMK50, MMK100, and MMK150 treatments.
Citation: American Journal of Veterinary Research 78, 11; 10.2460/ajvr.78.11.1245
Mean ± SD plasma dexmedetomidine (A) and MK-467 (B) concentrations over time for 8 healthy 5-year-old Beagles that received each of 5 premedication protocols (medetomidine [10 μg/kg, IV] alone [MED; solid line with black circles] and combined with MK-467 at each of 3 doses [50 {MKK50; dashed line with white squares}, 100 {MMK100; dashed line with white triangles}, and 150 {MMK150; dashed line with white circles} μg/kg, IV] and 15 minutes after glycopyrrolate [10 μg/kg, SC; MGP; solid line with black squares]) prior to anesthesia. There were at least 14 days between each experiment. Data were acquired at 5, 15, 35, 45, 60, and 90 minutes after administration of medetomidine and MK-467. Twenty minutes after medetomidine administration, anesthesia was induced with increments of ketamine (0.5 mg/kg, IV) and midazolam (0.1 mg/kg, IV) and maintained with isoflurane (ETISO, 1.2%) in oxygen. Isoflurane administration was discontinued 70 minutes after medetomidine administration, and dogs were allowed to recover. Plasma MK-467 concentrations were determined only for the MMK50, MMK100, and MMK150 treatments.
Citation: American Journal of Veterinary Research 78, 11; 10.2460/ajvr.78.11.1245
Mean ± SD plasma dexmedetomidine (A) and MK-467 (B) concentrations over time for 8 healthy 5-year-old Beagles that received each of 5 premedication protocols (medetomidine [10 μg/kg, IV] alone [MED; solid line with black circles] and combined with MK-467 at each of 3 doses [50 {MKK50; dashed line with white squares}, 100 {MMK100; dashed line with white triangles}, and 150 {MMK150; dashed line with white circles} μg/kg, IV] and 15 minutes after glycopyrrolate [10 μg/kg, SC; MGP; solid line with black squares]) prior to anesthesia. There were at least 14 days between each experiment. Data were acquired at 5, 15, 35, 45, 60, and 90 minutes after administration of medetomidine and MK-467. Twenty minutes after medetomidine administration, anesthesia was induced with increments of ketamine (0.5 mg/kg, IV) and midazolam (0.1 mg/kg, IV) and maintained with isoflurane (ETISO, 1.2%) in oxygen. Isoflurane administration was discontinued 70 minutes after medetomidine administration, and dogs were allowed to recover. Plasma MK-467 concentrations were determined only for the MMK50, MMK100, and MMK150 treatments.
Citation: American Journal of Veterinary Research 78, 11; 10.2460/ajvr.78.11.1245
Mean ± SD AUClast for dexmedetomidine, levomedetomidine, and MK-467 for the dogs of Table 1 after each treatment.
| Treatment | No. of dogs | Dexmedetomidine (min•ng/mL) | Levomedetomidine (min•ng/mL) | MK-467 (min•ng/mL) |
|---|---|---|---|---|
| MED | 8 | 155.1 ± 36.3† | 92.6 ± 23.2 | — |
| MMK50 | 7 | 138.9 ± 33.6 | 79.1 ± 20.3 | 7,170 ± 3,690 |
| MMK100 | 8 | 132.5 ± 49.0 | 83.0 ± 36.8 | 14,900 ± 5,910 |
| MMK150 | 8 | 101.0 ± 12.2* | 59.6 ± 7.3 | 21,700 ± 5,020 |
| MGP | 8 | 133.3 ± 35.5 | 80.0 ± 21.6 | — |
Data from only 7 dogs were evaluated for the MMK50 treatment owing to a missing sample for 1 dog.
Value differs significantly (P < 0.05) from the corresponding value for the MED treatment.
Value differs significantly (P < 0.05) from the corresponding value for the MMK150 treatment.
— = Not applicable.
See Table 1 for remainder of key.
Discussion
The dogs of the present study received lower doses of MK-467 in relation to medetomidine, compared with dogs of other studies.17,20–23 Results indicated that the low doses of MK-467 used in this study alleviated medetomidine-induced decreases in HR and CI in a dose-dependent manner in dogs during the premedication period prior to anesthetic induction (T0 to T20). Additionally, SC administration of glycopyrrolate 15 minutes prior to premedication with medetomidine (10 μ/kg, IV) did not attenuate the medetomidine-induced decrease in CI, and the HR and MAP were greatest for the MGP treatment throughout the premedication period.
