 Patients undergoing total knee arthroplasty with spinal anesthesia are often sedated to reduce patient discomfort due to large noises during surgery and also to reduce anxiety. Most commonly used sedatives include propofol and midazolam, but these agents often cause hypotension or respiratory depression, especially in elderly patients. Dexmedetomidine is a selective alpha 2 adrenergic drug, which acts as a sedative and also has analgesia effects. In contrast to propofol or midazolam, dexmedetomidine rarely causes respiratory depression, and therefore is often used in critically ill patients in the ICU and also in patients undergoing simple procedures. Hemodynamically, dexmedetomidine evokes a biphasic blood pressure response with a short hypertensive phase and subsequent hypotension. Bradycardia is also observed in many patients, which may lead to more serious outcomes in elderly patients who often have multiple comorbidities. Therefore, drugs to prevent bradycardia during dexmedetomidine infusion may help patients maintain a more stable hemodynamic state. The present study aims to compare the ability of atropine and glycopyrrolate to prevent bradycardia during dexmedetomidine infusion in elderly patients undergoing knee surgery with spinal anesthesia. 