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- Author or Editor: William W. Muir III x
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Objective—To determine the effects of xylazine on canine coronary artery smooth muscle tone.
Sample Population—Hearts of 26 healthy dogs.
Procedure—Dogs were anesthetized with pentobarbital, and vascular rings of various diameters were prepared from the epicardial coronary arteries. Vascular rings were placed in tissue baths to which xylazine was added (cumulative concentrations ranging from 10–10 to 10–4M), and changes in vascular ring tension were continuously recorded. Effects of the nitric oxide inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 5mM), the α1-adrenoceptor antagonist prazosin (10mM), and the α2-adrenoceptor antagonist atipamezole (10mM) on xylazine-induced changes in vascular ring tension were determined. Results were expressed as percentage of maximal contraction for each vascular ring preparation.
Results—Xylazine induced vasoconstriction of small (< 500-µm-diameter) and medium (500- to 1,000-µmdiameter) vascular rings but not of large (> 1,000-µmdiameter) rings. For large vascular rings, L-NAME, atipamezole, and prazosin did not significantly affect the contractile response to xylazine. For small vascular rings, the contractile response following addition of xylazine to rings treated with L-NAME was not significantly different from the contractile response following addition of xylazine to control rings, except at a xylazine concentration of 10–6M. Xylazine-induced vasoconstriction of small vascular rings was blocked by atipamezole, but the addition of prazosin had no effect on xylazine-induced vasoconstriction.
Conclusions and Clinical Relevance—Results suggest that xylazine increases smooth muscle tone of small canine coronary arteriesand that this effect is predominantly mediated by stimulation of α2adrenoceptors.( Am J Vet Res 2004;65:431–435)
Objective—To evaluate the effects of the α2-adrenoceptor agonist medetomidine on respiratory rate (RR), tidal volume (VT), minute volume (VM), and central respiratory neuromuscular drive as determined by inspiratory occlusion pressure (IOP) during increasing fractional inspired concentrations of carbon dioxide (FiCO2) in conscious dogs.
Animals—6 healthy dogs (3 males and 3 females).
Procedure—Dogs were administered 0, 5, or 10 µg of medetomidine/kg IV. We measured RR, VT, VM, and IOP for the first 0.1 second of airway occlusion (IOP0.1) during FiCO2 values of 0%, 2.5%, 5.0%, and 7.5% at 15 minutes before and 5, 30, and 60 minutes after administration of medetomidine.
Results—Increases in FiCO2 significantly increased RR, VT, and VM. The IV administration of 5 and 10 µg of medetomidine/kg significantly decreased RR and VM at 5, 30, and 60 minutes for FiCO2 values of 2.5% and 5.0% and at 30 and 60 minutes for an FiCO2 value of 7.5%. The IOP0.1 was decreased after 30 minutes only for an FiCO2 value of 7.5% in dogs administered 5 and 10 µg of medetomidine/kg. The IOP0.1 was decreased at 60 minutes after administration of 10 µg of medetomidine/kg for an FiCO2 value of 7.5%.
Conclusions and Clinical Relevance—The IV administration of medetomidine decreases RR, VM, and central respiratory drive in conscious dogs. Medetomidine should be used cautiously and with careful monitoring in dogs with CNS depression or respiratory compromise. (Am J Vet Res 2004;65: 720–724)
Objective—To compare the effects of lactated Ringer's solution (LRS) with those of a physiologically balanced 6% hetastarch plasma expander administered to isoflurane-anesthetized dogs with hypotension induced by blood withdrawal.
Animals—12 healthy Beagles.
Procedure—Blood was withdrawn from isofluraneanesthetized dogs (volume withdrawn measured) to a systolic arterial blood pressure (SAP) of 80 mm Hg. Six dogs each received either LRS or hetastarch solution (90 mL/kg/h, IV). Hemodynamic variables, pH, blood gas concentrations, PCV, serum electrolyte and total protein concentrations, and colloid osmotic pressure (COP) were determined at baseline, while SAP was 80 mm Hg, and after fluid treatment. The volume of fluid administered and rate of return of SAP to within 10% of baseline values were recorded.
Results—Mean ± SD volume of blood withdrawn to decrease SAP to 80 mm Hg was 173 ± 38 mL. Hemodynamic variables decreased after blood withdrawal but returned to baseline values more rapidly after infusion of a smaller volume of hetastarch solution, compared with the response to LRS infusion. Whereas PCV and serum total protein concentration decreased after administration of either solution, COP decreased only after administration of LRS. The total volume of hetastarch solution and LRS required to restore and maintain SAP to within 10% of baseline values was 1.1 ± 0.9 and 4.4 ± 1.7 times greater than the volume of blood removed, respectively.
Conclusions and Clinical Relevance—Compared with LRS infusion, smaller volumes of hetastarch solution normalized and maintained SAP without lowering COP in isoflurane-anesthetized dogs after blood withdrawal. (Am J Vet Res 2004;65:1189–1194)
Objective— To determine the hemodynamic effects of IM administration of romifidine hydrochloride in propofol-anesthetized cats.