Similar to the findings of other studies,2,4,13,15 the SVRI increased and arterial hypertension (MAP > 150 mm Hg) was detected after premedication with medetomidine alone (MED treatment) or 15 minutes after glycopyrrolate (MGP treatment). Furthermore, the mean SVRI for the MGP treatment was significantly greater than that for the MED treatment. That finding is in contrast to the results of another study10 in which glycopyrrolate administration did not significantly alter the SVRI of dogs sedated with romifidine, an α2-adrenoceptor agonist that is less potent than medetomidine. However, the reason the mean SVRI for the MGP treatment was greater than that for the MED treatment was probably associated with the fact that blood pressure increased disproportionately relative to changes in CO. The initial medetomidine-induced bradycardia (HR < 60 beats/min) was attenuated by the administration of glycopyrrolate in the present study, and the mean HR for the MGP treatment was significantly greater than that for the MED treatment throughout the premedication period, a finding that is in contrast to results of a previous study13 because none of the dogs became tachycardic (HR > 150 beats/min) during the MGP treatment. Nevertheless, the mean CI did not differ significantly between the MED and MGP treatments at any time during the premedication period, which suggested that glycopyrrolate did not enhance cardiac performance. We assumed that the increase in HR observed during the MGP treatment resulted from a shorter ventricular filling time, which, in conjunction with a simultaneous increase in arterial resistance, decreased stroke volume to a greater extent than that observed for the other 4 treatments. Thus, the increase in HR was not sufficient to maintain CI during the MGP treatment. Similar findings have been described for dogs that received atropine and dexmedetomidine concurrently.15 In the present study, we attempted to optimize the dose, route, and timing of glycopyrrolate administration to minimize undesirable cardiovascular effects. Also, glycopyrrolate was chosen rather than atropine because it does not cross the blood-brain barrier,28 and dogs pretreated with glycopyrrolate develop fewer undesirable cardiovascular anomalies, such as VPCs and sinus tachycardia, than do dogs pretreated with atropine or glycopyrrolate concurrently with medetomidine.12,13,15 Contrary to results of other studies,10,14 when the dogs of the present study received the MGP treatment, pretreatment with an anticholinergic 15 minutes prior to premedication with medetomidine did not prevent medetomidine-induced second-degree atrioventricular blocks. Additionally, the cardiac conduction abnormalities (eg, occasional single VPCs with pulse deficits or second-degree atrioventricular blocks) detected during the premedication period for the MGP treatment might have had a negative effect on HR or stroke volume, causing a subsequent reduction in the CI.
For the dogs of the present study, administration of MK-467 in combination with medetomidine alleviated medetomidine-induced increases in SVRI and MAP during the premedication period in a dose-dependent manner. In fact, the mean MAP was slightly decreased from baseline and the mean SVRI was affected the least when the MMK150 treatment was administered. Stroke volume, which is influenced by cardiac preload, contractility, and afterload, decreased for all treatments except MMK150. Interestingly, the mean CVP increased following administration of the MMK150 treatment, even though the initial medetomidine-induced increase in the MAP was attenuated. That increase in CVP may have been associated with varying distributions and functions of α2-adrenoceptors in arterial and venous vascular beds as well as a simultaneous decrease in CI.29
Although the mean respiratory rate for the dogs of the present study decreased after administration of each treatment, the mean Paco2 remained fairly stable throughout the premedication period, which suggested that the dogs were not hypoventilating. The mean Cao2 increased from baseline following the MED and MGP treatments, whereas the medetomidine-induced increase in Cao2 was alleviated by MK-467 in a dose-dependent manner. In another study,21 dogs that received a higher dose of MK-467 (250 μg/kg) than the highest dose (150 μg/kg) of the drug administered in the present study had a slight decrease in Cao2. Given that the mean Pao2 did not differ significantly among any of the treatments of the present study during the premedication period, it is possible that changes in Hba (data not shown) were responsible for the differences observed in mean Cao2. Small, albeit significant, decreases in Hba have been reported in dogs of other studies21,30 following concurrent administration of MK-467 and medetomidine. Moreover, administration of α2-adrenoceptor agonists to dogs can increase PCV and blood hemoglobin concentrations.10,31,32 Even if the Cao2 had increased after premedication with any of the medetomidine–MK-467 combination treatments (MMK50, MMK100, or MMK150), that increase would have been insufficient to compensate for the decrease in CI and maintain Do2I. Mean arterial lactate concentrations remained well below 2.5 mmol/L (the upper limit of the reference range for healthy dogs33) for the duration of the observation period for all 5 treatments, which was in agreement with the results of other studies17,19–22 in which dogs were administered a peripheral α2-adrenoceptor antagonist in combination with an α2-adrenoceptor agonist.
