An observational study showed that a slow heart rate (bradycardia) on arrival to the emergency department is a positive predictor of the presence of xylazine in patients with opioid overdose. Bradycardia has been reported in patients with opioid overdose. The study suggests that this symptom may signal the involvement of xylazine in opioid overdose. Xylazine is a substance often added to opioids. A slow heart rate helps doctors identify such cases in the emergency room.