Animals—15 adult domestic shorthair cats.
Procedure—Cats were randomly assigned to receive romifidine (0, 400, or 2,000 µg/kg, IM). Cats were anesthetized with propofol and mechanically ventilated with oxygen. The right jugular vein, left carotid artery, and right femoral artery and vein were surgically isolated and catheterized. Heart rate; duration of the PR, QRS, and QT intervals; mean pulmonary artery pressure; mean right atrial pressure; systolic, diastolic, and mean arterial pressures; left ventricular systolic pressure; left ventricular end-diastolic pressure; and cardiac output were monitored. Systemic vascular resistance, rate of change of left ventricular pressure, and rate pressure product were calculated. Arterial and venous blood samples were collected anaerobically for determination of pH and blood gas tensions (PO2 and PCO2).
Results—Administration of romifidine at 400 and 2,000 µg/kg, IM, decreased heart rate, cardiac output, rate of change of left ventricular pressure, rate pressure product, and pH. Arterial and pulmonary artery pressures, left ventricular pressure, left ventricular end-diastolic pressure, and right atrial pressure increased and then gradually returned to baseline values. Arterial blood gas values did not change, whereas venous PCO2 increased and venous PO2 decreased. Significant differences between low and high dosages were rare, suggesting that the dosages investigated produced maximal hemodynamic effects.
Conclusion and Clinical Relevance—Romifidine produces cardiovascular effects that are similar to those of other α2-agonists. High dosages of romifidine should be used with caution in cats with cardiovascular compromise. (Am J Vet Res 2002;63:1241–1246)
Objective—To evaluate the effect of medetomidine on minimum alveolar concentration (MAC), respiratory rate, tidal volume, minute volume (VM), and maximum inspiratory occlusion pressure (IOCPmax) in halothane- and isoflurane-anesthetized dogs.
Animals—6 healthy adult dogs (3 males and 3 females).
Procedure—The MAC of both inhalants was determined before and 5, 30, and 60 minutes after administration of medetomidine (5 μg/kg, IV). Dogs were subsequently anesthetized by administration of halothane or isoflurane and administered saline (0.9% NaCl) solution IV or medetomidine (5 μg/kg, IV). Respiratory variables and IOCPmax were measured at specific MAC values 15 minutes before and 5, 30, and 60 minutes after IV administration of medetomidine while dogs breathed 0% and 10% fractional inspired carbon dioxide (FICO2). Slopes of the lines for VM/FICO2 and IOCPmax/FICO2 were then calculated.
Results—Administration of medetomidine decreased MAC of both inhalants. Slope of VM/FICO2 increased in dogs anesthetized with halothane after administration of medetomidine, compared with corresponding values in dogs anesthetized with isoflurane. Administration of medetomidine with a simultaneous decrease in inhalant concentration significantly increased the slope for VM/FICO2, compared with values after administration of saline solution in dogs anesthetized with halothane but not isoflurane. Values for IOCPmax did not differ significantly between groups.
Conclusions and Clinical Relevance—Equipotent doses of halothane and isoflurane have differing effects on respiration that are most likely attributable to differences in drug effects on central respiratory centers. Relatively low doses of medetomidine decrease the MAC of halothane and isoflurane in dogs.
Objective—To compare the minimum alveolar concentration (MAC) of isoflurane required to prevent corticocerebral activation, autonomic responses, and purposeful movements after somatic or visceral stimulation in cats anesthetized with isoflurane.
Animals—17 healthy spayed female cats.
Procedure—Bispectral index (BIS), autonomic parameters, and purposeful movements were monitored before and after somatic or visceral stimuli in cats anesthetized with isoflurane. End-tidal (ET) isoflurane concentration was varied to determine MAC values for cortical arousal (MACBIS), autonomic responsiveness (MACBAR), and purposeful movement (MAC). Bispectral index values ≥ 60 were considered to represent corticocerebral activation.
Results—Minimum alveolar concentration for purposeful movement was significantly less than MACBIS and MACBAR for both somatic and visceral stimulation. Individual MAC values for somatic stimulation were not significantly different from respective MAC values for visceral stimulation. The percentage of cats that had a BIS response ≥ 60 was inversely related to the end-tidal isoflurane concentration.
Conclusions and Clinical Relevance—Corticocerebral arousal and subcortical autonomic reflexes occured at isoflurane anesthetic concentrations at which reflexive or purposeful movements were absent. These results suggested that isoflurane had a preferential effect on voluntary motor output at low end-tidal isoflurane concentrations, and that sensory pathways, subcortical sympathetic output, and cortical responsiveness are less susceptible to the anesthetic effects of isoflurane. Bispectral index values obtained after somatic or visceral stimulation were sensitive for the detection of early changes in cortical excitability. (Am J Vet Res 2003; 64:1528–1533)
Objective—To determine whether the prestimulation bispectral index (BIS) value or relative change in BIS after noxious stimulation can be used to assess the depth of isoflurane anesthesia in cats.
