Multicenter clinical comparison of sedative and analgesic effects of medetomidine and xylazine in dogs.

OBJECTIVE: To evaluate analgesic and sedative effects of medetomidine hydrochloride in dogs and to compare effects with those of xylazine hydrochloride. DESIGN: Randomized, controlled trial. ANIMALS: 184 dogs that required sedation or analgesia for completion of minor diagnostic or therapeutic procedures. PROCEDURE: Dogs were sedated with medetomidine, i.v. (750 micrograms/m2 of body surface area) or i.m. (1,000 micrograms/m2) or with xylazine, i.v. (1.1 mg/kg 10.5 mg/lb] of body weight) or i.m. (2.2 mg/kg [1 mg/lb]). Sedative effects were measured by scoring posture and response to noise. Durations of effects were determined by measuring time intervals between drug administration and changes in posture. Analgesic effects were measured by determining toe-pinch pressure needed to elicit a withdrawal response. Clinicians rated sedative and analgesic effects and ease with which diagnostic or therapeutic procedures could be performed. RESULTS: Posture and response to noise scores were significantly higher for dogs given medetomidine, i.m., than for dogs given xylazine, i.m., and for dogs given medetomidine, i.v., than for dogs given xylazine, i.v. Time to regaining sternal recumbency and time to regaining ability to stand were longest after i.m. administration of medetomidine. Toe-pinch pressures were not significantly different among groups. Clinicians rated overall analgesic and sedative effects as excellent significantly more often after administration of medetomidine than after administration of xylazine. Prevalence of adverse effects did not differ among groups. CLINICAL IMPLICATIONS: Medetomidine and xylazine, at doses tested, were effective and safe, but results of subjective measurements indicated that medetomidine provided better sedation and analgesia than did xylazine. Specific alpha 2-adrenergic antagonists (atipamezole, yohimbine) are available for control of adverse cardiovascular effects.

Effects of intrapleural pressure changes on canine left ventricular function.

Left ventricular diameter, pressure, and dP/dt, as well as aortic and intrapleural pressures, were recorded simultaneously from 10 acute anethetized mongrel dogs. Normal inspiration produced a significant decrease in end-diastolic diameter (46.9 +/- 1.2 to 43.1 +/- 1.0 mm) and end-systolic diameter (35.5 +/- 1.4 to 34.2 +/- 1.7 mm), reducing stroke diameter 2.5 mm. Airway occlusion (greater than or equal to 60%) produced a reduction in the decrease of end-diastolic diameter seen with normal inspiration and, to a lesser degree, in end-systolic diameter. Because only acute airway occlusion greater than or equal to 80% produced a significant increase in left ventricular afterload, the authors conclude that normal spontaneous inspiration results in a decrease in left ventricular preload due to pooling of blood in the pulmonary vasculature. The data suggest that this decrease in left ventricular preload is the predominant mechanism responsible for the inspiratory decrease in left ventricular stroke volume.