A 70-year-old man on ICU has a high-output state: cardiac index 5.5 L/min/m2, SVR 450 dyn·s/cm5, MAP 55 mmHg, ScvO2 82%, lactate 4.5 mmol/L. He has been on antibiotics for 48 hours for an abdominal source with adequate source control. Despite the high cardiac output and high ScvO2, lactate remains elevated. What is the most likely explanation for the persistent hyperlactataemia?