A 60-year-old man on ICU post-emergency laparotomy develops acute mesenteric ischaemia on day 3. CT angiography shows occlusion of the superior mesenteric artery. Lactate is 12 mmol/L, WCC 28 × 10⁹/L, and he has new-onset metabolic acidosis (pH 7.15). He is on noradrenaline 0.3 mcg/kg/min. What is the most important immediate management step?