Although monitoring the level of sedation or anesthesia achieved during the observation period following each treatment was beyond the scope of the present study, all dogs appeared to be well sedated after each treatment and a surgical plane of anesthesia (eg, inward rotation of eyes, loss of palpebral reflex, and relaxed jaw tone) was achieved. In another study21 in which anesthesia was induced with propofol, dogs premedicated with medetomidine and MK-467 (250 μg/kg) required a higher dose of propofol for successful endotracheal intubation than did dogs premedicated with medetomidine alone. In the present study, even though the mean AUClast of dexmedetomidine for the MED treatment was significantly greater than that for the MMK150 treatment, the total doses of ketamine and midazolam required for anesthetic induction did not differ between the 2 treatments. In dogs premedicated with medetomidine, administration of ketamine is associated with transient respiratory depression.34 Following ketamine administration, Paco2 was increased in all treatments, most likely owing to some ventilatory depression. However, the mean Paco2 was lowest for the MMK150 treatment. This was most likely because the mean plasma dexmedetomidine concentration at the time of anesthetic induction for the MMK150 treatment was lower than that for the MED treatment.
Fifteen minutes after anesthetic induction (T35), the mean SVRI and MAP were decreased from those immediately before induction (T15) and were close to or less than their baseline values for all 5 treatments of the present study. This finding was most likely a consequence of the vasodilatory effects of isoflurane7,8 and the sympatholytic effect of anesthesia in general. For each treatment, the decrease in SVRI observed following anesthesia induced with ketamine and midazolam and maintained with isoflurane caused the HR and CI to increase and return to levels near those at baseline. In another study35 involving isoflurane-anesthetized dogs (ETISO 1.22 ± 0.27%), low plasma dexmedetomidine concentrations (approx 0.2 ng/mL) caused a significant decrease in HR and increase in diastolic arterial pressure, compared with isoflurane alone. However, the plasma dexmedetomidine concentrations for the dogs of the present study were greater than those of that study35; thus, despite the changes in cardiovascular variables observed after the anesthetic induction, it is plausible that an ETISO of 1.2% alone may have been insufficient to completely attenuate all medetomidine-induced cardiovascular effects. Moreover, the CI during anesthesia for the MED treatment was significantly greater than that for the MGP treatment, presumably because the MED treatment induced a higher stroke volume index than did the MGP treatment. That finding was inconsistent with results of other studies in which HR and CI increased in romifidine-sedated dogs after pretreatment with glycopyrrolate (0.01 mg/kg, IM)10 and isoflurane-anesthetized dogs treated with dexmedetomidine (20 μg/kg, IV) and glycopyrrolate (40 μg/kg, IV, followed by 20 μg/kg, IV, q 30 min).3 In another study21 that had a similar design to the present study, except that a higher dose of MK-467 (250 μg/kg) was administered and anesthesia was induced with propofol, the decrease in SVRI observed after anesthestic induction was greater than that observed in this study and resulted in a low MAP (approx 60 mm Hg), whereas the MAP for the dogs of this study was maintained at approximately 70 mm Hg following administration of the highest dose of MK-467 (150 μg/kg). Additionally, the HR sustained during the MMK150 treatment was greater than that reported following administration of the higher dose of MK-467 in that other study.21 The differences observed between that study21 and the present study might have been a result of the different anesthetic induction agents used. In dogs premedicated with medetomidine, anesthestic induction with propofol results in a significantly lower HR and CI than anesthetic induction with ketamine and diazepam.36 In the present study, an increase in CI was generally associated with an increase in Do2I, which was consistent with the findings of the previous study.21
Some drugs can interfere with the LiDCO sensor37 and cause bias in CO measurements. It is currently unknown whether MK-467 or glycopyrrolate is compatible with the LiDCO sensor. Nevertheless, differences in the plasma dexmedetomidine concentrations among the treatments of the present study may have been a reflection of differences in CO and altered hepatic blood flow because the plasma clearance of dexmedetomidine is associated with CO.38 Thus, the plasma dexmedetomidine concentration data recorded in this study supported the CO results from the LiDCO sensor and suggested that CO was the least affected during the MMK150 treatment. Consequently, the AUClast for dexmedetomidine for the MMK150 treatment was significantly lower than that for the MED treatment. The AUClast for dexmedetomidine following administration of 150 μg of MK-467/kg in the present study was affected to a lesser extent than following administration of 250 μg of MK-467/kg in another study,39 in which plasma dexmedetomidine concentrations decreased by nearly 50% over time in unanesthetized dogs.
Residual effects of the anesthetic agents (propofol and isoflurane) used during instrumentation might have affected the results of the present study. However, in geriatric dogs (dogs in which the rate of drug clearance would be expected to be less than that in the healthy 5-year-old dogs used for this study), the mean residual duration of propofol is approximately 100 minutes,40 which was the approximate duration between propofol administration and collection of baseline measurements. Therefore, we believed that propofol administration during instrumentation had no effect on the study results. Additionally, at the time of intubation during each experiment, the ETISO was 0 before the vaporizer was turned on.
