A 72-year-old man with chronic hypertension and COPD presents to the emergency department with sudden onset of severe hypogastric and left lower quadrant pain. Blood pressure is 85/50 mmHg, heart rate 120 bpm. Abdominal examination reveals a pulsatile mass below the umbilicus and left flank ecchymosis (Grey Turner sign). Point-of-care ultrasound shows a large heterogeneous mass in the periumbilical region with no clear intimal flap. What is the most appropriate immediate management?