A 68-year-old man with a history of atrial fibrillation (not on anticoagulation) presents to the emergency department with sudden onset of severe pain in his left foot and calf. On examination, the left foot is pale and cold, with absent dorsalis pedis and posterior tibial pulses. Capillary refill is delayed. Sensory examination reveals diminished sensation in the foot. Motor power in the foot is intact. What is the most appropriate immediate next step in management?