Use of Detomidine Alone or Associated with Yohimbine in Horses: Cardiorespiratory Evaluation

Abstract

Background: Equine dentistry, an important veterinary field, saw significant advancements in the 1970s and 80s, allowing safer standing procedures with improved tools, techniques, and medications. The anesthetic protocol typically uses an α2-adrenergic agonist sedative like detomidine, which provides mild analgesia and sedation while keeping the horse standing, thus avoiding risks linked to general anesthesia. However, detomidine can alter physiological parameters during procedures. Yohimbine, an α2-adrenergic antagonist, functions as a reversing agent for α2-adrenergic agonists, reducing both their sedative effects and associated adverse outcomes.

Cases: The study evaluated 24 horses (12 males, 12 females) from Viamão city and the southern zone of Porto Alegre, Brazil, weighing between 340 and 580 kg, undergoing elective or therapeutic dental procedures including routine odontoplasty without extractions or local blocks. The animals fasted for 6 h before the procedure and were advised to consume only fibrous foods post-treatment to avoid gastrointestinal issues. Detomidine [0.02 mg/kg IV], an α2-adrenergic agonist inducing sedation, analgesia, and muscle relaxation, was administered to all horses, and yohimbine [0.1 mg/kg IV], an α2-adrenergic antagonist, to half of them, with evaluations of vital signs (heart rate, respiratory rate, body temperature) at baseline, 20, and 50 min. Results showed that heart rate was the only parameter with statistically significant changes, while other parameters remained stable. Environmental factors, like exposure to open, unshaded areas, influenced the animals' physiological parameters, potentially affecting results due to the limited sample size. The limited sample size was noted as a constraint, as random effects could disproportionately affect results. Statistical analysis used a two-tailed test with an alpha of 0.05 and 11 degrees of freedom, reflecting the sample size per group. These findings highlight the importance of controlled conditions and adequate sample sizes for future studies.

Discussion: This study confirms that detomidine is a safe sedative for cardiorespiratory evaluation in equine dental procedures, provided that vital signs are monitored before, during, and after its use. The benefits of combining detomidine with an α2-adrenergic antagonist like yohimbine, which effectively reduces adverse effects, such as gastrointestinal hypomotility and bradycardia. Detomidine acts by binding to α2-adrenergic receptors, decreasing catecholamine release and central nervous system excitation. This mechanism explains the physiological responses, including decreased heart rate (HR), respiratory rate (RR), and motor activity. The study observed significant heart rate changes in both groups at 20 min post-detomidine administration, with further changes in the yohimbine group at 50 min due to the antagonist's rapid reversal of sedation and bradycardia. While detomidine affects thermoregulation, no body temperature changes were noted, a known side effect, underscores the benefit of using an antagonist to mitigate such risks. Detomidine, with an action lasting 60-120 min, is safe at low doses (0.002 - 0.02 mg/kg IV or IM). Gastrointestinal effects, including mild hypomotility, were noted but did not worsen post-procedure. The results indicate that while detomidine is generally safe for equine dental procedures, proper preparation and continuous monitoring of vital parameters are essential. Additionally, yohimbine demonstrates significant benefits as a reversing agent by mitigating adverse effects such as bradycardia and gastrointestinal hypomotility.

Keywords: equine, α2-agonist, reverser, sedative, equine dentistry.