For the dogs of the present study, the mean SAP at baseline was increased from the reference range (110 to 150 mm Hg) for all 5 treatments. During baseline measurements, dogs were gently maintained in lateral recumbency to protect the patency of the arterial cannula because it was closely connected to the lithium sensor. Even though the dogs appeared to be relaxed at baseline, the high mean SAP was most likely a result of physical restraint.
Because the present study was purely experimental, dogs were not surgically or otherwise stimulated during any of the experiments. Clinical studies are necessary to evaluate the cardiovascular effects of MK-467 in dogs subsequently anesthetized with isoflurane. Also, further investigation is necessary to develop options for maintaining blood pressure in dogs that become hypotensive while anesthetized with isoflurane following premedication with medetomidine and MK-467.
Results of the present study indicated that premedication of dogs with medetomidine (10 μg/kg, IV) and 150 μg of MK-467/kg (IV; MMK150 treatment) provided the greatest cardiovascular stability during the premedication period, compared with premedication of dogs with medetomidine alone or in combination with MK-467 at doses of 50 and 100 μg/kg or 15 minutes after SC administration of glycopyrrolate (10 μg/kg). However, the MMK150 treatment partially, but not completely, alleviated medetomidine-induced hemodynamic changes (eg, bradycardia) during the premedication period. Additionally, although pretreatment of dogs with glycopyrrolate prior to medetomidine resulted in an increase in HR, it did not attenuate the medetomidine-induced decrease in CI during the premedication period or during isoflurane anesthesia. From a clinical standpoint, administration of medetomidine and MK-467 alleviated medetomidine-induced cardiovascular changes in dogs prior to anesthesia and might cause fewer adverse effects than glycopyrrolate administration.
Acknowledgments
The authors thank Merck, Sharp & Dohme Corp, Rahway, NJ, for donating the MK-467; and Vetcare Ltd, Mäntsälä, Finland, for donating the medetomidine and providing funding support for the materials used in the study.
Statistical analyses of cardiopulmonary data were performed by 4Pharma Ltd, Helsinki, Finland.
Presented in part as an abstract at the spring meeting of the Association of Veterinary Anaesthetists, Lyon, France, April 2016.
ABBREVIATIONS
| ABP | Arterial blood pressure |
| AUClast | Area under the time-concentration curve to the last sampling point |
| Cao2 | Arterial oxygen content |
| CI | Cardiac index |
| CO | Cardiac output |
| CVP | Central venous pressure |
| Do2I | Oxygen delivery index |
| ETISO | End-tidal isoflurane concentration |
| Hba | Arterial hemoglobin concentration |
| HR | Heart rate |
| LiDCO | Cardiac output measured by use of lithium dilution |
| MAP | Mean arterial pressure |
| SAP | Systolic arterial pressure |
| SVR | Systemic vascular resistance |
| SVRI | Systemic vascular resistance index |
| VPC | Ventricular premature contraction |
Footnotes
Dorbene (1 mg/mL), Laboratories Syva Sau, León, Spain.
Merck, Sharpe&Dohme, Philadelphia, Pa.
Robinul (0.2 mg/mL), Meda Pharma GmbH &Co, Bad Homburg, Germany.
Vetofol (10 mg/mL), Norbrook Laboratories Ltd, Newry, Northern Ireland.
Isoflo, Orion Pharma Ltd, Turku, Finland.
Terumo Europe NV, Leuven, Belgium.
CV-12702, Arrow International, Reading, Pa.
Ringer-Acetat Baxter Viaflo, Baxter Ltd, Helsinki, Finland.
Ketaminol Vet (50 mg/mL), Intervet International BV, Boxmeer, The Netherlands.
Midazolam (5 mg/mL), Hameln Pharma Plus GmbH, Hameln, Germany.
Anesco Inc, Georgetown, Ky.
S/5 Anesthesia Monitor, GE Healthcare, Helsinki, Finland.
Quick Cal Calibration gas, GE Healthcare, Helsinki, Finland.
Gabarith PMSET, Becton Dickinson, Sandy, Utah.
LidCO Plus Hemodynamic Monitor, LidCO Ltd, Cambridge, England.
Pico50, Radiometer, Copenhagen, Denmark.
ABL 855, Radiometer, Copenhagen, Denmark.
Metacam (5 mg/mL), Boehringer Ingelheim Vetmedica, Ingelheim am Rhein, Germany.
Phoenix 64, version 6.3, Pharsight Corp, Certara USA Inc, Princeton, NJ.
SAS, version 9.3, SAS Institute Inc, Cary, NC.
SPSS, version 22, IBM SPSS Inc, Chicago, Ill.
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