Animals—17 healthy female cats.
Procedure—Electroencephalogram (EEG) patterns and BIS values were examined in cats that received increasing end-tidal (ET) isoflurane concentrations. Subsequently, BIS values were determined before and after either a noxious somatic or visceral stimulus in cats that received ET isoflurane concentrations ranging from 1.8% to 2.4%. Electrical stimuli of the tail base and bladder distension to 50 cm of water were the somatic and visceral stimuli, respectively.
Results—The resting BIS at ET isoflurane concentrations from 1.4% to 1.9% steadily decreased concurrently with increasing degrees of EEG suppression. Prestimulation BIS values, however, were not related to 1.8% to 2.4% ET isoflurane concentrations and not useful for prediction of BIS values or hemodynamic and movement responses after a noxious stimulus. The poststimulation BIS value and the difference between mean BIS values before and after stimulation were inversely correlated with increasing ET isoflurane concentrations. Poststimulation BIS values > 60 were observed at ET isoflurane concentrations greater than those associated with a movement response after a stimulus.
Conclusions and Clinical Relevance—The prestimulation BIS value has limited use in assessing anesthetic depth in cats during isoflurane anesthesia. The change in BIS values after a noxious somatic or visceral stimulus was a reliable measure of anesthetic depth and may be a useful measure of early arousal from the hypnotic state. (Am J Vet Res 2003;64:1534–1541)
Objective—To compare effects of electroacupuncture and butorphanol on hemodynamic and respiratory variables and rectal analgesia in mares after controlled rectal distention.
Animals—8 healthy mares.
Procedure—Each horse received saline (0.9% NaCl) solution (0.01 mL/kg, IV; control treatment), butorphanol tartrate (0.1 mg/kg, IV), or 2 hours of electroacupuncture (EA) at acupoints Bladder 21, 25, and 27 on both sides of the vertebral column, Bai hui, and Stomach 36 (right side only). Order of treatments in each mare was randomized. At least 7 days elapsed between treatments. A balloon was inserted in the rectum of each mare, and controlled distention of the balloon (pressures of ≤ 220 mm Hg) was used to measure nociceptive rectal pain threshold. Rectal temperature and cardiovascular and respiratory variables were measured before (baseline) and 5, 15, 30, 60, 90, and 120 minutes after onset of each treatment.
Results—Butorphanol produced greater increases in rectal pain threshold, compared with EA (mean ± SD, 214 ± 24 vs 174 ± 35 mm Hg of balloon pressure). Electroacupuncture produced minimal cardiovascular and respiratory changes. Although clinically not important, butorphanol produced moderate significant increases in heart and respiratory rates, arterial blood pressure, and rectal temperature and decreases in arterial oxygen tension. Arterial pH, carbon dioxide tension, bicarbonate concentrations, base excess, Hct, and concentration of total solids were not significantly different from baseline values after EA, butorphanol, and control treatments.
Conclusions and Clinical Relevance—Electroacupuncture and butorphanol (0.1 mg/kg, IV) may provide useful rectal analgesia in horses. (Am J Vet Res 2003;64:137–144)
Objective—To determine the analgesic, hemodynamic, and respiratory effects induced by caudal epidural administration of meperidine hydrochloride in mares.
Animals—7 healthy mares.
Procedure—Each mare received meperidine (5%; 0.8 mg/kg of body weight) or saline (0.9% NaCl) solution via caudal epidural injection on 2 occasions. At least 2 weeks elapsed between treatments. Degree of analgesia in response to noxious electrical, thermal, and skin and muscle prick stimuli was determined before and for 5 hours after treatment. In addition, cardiovascular and respiratory variables were measured and degree of sedation (head position) and ataxia (pelvic limb position) evaluated.
Results—Caudal epidural administration of meperidine induced bilateral analgesia extending from the coccygeal to S1 dermatomes in standing mares; degree of sedation and ataxia was minimal. Mean (± SD) onset of analgesia was 12 ± 4 minutes after meperidine administration, and duration of analgesia ranged from 240 minutes to the entire 300-minute testing period. Heart and respiratory rates, rectal temperature, arterial blood pressures, Hct, PaO2, PaCO2, pHa, total solids and bicarbonate concentrations, and base excess were not significantly different from baseline values after caudal epidural administration of either meperidine or saline solution.
Conclusions and Clinical Relevance—Caudal epidural administration of meperidine induced prolonged perineal analgesia in healthy mares. Degree of sedation and ataxia was minimal, and adverse cardiorespiratory effects were not detected. Meperidine may be a useful agent for induction of caudal epidural analgesia in mares undergoing prolonged diagnostic, obstetric, or surgical procedures in the anal and perineal regions. (Am J Vet Res 2001;62:1001–1007)